These are some common FAQ’s relating to the Kepler telescope, it’s mission history, and recent discoveries.
For more information on the Kepler telescope, please visit their website at: http://kepler.nasa.gov/
A. Kepler Mission Science
A1. How will Kepler detect planets?
The Kepler Mission is designed to detect planets as they pass in front of their stars which causes a tiny dip in the stars’ light. (See Occultation-Graph animation (QuickTime, 1 MB).
Kepler will look at just one large area of the sky in the constellations Cygnus and Lyra. Over the course of the mission, the spacecraft will simultaneously measure the variations in the brightness of more than 100,000 stars every 30 minutes, searching for the tiny “winks” in light output that happen when a planet passes in front of its star. The effect lasts from about an hour to about half a day, depending on the planet’s orbit and the type of star. The mission is designed to detect these changes in the brightness of a star when a planet crosses in front of t, or “transits the star.” This is called the “transit method” of finding planets. Transits are only seen when a star’s planetary system is nearly perfectly aligned with our line of sight. For a planet in an Earth-size orbit, the chance of it being aligned to produce a transit is less than 1%.
A2. What is the threshold for a detection as a planet transits a star?
Transits, which cause dips in a star’s light, are minuscule compared to the brightness of the star, and challenging to detect. For an Earth-size planet transiting a Sun-like star the change in brightness is only 84 parts per million (ppm). That is less than 1/100th of 1%. For a Jupiter-size planet, the transit causes the star light to dip 1 to 2%. The figure shows to scale both a Jupiter transit across an image of our sun on the left and an Earth-size transit to scale on the right. The size of the effect for an Earth is similar to the dimming one might see if a flea were to crawl across a car’s headlight viewed from several miles away.
A3. Do you need several transits to find a planet?
The discovery of a planet is confirmed by observing several transits that have the same depth (dip in star light), duration (time to transit the star), and period (same amount of time between successive transits). A single event that looks like a transit is not enough. It must be confirmed by observing repeated transits.
A4. Will all the stars Kepler observes have transiting planets?
Transits are only seen when a star’s planetary system is nearly perfectly aligned with our line of sight. There is no preferred alignment of the plane of planetary systems. The orbits of the planets can be at any angle to our line of sight. Therefore, the Kepler Mission is designed to look at more than 100,000 stars to find the small percentage that will actually show transits. For a Jupiter-size planet orbiting close to its star, the chance of a transit is about 1 in 10 (10%). For an Earth-size planet in an Earth-size orbit, the chance of it being aligned to produce a transit is less than 1%.
For Kepler to detect a transiting planet, its orbit plane must be lined up with our line of sight. Most of the time, the extrasolar planets’ orbital planes do not line up. For Earth-size planets around Sun-like stars, the chances of randomly oriented orbital planes being in the correct orientation for Kepler to see a transit is about 0.5%. That is why the design of Kepler called for a very wide field telescope to be able to observe more than 100,000 stars. If all those stars had Earth-size planets, about 500 (100,000 x .005) would be in the correct orientation to transit. Statistically, we can infer that every planet Kepler detects represents hundreds more planets that are out there but not detectable due to inopportune orbital orientation. (See also web page on Characteristics of Transits specifically the section on “Geometric Probability”.)
A5. Why can’t Earth-size planetary transits be observed from the ground?
There are two major reasons why these observations can’t be done from the ground:
- The motions in the atmosphere are constantly bending the rays of light from each star into different directions. This is why stars appear to twinkle. If you can see the change with your eye, you already know that the apparent brightness is changing by more than 50% (one stellar magnitude). With a lot of effort and for a very small region of the sky, astronomers have been able to measure changes as small as one part in 1,000 by comparing each star in a group to the whole group. This precision is still not good enough to find Earth-size planets, but should still be okay for detection of giant planets from the ground.
- To detect a planetary transit as short as 2 hours out of a year requires measuring the brightness of the stars continuously. You can’t blink! That means that you would need to set up dedicated telescopes in many places around the globe, so that there would be at least one of them on the night side of the Earth at all times. But, as the Earth orbits the Sun, the available night sky continuously changes. So there is no one part of the sky that can be continuously monitored throughout the year. In addition, the inevitable bad weather and the moon makes the prospects for ground based observing even that more inefficient. This would end up being a very expensive operation, even if the stars didn’t twinkle. To detect Earth-size planets, space is the necessary place to be.
A6. Don’t the stars vary more than the change caused by a transit?
Yes, the stars do vary in brightness all the time. In fact it is almost impossible to make a perfectly constant source of light. Fortunately, the stars we are most interested in are stars like our Sun. They vary less than the change in brightness caused by an Earth-size planetary transit on the same time scale as a transit (a few hours).
Our Sun varies over many time scales: There are Maunder minimums, which do not occur for many centuries or longer and have caused “mini ice ages” even as recently as during the 17th century. There is an eleven-year “solar cycle” of minimum and maximum activity. The largest short-term variations are caused by “sun spots” that appear and fade, and rise and set as the Sun rotates with a period of four weeks.
Planetary transits have durations of a few hours to less than a day. The measured solar variability on this time scale is 1 part 100,000 (10 ppm) as compared to an Earth-size transit of 1 part in 12,000 (80 ppm). Even then, most of the variability is in the UV, which is excluded from the measurements by the Kepler Mission . (For more information, read about “ Stellar Variability.”)
A7. What results are expected from Kepler science?
To estimate possible results for the Kepler Mission science some assumptions must be made:
- One-hundred thousand main-sequence stars are monitored and variable stars are excluded;
- On average, two Earth-size or larger planets exist in the region between 0.5 and 1.5 AU. (An AU is an astronomical unit, the average distance from the Sun to the Earth.)
- About 0.5% of star-planet systems are aligned to allow transit observations of planets in or near the habitable zone of the star. (The habitable zone is the region where water can exist as a liquid on the surface of a planet.)
With those assumptions the number of detections still depends on what size planets are common. We may find
- About 50 planets if most are about 1.0 Earth radius in size
- About 185 planets if most are about 1.3 Earth radii in size
- About 640 planets if most are about 2.2 Earth radii in size
(Or possibly some combination of the above)
- About 12% of the cases with two or more planets per system
Most likely results will be some combination of the above. There will also be hundreds of giant planets found and many instances with two or more planets per system. (For more on expected results see the Expected Results page.)
A8. When do you think will the first Earthlike planet orbiting a Sun-like star be discovered?
Kepler is seeking evidence of Earth-size planets in the habitable zone of Sun-like stars. To confirm an Earth, the science mission team requires a minimum of 3 transits of the same period, depth and duration. In the case of the Sun-like star, the period of the planet would be about the same as the Earth: one year. Thus, it will require a minimum of 3 years (and likely longer) to find an Earth-size planet in the habitable zone, and confirm the observations with 3 transits. Kepler launched in 2009, and the soonest we anticipate announcing an Earth-size planet orbiting a Sun-like star would be sometime in 2012-2013. For more on expected results see the Expected Results page.
A9. When will the Kepler Mission Science Office release data on planets detected by transits?
It is NASA policy to release all scientific information as quickly as consistent with their calibration, validation and meeting mission goals.
Whenever possible, the Kepler team attempts to confirm a planet candidate via radial velocity (spectroscopic) methods from ground-based observatories. The Kepler field of view is in the Cygnus region of the sky, only visible in the summer from the northern hemisphere. For that reason, the year’s discoveries are announced well after the summer observing season.
For the data to be released in a form that is of value and that maintains the scientific integrity of the mission, it is released in a processed format, not simply the raw strings of bits returned by the spacecraft. It takes several months for data to be validated and especially for mission integrity, false positive events—ones that look like transits but are caused by other phenomena such as grazing binary stars—must be checked through ground-based observations of the stars.
Data for each 3-month observation period will be made public within one year of the end the observation period. For stars that get dropped from the planet search program, data will be made public within 2 months of their being dropped.
The data from the Kepler Mission is archived at the Kepler archive in the Multimission Archive at STScI (MAST) and is expected to be supported for ten years after the end of mission.
A10. When and if we find these exoplanets, what next? Will there be a manned mission to those planets?
NASA is not contemplating a manned mission to any stars, because it would take so long — even the closest star, at 4 light-years (LY) distance, would take thousands of years to reach at any speeds we can attain now. The next steps will be to detect light directly from the planet—enough to obtain a spectrum that would tell us what type of atmosphere the planet has, which would give clues as to whether or not the planet actually has life. NASA missions that are being planned that have some of these capabilities are the Space Interferometry Mission (SIM) and the James Webb Space Telescope (JWST).
A11. Can the data from this mission support other research programs?
Obtaining astrophysical information about each star is a natural byproduct of detecting planetary transits. The following are some of the potential uses for these data:
Phenomena Information Obtained Stellar rotation rates Variation in rates with stellar type p-mode oscillations Window on stellar interior:
mass, age, metallicity of stars
Characteristics of solar-type stars Determine what is a “normal” star Frequency of Maunder minimum Earth climate implications Stellar activity Star spot cycles, white light flaring,
Cataclysmic variables Pre-outburst activity, mass transfer Eclipsing binaries Detection of high-mass ratio binaries Active Galactic Nuclei variability “Engine” size in BL Lac, quasars and blazars
The data from the Kepler Mission is archived at the Kepler archive in the Multimission Archive at STScI (MAST).
For further information, see “ Related Science.”
A12. How does the Kepler Mission contribute to the Origins missions SIM and TPF?
The Kepler Mission contributes in several ways to both the Space Interferometry Mission (SIM) and the Terrestrial Planet Finder (TPF) mission: The Kepler Mission determines the frequency of terrestrial and smaller planets in a larger volume of our Galaxy than available to either SIM or TPF and thereby determines the expected number of planets that either SIM or TPF might observe. If terrestrial planets are rare in the extended solar neighborhood, then the capabilities of both these missions will need to be increased. From the distribution of planets among the different stellar types observed, SIM and TPF will know which types of stars in our immediate solar neighborhood are most likely to have planets. Although neither of these missions will be able to detect terrestrial planets as far away as the Kepler Mission can, the Kepler Mission does identify planetary systems already known to have terrestrial planets. SIM and TPF can observe these systems to determine if there are other larger planets which would not have been seen transiting their parent stars, thereby providing a more complete picture of the composition of planetary systems having known terrestrial planets. The systems found by the Kepler Mission to have terrestrial planets can be examined in the infrared to measure the amount of zodiacal light within each system. If large amounts of zodiacal light are common, then it may be difficult for TPF to image any planets. Results from the Kepler Mission provide the sustained impetus to fund the much more ambitious TPF as stated by the National Academy of Sciences (NAS) decadal survey report Astronomy and Astrophysics in the New Millennium (p. 7), which calls for building the TPF space mission “predicated on the assumption … that, prior to the start of TPF, ground- and space-based searches will confirm the expectation that terrestrial planets are common around solar-type stars.”
A13. If Kepler were positioned around a distant star, at what distance could it detect earth?
I am curious about how easily the ephemeral “others” might find us, if they exist.
Most of the planets Kepler will detect will typically be between about 100pc and 1kpc (1parsec=3.1 light years). So If someone had the equivalent of Kepler in a solar system at say 500 pc they could detect Earth. But… that star would have to be within about 1/2 degree of being along the ecliptic plane on the sky in order for them see see Earth transiting the Sun.
A14. Do you think any planets could support life, and if so, what characteristics would they have to have?
Actually, that’s the main goal of the Kepler mission — to find planets that could support life. And yes, I think there must be some planets that could support life.
The simplest requirement for a planet to have life (carbon-based like on Earth), is for there to be liquid water. That means the temperature must be above the freezing point of water (0 degrees C or 32 degrees F) and below the boiling point (100 degrees C or 212 degrees F). That leads to other requirements: the planet must stay far enough away from the star, yet close enough to be in that temperature range for liquid water. This zone of distances from a star where liquid water can exist is known as the habitable zone, since that’s the region that living things could inhabit.
Another requirement is for the planet to have enough air, but not too much air (like the giant planets have). This depends mostly on the planet’s size. It must be big enough to have sufficient gravitational pull to hold onto air molecules. At less than 0.8 Earth radii (0.5 Earth masses) the planet would not have enough surface gravity to hold on to a life sustaining atmosphere. That rules out a Mars size planet (about 0.5 Earth radii). At about 2 earth radii (8 Earth masses), a planet would have enough surface gravity to hold onto hydrogen and helium and turn into a gas giant. This depends on composition of the planet, that is fraction of silicates, metals etc. If it’s Neptune size (about 4 times Earth diameter) or bigger it’s definitely getting to have too much gravity and will hold onto way too much atmosphere. If such giant planets had very large moons—more than half the Earth’s diameter—then those moons might support life, if the whole moon-planet system was in the habitable zone of the star.
A15. In the Kepler Planet Candidate Data Explorer, KOI-1902.01 has a planet temperature of -123°F. KOI-401.02 is 212°F? How can planets with such extreme temperatures be classified in the habitable zone?
The definition of habitable zone is often stated simply as saying temperature must be between 0 and 100 °C. However, actual conditions on real planets (or moons) vary, as we can see in our solar system, where it looks like Europa has liquid water under its icy crust. For that reason, the range of temperatures included in habitable range is expanded some from the nominal values of freezing point and boiling point of water 273-373K (0-100°C). The 212°F temperature you mentioned is the boiling point of water…right at the edge of habitable. Some microorganisms can live at such temperatures, which technically qualifies as habitable.
The calculations for the table start with the power of starlight intercepted by the planet (in units of that received by Earth, known as the solar constant). We further assume that the planet actually reflects 30% of the energy falling on it (an albedo or reflectivity of 0.3). What is absorbed by that planet is balanced by “blackbody radiation” given off by the planet that results in an average equilibrium temperature for the planet. We cannot know the actual ground temperature from Kepler data, but we can calculate an equilibrium planet temperature for an “ideal” blackbody. But that does not take into account other factors like presence of greenhouse gases in the atmosphere or internal heat sources, not to mention actual albedos that can differ radically from 0.3. Greenhouse gases can raise the planet’s temperature by anywhere from a few degrees to hundreds of degrees (in the case of Venus e.g.). Greenhouse warming for Earth is 33 K.
Clearly the concept of habitable zone is not an exact science.
B1. Where is Kepler pointed?
Kepler will look at just one large area of the sky in the constellations Cygnus and Lyra. The star field for the Kepler Mission was selected based on the following constraints:
- The field must be continuously viewable throughout the mission.
- The field needs to be rich in stars similar to our sun because Kepler needs to observe more than 100,000 stars simultaneously.
- The spacecraft and photometer, with its sunshade, must fit inside a standard Delta II launch vehicle. The size of the optics and the space available for the sunshield require the center of the star field to be more than 55-degrees above or below the path of the sun as the spacecraft orbits the sun each year trailing behind the Earth. The Sun, Earth and Moon make it impossible to view some portions of the sky during an orbital year. Thus, Kepler looks above the ecliptic plane to avoid all these bright celestial objects.
These constraints limited scientist to two portions of the sky to view, one each in the northern and southern sky. The Cygnus-Lyra region in the northern sky was chosen for its rich field of stars somewhat richer than a southern field. Consistent with this decision, all of the ground-based telescopes that support the Kepler team’s follow-up observation work are located at northern latitudes. The star field in the Cygnus-Lyra constellations near the galactic plane that meets these viewing constraints and provides more than 100,000 stars to monitor for planetary transits. (For additional information, read “ Target Field of View.”)
B2. Why did you choose the area around Cygnus and Lyra?
We need a region of the sky that is both rich in stars, and one where the Sun does not get in the way throughout the entire orbit of the Kepler spacecraft. Cygnus is far enough north of the plane of Earth’ orbit (the ecliptic) that the Sun will not encroach on Kepler’s view, yet is in a very star-rich part of our Milky Way galaxy. (For additional information, read “ Target Field of View.”)
B3. How far–in miles–are the target stars from Earth?
The stars that Kepler is observing are in the range of a few hundred to a few thousand light-years away. One light year is about 6 trillion (6,000,000,000,000) miles.
B4. What is the typical distance to the stars where Kepler will find Earth-size planets?
Kepler will be looking along the Orion spiral arm of our galaxy. The distance to most of the stars for which Earth-size planets can be detected by Kepler is from 600 to 3,000 light years. Less than 1% of the stars that Kepler will be looking at are closer than 600 light years. Stars farther than 3,000 light years are too faint for Kepler to observe the transits needed to detect Earth-size planets. (For further information, read “ Dependencies of Detectable Planet Size.”)
B5. How long will it take Kepler to get to its target stars in Cygnus?
The Kepler spacecraft is not traveling to the stars in Cygnus. It will orbit our own Sun, trailing behind Earth in its orbit, and stay pointed at Cygnus starfield for 3.5 years to watch for drops in brightness that happen when an orbiting planet crosses (transits) in front of the star. Cygnus was chosen because it has a very rich starfield and is in an area of sky where the Sun will not get in the way of the spacecraft’s view for its entire orbit.
B6. How does Kepler decide which stars to study?
The bulk of the stars that were selected are more or less sun-like, but a sampling of other stars were included as well. One of the most important factors was brightness. Detecting minuscule changes in brightness caused by transiting planet is impossible if the star is too dim and/or noisey. Kepler is monitoring stars that are as faint as 16th magnitude, although stars fainter than about 14.5 magnitude are very difficult to perform follow-up observations on. Even for stars fainter than 12th magnitude, they will have to be quieter than the sun or the planets larger than earth to be able to detect transits. Also, there are only a few hundred stars brighter than 9th magnitude.
Another factor in selection was stellar type which is related to the star temperature, size, and mass (see http://en.wikipedia.org/wiki/Stellar_classification). The Sun is a type G2 dwarf star—effective temperature: 5778 K. For stars much hotter (larger and more massive) than the Sun an Earth-size transit is much smaller and more difficult to detect, but can be done if the star is bright and quiet. The graphs below show roughly how many Kepler target stars there are for various temperatures and for various brightnesses.
B7. Since Kepler has thruster modules, is it possible to move the spacecraft to look at a slightly different region of space, and what other regions would provide the mission with potential targets to examine?
The thrusters are not really for orienting the spacecraft. Orienting the spacecraft is accomplished by reaction wheels that can be thought of as a set of specialized gyroscopes. Altering the speeds of the gyroscopes controls the rotation of the spacecraft. Sunlight striking the spacecraft imparts spin to the spacecraft which the reaction wheels continuously compensate for until they reach their maximum speed (called saturation). At this point the thrusters are fired to spin down (ordesaturate) the reaction wheels. There are 4 reaction wheels arranged like a tetrahedron. This is a common technique to provide redundancy in case one wheel fails, so that one can still control around three axes. Desaturating the reaction wheels happens about every 3 days. The reaction wheels also do the job of rolling the spacecraft 90 degrees every 3 months to keep the solar panels pointed at the Sun.
It’s essential that the telescope point at the exact same field of view throughout the mission. That is because it’s not sufficient to detect only one planet transit to establish discovery of a planet. Multiple transits are required. And for planets in the habitable zone of a Sun-like star, those transits would only occur every year or so. That’s why the mission duration is at least 3.5 years—-to find habitable planets around Sun-like stars. If we pointed the telescope somewhere else, we would have to observe this new field of view for 3.5 years or more to reach our science goals. Kepler has shown us, that planets seem to be fairly common, so any other field is likely to show many planet transits such as the current field of view.
Unlike other missions that are observatories designed to look anywhere on the sky, The Kepler hardware (focal plane orientation, sunshade, solar panels, radiator, etc) were designed for this specific star field. The spacecraft could be pointed elsewhere, but sun angle, thermal, power and other things would have to be studied first. The mission design was optimized for this star field which was studied extensively before hand to identify the stars to target. The operations have been tuned to this orientation. Hence, it would be extremely costly and disruptive to the existing science program to point anywhere else on the sky. (Also, see FAQ B1: Where is Kepler pointed?)
B8. What are alternate names for the Kepler host stars?
That would be helpful for amateurs to be able to make finding charts for observing.
One of the best techniques for finding alternate star names is to use the SIMBAD database at http://simbad.u-strasbg.fr/simbad. (Use the “Basic Search” and enter the Kepler name, e.g. Kepler-4, and the result will include other know names (Identifiers).
C1. What is the main instrument on the Kepler spacecraft?
The sole instrument aboard Kepler is a photometer (or light meter), an instrument that measures the brightness variations of stars. The photometer consists of the telescope, the focal plane array, and the local detector electronics. Kepler is a 0.95-meter (37-inch) aperture Schmidt-type telescope with a 1.4-meter (55-inch) primary mirror. For an astronomical telescope, Kepler’s photometer has a very wide field of view: it’s about 15 degrees across. It would take 30 Moons lined up in a row to span the Kepler field of view. The photometer features a focal plane array with 95 million pixels. The focal plane array is the largest camera NASA has ever flown in space. For further information, (read “Photometer and Spacecraft”)
C2. How does Kepler collect starlight to measure transits?
(See Optical Path Animation (QuickTime, 5.67 MB).
The Kepler spacecraft is a single-purpose Schmidt telescope. It stares continuously at a large field of view (see “Where is Kepler pointed?”) to observe more than 100,000 stars simultaneously. The starlight enters the telescope, reflects from the primary mirror to the focal plane array of 21 modules each with two 50×25 mm 2200×1024 pixel CCDs. The pixels (picture elements) collect the photons of light from the stars. Every 6 seconds, the array “reads out” the number of photons in each pixel to an onboard computer for storage and initial processing. For the selected stars, the data (photon counts) accumulates in an on board computer, and is transmitted to Earth once each month. (For more information, see the next question, “How do CDDs (charge coupled devices) work?”)
C3. How do CCDs (charge coupled devices) work?
In a CCD, the silicon region is divided electrically into small individual picture elements or pixels with about four hundred elements per cm in each direction, like a very finely divided sheet of graph paper. The free electrons are kept from moving around by permanent channel stops (the vertical lines in the figure) and externally applied voltages (the horizontal lines in the figure). Each pixel can then be thought of as an individual bucket or well that collects electrons.
As shown in the animation, first the CCD is exposed to light from a telescope or camera lens. Overtime this produces an image made up of electrons in the CCD.
To readout an image that has been captured with the CCD requires shifting the information out of the pixels. First, the columns of pixels are all shifted down one row. The bottom row of pixels is shifted into a readout register. Each pixel in the readout register is shifted out to an amplifier and the number of electrons in each pixel are recorded. This produces a series of 1’s and 0’s that represent the image. This is repeated over and over until all the pixels have been read. The stream of 1’s and 0’s is then digitally processed to reproduce the image that is later displayed.
In the Kepler Mission the 1’s and 0’s are recorded onboard the spacecraft and sent to the ground, where the data are processed to look for changes in the brightness of each star that may be caused by a planetary transit. (For further information, (read “ Photometer and Spacecraft.”)
C4. How many pixels are dedicated to each target star?
The image of a star is spread across a “postage stamp” of 30 pixels.
C5. How are individual target stars imaged by the Kepler CCDs?
The star images are spread across a “postage stamp” of 30 pixels. By spreading the light across a set of pixels, Kepler captures all of the photons from a target star without a single pixel saturating (filling up), which would produce faulty data. A second reason for spreading the light across a “postage stamp” of pixels is to compensate for any spacecraft movement. By surrounding the image of the star with sufficient pixels, any tiny movement of the spacecraft will not push the star beyond its “postage stamp” and the mission scientists can be assured that the measurements are accurate.
C6. Does a starlight always land on the same pixels?
Every three months, the spacecraft is reoriented (rotated one-quarter turn) to keep the solar panels pointed toward the Sun. During each 3-month observing period, the starlight does land on the same set of pixels. But after 3 months, when the spacecraft has been rotated, the light from each target star is collected by a new set of pixels on a different CCD. The overall arrangement geometry of the CCDs was designed so that the configuration is nearly identical after each 3-month rotation, except for the center CCD module. For the center module, the stars simply move to a different set of pixels on the same CCD.
C7. Do the images of stars overlap on the CCDs?
More than 100,000 target stars were selected in the Kepler field of view. These stars were chosen so that they do not overlap other stars or more distant background galaxies. The data from each selected star is carefully reviewed to eliminate binary stars, and distant field stars and galaxies. The data for each target star is separate from the data for other nearby target stars.
C8. How is the data from the pixels downloaded?
The Kepler data is stored onboard, and downloaded once per month. It is transmitted from the spacecraft to Earth via NASA’s Deep Space Network. The NASA Deep Space Network – or DSN – is an international network of antennas that supports interplanetary spacecraft missions and radio and radar astronomy observations for the exploration of the solar system and the universe. The DSN currently consists of three deep-space communications facilities placed approximately 120 degrees apart around the world: at Goldstone, in California’s Mojave Desert; near Madrid Spain; and near Canberra Australia. This strategic placement permits constant observation of spacecraft as the Earth rotates, and helps to make the DSN the largest and most sensitive scientific telecommunications system in the world. From the DSN, the data flows to the Mission Operations Center in Boulder, CO, then to the Data Management Center in Baltimore, MD, home of the Hubble Space Telescope Science Institute. The raw data is archived at the Data Management Center, and then transmitted to the Kepler Science Operations Center (SOC) at NASA’s Ames Research Center in California. At the SOC, the data is processed and analyzed to produce calibrated light curves. These light curves can reveal the presence of a planet.
Does all of the data collected by the Kepler CCD array get sent back to Earth? No, only the data for selected stars gets sent back to Earth. The Kepler instrument detects light using a set of 21 CCD modules each with two specialized CCDs (the focal plane array) that cover the field of view (105 degrees square). Each “snapshot” produces the equivalent of a 95 megapixels image every 6 seconds. The onboard computer is programmed to keep only the data for the targeted stars, and discard the rest. The onboard computer adds together the 6-second data snapshots into 30 minute observations, and stores that information. The data for non-targeted stars and galaxies is discarded before transmission to Earth.
C9. Couldn’t the mission be done with a smaller photometer and cut the cost?
A representation of the scientific performance versus project cost is shown in the figure. A well conceived project is at A with maximum possible science per dollar available. Many times, those who fund a program perceive the project to be at B, where costs can be cut without much loss in science; while the science team tries to believe that they are at C, where more science can be achieved at little extra cost. Good clever scientists and engineers might be able to get to point D, but this is unusual. Project managers worth their weight in gold are those who can push toward E, keeping the performance, but saving on cost. Any project headed from A to B A toC or A to F is doomed to be canceled or should be canceled.
For the Kepler Mission to work, 100,000 main-sequence stars must be monitored to a differential photometric precision of 1:50,000 every 6.5 hours. Substantially fewer stars and the results may turn out to be ambiguous. The necessary precision requires recording ten billion photons from each star every 6.5 hours. Thus, a smaller photometer would mean either fewer stars at the required precision or poorer precision for most of the stars and thereby the inability to detect Earth-size planets. Also, the photometer would need to be much smaller before other costs, such as the launch vehicle, would begin to drop significantly.
Our project manager has worked hard at both increasing the performance by increasing the downlink data rate to permit monitoring 100,000 stars (originally we planned to monitor only 5000 stars) and in reducing the cost by changing the orbit to an Earth-trailing heliocentric orbit and thereby eliminating an expensive propulsion stage needed to get to an L2 halo orbit. This also allowed us to use a smaller and less costly launch vehicle. In essence, we have already pushed the cost-performance curve in both the D and E directions.
D1. Ball Aerospace & Technologies Corp. is responsible for developing the Kepler flight system. Which other companies contributed to this mission?
Ball Aerospace & Technologies (Colorado) E2V (England), Corning Incorporated (New York), Brashear LP (Pennsylvania), and……
D2. Who built the 1.4 m (55 inch) primary mirror and what is it made of?
Corning Incorporated (New York) provided the mirror blanks of fused silica for the primary and corrector plate. Brashear LP (Pennsylvania) figured the primary and corrector plate for the Schmidt telescope that is the main instrument. Ball Aerospace and Technologies Corporation (Colorado) constructed the spacecraft.
D3. How big is the Kepler spacecraft?
The overall size is about 2.7 meters (nine feet) in diameter and 4.7 meters (15.3 feet) high.
D4. How much did the Kepler spacecraft weigh at launch?
The total mass at launch was 1052.4 kilograms (2,320.1 pounds) consisting of 562.7-kilograms (1240.5-pounds) for the spacecraft, 478.0-kilograms (1043.9-pounds) for the photometer, and 11.7 kilograms (25.8 pounds) of hydrazine propellant.
D5. What powers the Kepler spacecraft?
Power is provided by four non-coplanar panels with a total area of 10.2 square meters (109.8 square feet) of solar collecting surface area. Combined, the 2860 individual solar cells can produce over 1,100 Watts. Power storage is provided by a 20 Amp-hour rechargeable lithium-ion battery. The spacecraft must execute a 90 degree roll every 3 months to reposition the solar panels to face the Sun while keeping the instrument aimed at the target field of view. (See animation.)
D6. Why is the high-gain antenna fixed on the spacecraft rather than on a gimbaled arm?
Early in the spacecraft design phase, a decision was made to mount the antenna directly on the spacecraft. There were two reasons: it reduced the mission design and construction cost, and it reduced risk of failure of the arm to extend the antenna after launch.
D7. What are the new developments that now make this mission possible?
Two recent research results have enabled the practicality of the Kepler Mission:
- The demonstration that charge coupled devices (CCDs) have the needed photometric performance to make the measurements. All sources of noise (photon shot noise, stellar variability (see above), CCD noise and pointing jitter) when combined must be less than one part in 50,000 (20 ppm); four times less than the effect of an Earth-size transit. The required CCD performance with all the known noise sources has been achieved in recent laboratory measurements along with the detection of Earth-size transit signals (Koch, et al. 2000). Thus, CCDs can be used to simultaneously measure tens of thousands of stars at one time.
- Until recently, no one knew what the variations in stellar brightness were on the time scale of a fraction of a day. This information is now available for one star, our Sun. These data indicate that on the time scale of a transit, the variability is typically ten times less than the effect being measured. Fortunately, our Sun is one of the more common stellar types, and we expect other solar-like stars to behave in a similar fashion.
D8. What obstacles had to be overcome in development of the mission?
The main obstacles were (a) technical challenges, to achieve instrumentation and systems operations precise enough to detect transits of Earth-size planets, (b) budgetary restrictions and alterations, and (c) procuring highest quality materials and components. Two CCD manufacturers were initially contracted to make CCDs for Kepler mission and ultimately, one of them was able to produce the quality CCDs required for the mission. Budget constraints caused slight shortening of the nominal mission length from 4 years to 3.5 years and also triggered the decision to replace the gimbaled communications antenna with a fixed one.
D9. Why not use the Hubble Space Telescope (HST)?
There are three basic reasons why the HST could not be used to look for planets in the way described here:
- The field of view (FOV) of the HST is too small to observe a large number of bright stars. The FOV of the HST is about the size of a grain of salt held at arm’s length. There is almost never more than one bright star in the HST FOV at any one time. However, the FOV of the Kepler Mission photometer is about the size of both of your open hands held at arm’s length. Or another way of looking at it is, that it is about equal to the size of two “dips” of the Big Dipper.
- The brightness of every target star has to be measured continuously, not just once in a while, since one does not know when to expect a transit to happen. The HST is for the use of the entire astronomical community to address thousands of questions and would not be dedicated to just one question requiring continuous use for up to four years.
- The HST does not have a specially designed photometer observing over 100,000 stars simultaneously with the precision required for the measurements needed to detect Earth-size transits.
The HST has been used by Ron Gilliland to look for transits of giant planets with periods of only a few days in the globular cluster 47 Tuc, a region of very high star density. No transits were detected.
D10. Are there other photometry missions?
There are two other photometry mission, one that is currently operating: MOST and COROT. However, they are considerably less capable than the Kepler Mission, since their primary science mission is to measure the properties of stars. COROT has 1/10 the collecting area for photons, 1/20th the field of view of the sky and stares at a given star field for 1/10 the amount of time that the Kepler Mission stares. MOST is an even smaller mission and less capable for planet detection. MOST was launched on June 30, 2003 and has produced spectacular photometric results on microvariability of stars – asteroseismology. COROT was launched on 27 December 2006. COROT also does asteroseismology, and has found extrasolar planets, though not Earth-size planets in the habitable zone.
D11. What is the mission cost?
The Kepler Mission life cycle cost is approximately $600 million. This includes the design, construction, launch and operation of the spacecraft as well as the scientific analysis of the data. The Mission involves scientists and engineers across the United States, Canada and Europe (see http://kepler.nasa.gov/about/team.html). The Kepler science office is at NASA Ames Research Center.
E1. What type of rocket was used to launch the spacecraft?
The Kepler spacecraft lifted off March 6, 2009 aboard a Delta II rocket from Cape Canaveral Air Force Station in Florida. Launch occurred at 10:49 p.m. EST.
E2. How long will the mission last?
The Kepler Mission is scheduled to observe for a minimum of 3.5 years. The spacecraft is designed to observe up to 6 years. Generally, NASA’s Science Mission Directorate reviews active missions, and makes the decision to extend missions based upon the opportunity for further scientific discoveries. (For more information, read “Launch Vehicle and Orbit.”)
E3. Where does Kepler orbit?
Kepler is in a heliocentric (Sun-centered) orbit. Kepler’s orbit was chosen to enable continuous observation of the target stars. This requires that the field of view of Kepler never be blocked. For a spacecraft in low-Earth orbit, nearly half of the sky is blocked by the Earth and the obscured region is constantly changing. The most energy efficient orbit beyond Earth orbit is a heliocentric (Sun centered) Earth-trailing orbit. An Earth-trailing heliocentric orbit with a period of 371 days provides the optimum approach to maintaining a stable trajectory that keeps the spacecraft within telecommunications capability. Another advantage of this orbit is that it has a very-low disturbing torque on the spacecraft, which leads to a very stable pointing attitude. The spacecraft must execute a 90 degree roll every 3 months to reposition the solar panels to face the Sun while keeping the instrument aimed at the target field of view. (See animation.)
Not being in Earth orbit means that there are no torques due to gravity gradients, magnetic moments or atmospheric drag. The largest external torque then is that caused by light from the sun. This orbit also avoids the high-radiation dosage associated with an Earth orbit, but is subject to energetic particles from cosmic rays and solar flares. (For more information, read “ Launch Vehicle and Orbit”)
E4. Where is Kepler now?
Kepler has been added to the Jet Propulsion Lab (JPL) Horizon database http://ssd.jpl.nasa.gov/horizons.cgi
E5. How does Kepler communicate with Earth?
The telecom subsystem will be used for receiving commands and for transmitting engineering, science and navigation data back to Earth. It is designed to operate out to a distance of 96 million kilometers (about 60 million miles). The system uses a parabolic dish high-gain antenna for transmitting, two receiving low-gain antennas and two transmitting low-gain antennas. The system can receive commands from Earth at speeds ranging from 7.8 to 2,000 bits per second, and can send data to Earth at speeds from 10 to 4.3 million bits per second. This transmission capability is the highest data rate of any NASA mission to date.
Telecommunications and navigation support for the mission are provided by NASA’s Jet Propulsion Laboratory (JPL) and NASA’s Deep Space Network (DSN). During the science phase of the mission, Kepler will perform its data-gathering duties automatically. Twice a week, the operations team contacts the spacecraft to assess its health and status and upload any new command sequences. Once per month, the spacecraft stops taking data for one day, re-orientates the spacecraft to point the high-gain antenna at the Earth and downlinks the science data. Every three months, the spacecraft also must be rotated 90 degrees about the optical axis to maintain the maximum exposure on the solar array and to ensure the spacecraft’s radiator is pointing towards deep space. After rotation, the instrument requires a new star pixel map for the 100,000 target stars and the 87 fine guidance sensors stars. (Insert image of spacecraft with antenna labeled–
E6. What happens in 61 years when the spacecraft is closest to Earth?
Like Earth, the Kepler spacecraft orbits the Sun (heliocentric orbit). But, Kepler is in an “earth-trailing” orbit, taking 371 days to orbit the Sun. After 61 years, it will be in the vicinity of the Earth, but not collide. With a smile, Mission Principal Investigator Bill Borucki says “My grandchildren will retrieve Kepler, and place it in the new National Air and Space Museum on the Moon!” Kepler may simply be left in it orbit.
E7. If the Kepler Mission extends beyond 3.5 years, will the spacecraft be pointed to a different part of the sky?
If the Kepler Mission is extended, it will continue to observe the same portion of the sky (same field of view). To confirm a planet, the science mission team requires a minimum of 3 transits of the same period, depth and duration. An extended mission would enable the discovery of planets on longer orbits at greater distances from their parent stars.
E8. Who manages the Kepler Mission during operations?
Mission operations during both commissioning and science operations phases of the mission involve several organizations, including: NASA’s Ames Research Center, Moffett Field, Calif., which will conduct Mission management and operate the Science Operations Center (SOC); The Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado in Boulder, Colo., which is the site of the Mission Operations Center (MOC); Ball Aerospace & Technologies Corp., also located in Boulder will use its Flight Planning Center (FPC) to provide engineering support; NASA’s Jet Propulsion Laboratory, Pasadena, Calif. will use its Deep Space Network (DSN) for navigation and communication; Space Telescope ScienceInstitute (STScI) in Baltimore, Md. will provide the data management services.
F1. When was the idea to the Kepler Mission born?
Bill Borucki and A. L Summers first proposed finding planets with transit photometry in 1984 in an article titled “The photometric method of detecting other planetary systems.” published in Icarus, 58, 121.
F2. Why did you name the Kepler Mission after the German astronomer Johannes Kepler?
NASA’s Kepler Mission was named in honor of Johannes Kepler because he was the first person to describe the motions of planets about the Sun in such a way that their positions could be precisely predicted. He derived three laws of planetary motion from observational data taken by Tycho Brahe. Kepler’s first two laws of planetary motion were published in 1609, 400 years prior to launch. Ten years later, he published his third law of planetary motion, which describes how the orbital period (year) of a planet is proportional to the semimajor axis (distance) from the Sun. The fact that Johannes Kepler derived his laws from data made him the first astrophysicist, and the Kepler Mission honors him for this accomplishment. The Mission also uses Kepler’s third law to determine the size of planetary orbits from the periods discovered by observing repeated transits.
G1. Where can I go on the net to see photos taken by the Kepler telescope?
Kepler images are posted on the multimedia section of the Kepler website. Full images of the Kepler starfield are only downloaded every quarter. You can see full starfield images in the Kepler multimedia section under First light and Kepler field of view for each season. Kepler is not designed to get “pretty pictures” like the Hubble Space Telescope, Spitzer, and the planetary missions. The Kepler photometer has especially large pixels, many times the size of those in the Hubble Space Telescope detector. The giant-size pixels make possible the enormous field of view that Kepler has. For routine data acquisition, Kepler is programmed to download information from pixels immediately adjacent to each of those 156,000 stars. This produces extremely pretty light curves, but no “pretty pictures.”
G2. How can I locate photographs, graphics, animations and images from the Kepler Mission?
You can find a great variety of images and other materials throughout the Kepler website Multimedia section.
G3. How can the public or my classroom be involved?
The Kepler Mission provides several opportunities for teachers, students and the public to get involved. Kepler’s Education and Public Outreach program includes activities and materials for teaching and learning, and information on public outreach through the Night Sky Network, StarDate, and much more.
Q: I was confused by subject table found at following link: http://kepler.nasa.gov/Mission/discoveries/ .The table shows comparison data for the Earth and Jupiter. The temperature data for the Earth & Jupiter do not agree with NASA factsheet for planetary data. In particular, the NASA fact sheet shows the mean temperature of the Earth as +15 deg C or 288 K. Why does the subject discoveries table show the Earth’s temperature below the freezing point of water at 255 K ??? Does this mean you are measuring a Global Cooling or just listing some temperature other than mean temperature ?
The planet temperatures calculated do not take into account atmosphere at all, because the effect of atmosphere is very dependent on composition of atmosphere and we have no way of determining that. If Earth did not have atmosphere, indeed it’s temperature would be below freezing point of water. In the case of Jupiter, I have to add the fact that our computed planet temperatures are based on an assumption that the planet is in equilibrium with only radiation from it’s star balanced by its own black body radiation. Jupiter has not only an atmosphere to complicate things, but a significant internal heat source as well.
- Should I send technical questions to the MAST help-desk?
Yes. For best service send questions on technical issues (i.e. not relating to the GO proposal program) to the MAST helpdesk at firstname.lastname@example.org. These will be screened within a work day and forwarded to the appropriate Kepler Project office if MAST should not or cannot answer them.
- Can Kepler be pointed to observe another region of the sky?
No. The Kepler field of view (FOV) is centered on the sky position RA = 19h22m40sand Dec +44o30‘00“. This pointing will not be altered during the course of the mission.
- Are there other sources of public information that describe photometric or spectroscopic observations of objects in the Kepler field?
Readers should consult the Related Sites link in the left gutter of the MAST/Kepler home page. We expect this list of links to grow with the mission as ground-based exoplanet hunters post their results.
- What are the spatial properties of a point source on the Kepler focal plane?
The focal plane scale of the instrument is about 3.98 arcsec/pixel. Although Kepler is not meant to be an imaging instrument, each detector module has been focused independently. Some changes in focus and image quality are possible during the mission lifetime. According to the Kepler Instrument Manual, the goal is for 95% of the light (“Encircled Energy”) to fall on an area of 7 pixels or less, and the latest information from the project indicates that the actual 95%-circle is 4.2 arcsec (a little more than 1 pixel) . Note that actual image characteristics may change subtlely from quarter to quarter, or even within quarters, because of differential aberration of starlight. The aberration effect causes shifts up to +/-0.25 pixel and can shuffle the distribution of flux among neighboring pixels. In its extraction of flux from the stellar image, the Kepler pipeline processing software should correct for this effect.
- Where can I go to find recent status reports on the Kepler satellite?
NASA provides an official site for mission updates.The homepage of the Kepler Asteroseismic Science Consortium (KASC) also lists mission status activities of the satellite and data collection process. An Elapsed Mission Clock is furnished on this site to provide browsers of just where “today” fits into the mission lifetime.
- How do I determine what the schedules and writing instructions are for the next Guest Observer (GO) cycle?
NASA releases a ROSES (Research Opportunities for Space and Earth Sciences) document annually that announces the research programs its grants offices are supporting and the schedule and instructions for the proposal cycle. MAST recommends that users consult the official announcement at the NASA 2012 ROSES site. For details on the Kepler GO proposal Cycle 5 see the Table 3, Appendix D7. At this writing NASA has not announced the proposal due date for this proposal cycle.Information from the GO office takes precedence over this response.
- Where do the targets on the Kepler Target Search page come from?
Objects visible on the Target Search page were provided from a list of objects, the great majority of which are stars, in the Kepler Input Catalog. See the question under “Catalogs” on the KIC.Information from the GO office takes precedence over this response.
- What is the procedure for proposing for extragalactic or other objects not returned in the Kepler Target Search results page?
GO proposers are referred to the Kepler GO program web site for information. In general, there is no prohibition on targets within the appropriate magnitude range. However, for extended objects the number of pixels required to cover an object is a limited resource for the Project. Proposers interested in proposing these types of targets should consult the Proposal instructions and FAQs posted by the Kepler GO Office for guidance in proposing such targets.A FAQ from the GO target informs users of important consequences of requesting customized apertures, e.g. for extended sources.
- What are the Kepler “seasons” all about?
Kepler was launched into an Earth-trailing heliocentric orbit of a little longer than a year (372.5 days), and this fact has a number of consequences. First, the spacecraft slowly becomes more distant from Earth. This fact ultimately places a hard limit on the mission lifetime. Second, in order to maintain orientation of the solar array toward the Sun, the satellite must roll on its pointing axis four times a year. Each of these intervals is called a season or quarter. The Project designates four seasons, numbered 0, 1, 2, 3, which repeat each Kepler year. New “quarters” start with the beginning of data collection for each new season and run continuously (1, 2, 3, 4, 5, etc.). The exception to this statement is that the Project has designated the commissioning period, ending on May 12, 2009 as “Quarter 0.” This quarter has no analog in the seasons assignments. To find out the predicted and actual season start dates (which differ from one another by as much as a few days) users may consult MAST’s seasons table, See also the Quarter calender table under the Data Retrieval section. Date for seasons further into the the future will be posted as new predicted and actual dates are communicated by the project.During a given season the CCDs are fixed in position such that light from any object in the FOV falls on the same group of pixels. Kepler’s 42 CCDs (contained in 21 modules) have a four-fold symmetry such that when the satellite rolls 90o a new CCD sees almost exactly the same area of the sky as an old one did during the previous season.
- Are all objects in the Kepler field available for proposing in the current GO cycle?
According to a the revised Project policies, dated Dec. 18, 2009, all objects in the Kepler Field of View are now available for GO observations, as long as the purpose of them relates to non-exoplanetary science. Proposers should check with the GO Office for the suitability of proposing bright objects.Because of this revised policy the purpose of the Availability Flag has been changed. Its value now equals 0 for KIC targets not observed or planned to be observed, =1 if observations are planned and/or taken but not yet archived, and = 2 if data for the target have already been archived at MAST.
- How do I retrieve Kepler data?
First, locate the data of interest, either proprietary or public, via the Kepler Data Search & Retrieval page. On this form specify one or more search criteria. Typical search selections are based on time of observation (MJD, at mid-cadence), target name (e.g., 2MASS id), proposal id (aka “investigation id”), stellar parameters, such as Teff, or target position in the sky. Note that kepler_id and target name are different identifiers. For standard target names the resolver (i.e., SIMBAD or NED) will return the target coordinates, which will then be used in the search. Uploads of target lists follow the same formats allowed for most other MAST missions. Lists are uploaded by clicking the File Upload form on the link at the upper right of the search page and selecting the list tab. (See the Help for this form and the Kepler Archive Manual for further details.)Initiate the search by clicking on the Search button. The results of the search will be displayed.To initiate data retrieval, click in the open box next to the desired data in the Search Results table. Proprietary observations are marked by a yellow strip and can be downloaded only by the Kepler Project PI for Key Project Data, the GO for GO proposals, and those collaborators for which the PI or GO have requested access via the archive help-desk Submit the list of data by clicking the “Submit marked data for retrieval” tab. Data retrieval follows in the same way as for the HST and FUSE missions. The retrieval Options page is displayed. The user may request the data be staged to an ftp area or sent directly to the user’s computer. Either way, notification is sent by email when the process is completed. Anonymous retrieval is available for non-proprietary data only. Help on the retrieval options is available by clicking on any of the field option links. As of this writing the options for data products include light curve and pixel target files, in each case for whole Kepler quarters.Problems and questions should be directed to the archive help desk at email@example.com.
- How do I determine when the proprietary date for a target expires?
The date at which data become public will be visible to any user via the Kepler Data Search & Retrieval page. If an observation is still proprietary, it will be color-coded yellow in the Results Table, and the release date will be in the future. According to NASA policy, the expiration date for Guest Observer data corresponds to one year from the date the data are archived. If you are not the PI or CoI, and therefore do not have access privileges, be sure to recheck on the Search & Retrieval page to confirm the status as the date approaches to insure that the date has not changed.For restricted Key Project data the proprietary date schedule is more complicated. The proprietary period starts with the start of science data collection for the Mission. A schedule of the expected release dates can be found in the following FAQ (#32).
- When does the proprietary period end for Kepler Data?
The table below lists the release dates for Kepler Key Project data, also known as the exo-planetary search data. The specific release dates for data have been changed a number of times since launch. Until October 28th (see below) the nominal proprietary period for KASC and GO data is 1 year from ingest of the data into the archive.
Quarter Data release date 1 6/15/10 2 2/01/11 3 9/23/11 4, 5 and 6 1/7/12 7, 8 and 9 7/28/12 10 – 13 10/28/12
The Q10-13 marks the end of the proprietary era for Kepler science data. All Kepler science data after the release of Q13 data, including Key Project, KASC, and GO data, will be nonproprietary (publicly available). We expect this condition to hold through the end of the mission.
- Why/when should I register to retrieve data?
You should register only if you want to retrieve proprietary data, or if your existing archive account name is different from that on your proposal. The system that handles the proprietary rights for Kepler data requires an archive account and password be entered before retrieving proprietary data. This will ensure that proprietary data are sent only to the GO or Kepler Team member authorized to retrieve the data.Note that registered users must also be authorized to receive proprietary data. For Kepler,an automated system has been set up so that authorization for the PI and co-Is of a given investigation will be handled by software for all PIs and co-Is that have registered before their data are taken using the same name as on their proposals.Use the on line form to register (i.e., get an archive account and password).GOs and Kepler Team members may register as soon as they receive an investigation_id/proposal_id from the GO Office or the Kepler Project. You must register and be authorized before retrieval of your proprietary data will be successful.
PIs and co-Is who do not register in time will need to follow the standard archive registration policy as follows. The PIs should request authorization for themselves when they register for their account. Only the PI for an investigation may authorize anyone to access the data. If a Co-I wishes access to their data, they must have the PI on the investigation send e-mail to firstname.lastname@example.org stating the investigation ID and the identities of anyone who should be able to retrieve the data.
- What is the policy for public release of images over the whole Kepler field of view (“Full Frame Images”) and calibration files?
The Kepler project typically takes one Full Frame Image (FFI) per month, and these are calibrated and sent to the MAST archive for public use. Although originally carried out about once per month, the delivery schedule has been changed to once per quarter, meaning that generally three FFIs are delivered at a time. Note as stated elsewhere that a set of FFIs (the “Golden Eight”) were taken over a span of a day or so early in the telescope’s commissioning period. The FFIs may be retrieved at the FFI retrieval site.All calibration files, which are known as the Focal Plane Characterization Model, are public. Users should note that the calibration is applied across the field of view, not on a target by target basis. Thus the Focal Plane Characterization Model is applied to cadence data, not light curve or target pixel data. The times of the FPC Model and the FFI files are expressed as MJD (at mid-cadence).
- May I request data over arbitrary lengths of time, assuming the data for that target is non-proprietary?
Yes, but note well how the time is specified. Recall that light curves are produced on a quarterly basis. If the input start time in the query falls within a quarter, all light curves whose start times occur before the input start time and whose end times occur afterthe input start time, will be missed. For example, a query where start time > 26 May 2009 will miss all data with cadence start times of 12 May 2009.
- What are the start dates for the Kepler quarters?
The planned start dates (UTC) for the Kepler quarters are given in the Table below. The planned end times are typically one day before the start date of the next quarter.Jun 27
Quarter Planned Start Date Actual Start Date 1 May 13, 2009 May 13, 2009 2 June 20, 2009 Jun 18 3 Sept 17, 2009 Sept 18 4 Dec 17, 2009 Dec 19 5 Mar 19, 2010 Mar 19, 2010 6 June 23, 2010 June 23 7 Sept 23, 2010 Dec 22 8 Dec 22, 2010 Mar 24, 2011 9 Mar 24, 2011 Jun 27 10 June 27, 2011 Sep 27 11 Sept 29, 2011 Jan 04, 2012 12 Dec 29, 2011 Mar 28 13 Mar 28, 2012 Jun 27 14 Jun 28, 2012 Sep 27 15 Sep 28, 2012 Nov 15 16 Dec 29, 2012 Mar 28, 2013 17 Mar 29, 2013 Jun 27 18 Jun 28
- How do I find public data in the archive?
To find public data in the Kepler archive, go to the Data Search and Retrieval page. In the “Release Date” field on this page, enter the less than sign followed by the current date in the form MMM dd YYYY. For example < Oct 30 2009. The results page will show all Kepler data with a release date prior to the current date. By definition, these data are public. Users are also directed to a public download page, either from a direct link at the top of the MAST/Kepler home page or by navigating to the public light curve page, which details a few methods of downloading public data.
- What is a Golden FFI?
The “Golden Eight” FFIs were obtained during commissioning after the telescope had been focused, thermal stability obtained and pointing established. The Kepler Project considers these 8 FFIs to be the best FFIs Kepler will ever obtain. Here are the golden FFI filenames.
These files may be retrieved from the archive via the FFI search form. The size of a FFI file is approximately 390 MB.
- What is a Custom Aperture File Observation and how do I find them?
The vast majority of Kepler observations are taken with one of the standard apertures. However, for bright stars, where there is saturation and bleeding along the CCD columns, multiple objects that can not be reasonably separated into individual apertures (e.g., a star cluster) and objects that do not appear in the KIC, a custom aperture is defined and used for the observation. These observations, called custom aperture observations or CAF observations, are handled differently from usual Kepler observations and are assigned a kepler_id greater than 100,000,000 (for example, 100001645).The following apply to different subsets of CAF observations.
- For many CAF observations, the kepler_id changes from quarter to quarter, even if the target and investigation are the same.
- For CAF observations of clusters, the kepler_id may remain the same for a number of quarters.
- For CAF observations of multiple targets and non-KIC targets, the coordinates change from quarter to quarter, as they are the geometric center of the aperture.
- For CAF observations of objects with KIC ids (usually bright stars), the coordinates in the archive are taken from the KIC and remain the same from quarter to quarter, although the kepler_ids may not.
- For CAF observations of bright targets the kepler_id is supposed to change from quarter to quarter. However, kepler_id = 100001645 violates this rule.
For the archive user, a cone search is an obvious tool for finding CAF observations. In the case of a target with different coordinates in each quarter, the difference in the coordinates needs to be considered when specifying the cone search radius. In the case of bright objects, say 16 Cyg, the coordinates provided by the resolver (SIMBAD at CFA, for example) may differ from those in the KIC and so require a larger search radius to find the observations. We recommend the user try a range of radii. The current default is 0.02 arcminutes.
- I have the kepler_id (aka KIC id) for CH Cyg. Why don’t I see the custom aperture observations when I search on the kepler_id?
At this time, there is no way to search for the CH Cyg kepler_id (11913210) and have the CAF observations returned. A cone search is required. For this example, enter CH CYG as the “target name” and use “Simbad at cfa” as the resolver. As of August 12, 2011, 27 rows are returned, ranging from a single Q0 observation through Q8 with 4 observations. The default search radius (0.02) was used.While it appears that not all of these observations are of CH Cyg (why are there 3 long cadence observations in Q6?), all observations are of CH Cyg. This is an artifact of the scheduling system, which requires a long cadence for any short cadence. Since each short cadence in Q6 has a different kepler_id, 3 long cadences, each with a different kepler_id, were produced. These 3 long cadence files are basically the same. (Cosmic rays can be calculated differently for the three files however.)
- I did a cone search, and these multi-quarter, multiple object CAF observations of kepler_id 100000935 were returned. Why is the angular separation the same when the coordinates for each observation are different?
MAST uses a Hierarchical Triangular Mesh (HTM) to improve search times. There is one set of HTM indices per target. The indices are updated quarterly when the new Kepler Target Catalog (KTC) is received. All angular separations shown in cone search results are based on deviations from the HTM position. If the entered/resolved coordinates are for the latest KTC received, all entries for a given target are returned with a 0.0 angular separation. Unfortunately the latest KTC coordinates are NOT necessarily the coordinates for the latest archived data, and there is no way for the user to determine which quarter’s data corresponds to the HTM indices.
- What is the “KIC” and how does it relate to what I see on the MAST/Kepler search pages?
The KIC, or Kepler Input Catalog, is the primary source of information about objects observed as part of the ground-based Kepler Spectral Classification Program (SCP) in preparation for the selection of Kepler PI and GO targets. The KIC lists objects down to 21st magnitude, but it is not complete to this limit. Light from only about 1/3 of these objects, some 4.4 million, falls on the Kepler CCD detector. A small number of the KIC objects are calibration objects distributed across the sky. For this reason the full KIC should never be used for Kepler target selection. (By “target” we mean any fixed celestial object observable by Kepler). Information from the KIC is combined with data in two other catalogs to allow searches on the Kepler Target and Data Search pages; see next FAQ.MAST provides a portal to those who want to see the contents of the full KIC. Also,Release Notes for this compilation have been posted by the SCP team.
- Are there other Kepler catalogs delivered to MAST?
The Project delivers to MAST the Kepler Target Catalog (KTC) and the Characteristics Table (CT). The KTC is basically an observing log, containing both the observed and planned targets. The KTC catalog is delivered to MAST quarterly. The CT contains object-specific information pertaining to the Kepler objects, such as the column and row pixel location. The CT was delivered early in the project mission, and a new version was delivered and made public in June, 2011 (note that some new fields were added and others deleted!). Access to information in these catalogs is via MAST’s Kepler Search and Retrieval page and Kepler Target Search page. (See for example the “Field Descriptions” link on these pages.)In addition to these catalogs, plans are being made to enhance the effective magnitude limits of targets to stars fainter than the KIC routinely covers (i.e. fainter than 17th magnitude). General users as well as Kepler GO and ADAP program proposers should stay tuned to the MAST’s High Level Science Products page for these enhancements.Please pay attention to news updates on the MAST and Kepler news corners for the announcements of formally delivered catalog material.
- Are there objects visible to the Kepler detectors that are not in the KIC?
Yes. The KIC includes objects as faint as 21st magnitude, although its coverage is not complete to this brightness level. In particular, variable sources and extended sources with low surface brightness may not be in the KIC because they were not in the ground-based catalogs that were used to make it.
- What are the objects with kepler_ids greater than 10,000,000?
There are two types of objects with kepler_ids greater than 10,000,000. The vast majority are CCD monitors, which are engineering targets defined by the Kepler Project. The remainder are astronomical objects that have been or will be observed by Kepler, but which do not appear in the KIC, or groupings of KIC objects that can not be adequately resolved by Kepler. Examples include just a few GO targets, faint flare stars, planetary nebulae, and subclusters of stars. The Project is expecting to provide only positional information for such objects.
- What is the “Kepler magnitude”?
The “Kepler magnitude” is not to be confused with satellite flux measurements from the satellite. It is a magnitude computed according to a hierarchical scheme and depends on what pre-existing catalog source is available: SCP, Tycho 2, or photographic photometry, in order of preferred selection. For SCP objects (see next FAQ) the Kepler magnitude is further defined according to which of the Sloan grimagnitudes are available for a given star. These dependencies can be found in the Brown et al. (2011AJ….142..112B) paper. When all gri magnitudes are available the relations depend on the gri magnitudes and (g-r) color according to the following prescription: if (g-r) ≤0.3 then kepmag = 0.25g+0.75r, while if (g-r) >0.3 then kepmag = 0.3g+0.7i. We note that for the small fraction of objects for which Sloan photometry is not available the Kepler magnitude is taken from photographic catalogs. The errors of photographic Kepler magnitudes are typically ±-0.2-0.3 mags.
- What is the Spectral Classification Program (SCP)?
Early on, the Kepler Project realized the need for homogeneous ground-based observations that would provide information about all the stars in its detector’s field of view. The SCP, a project commissioned to the Harvard-Smithsonian Center for Astrophysics (led by Dr. D. Latham), realized the solution to this need by providing colors through copies of the filters used for the Sloan survey (plus an additional “metallicity” filter). “SCP objects” are defined for which the SCP_ID field is listed; its value is the 2MASS identifier (TMID). For an object to be an SCP object it must have 2MASS JHK colors. However, the SCP program succeeded in observing nearly all “SCP objects” in at least one of the 4 ground-based griz filters of the Sloan photometric system. Thus, in practical terms SCP stars are those that have been observed by the 2MASS project and through at least one of the Sloan filters. Some 98% of KIC objects on the Kepler detectors have been observed through one or more of the Sloan filters or had synthetic g and r magnitudes determined from the B and V band photographic photometry of earlier catalogs.Additional caveats are the following: (1) not all KIC stars observed by 2MASS are SCP objects; these do not have Sloan filter magnitudes, (2) the g and r magnitudes of many faint KIC objects were computed from photographic catalog magnitudes. To find out whether stars in your object list have been observed photoelectrically or photographically, highlight the “Keplermag_Source” item in the output menu of either of the Search forms and then click “Add.”The colors and color-derived quantities shown in MAST’s Kepler Search pages are derived from the SCP program. A copy of the SCP plan is available. And: as noted in another FAQ, users should be aware that the INT_KIS Sloan i magnitudes for Kepler objects are in serious disagreement with the KIC i magnitude. However, GO proposers should continue to use Kepler magnitude values for KIC objects listed in the KIC.
- How do I search for an object by RA in decimal hours?
The Kepler search forms have fields for entering Right Ascension and Declination, and they accept (only) decimal degrees or sexagesimal (hhmmss) formats. To search for RA in decimal hours, go to one of the “user-specified field” pulldown menus, select the entry “RA_hours (J2000)”, and then enter values in the corresponding “Field Descriptions” box. Note that currently the File Upload search option accepts only RA in decimal degrees or sexagesimal format. For more help, click any of the form element labels on the search form.Note that the coordinates in the search results will always be displayed in sexagesimal format unless one clicks the “Hours” button in the “Output Coords” field element.
- Is information about the physical characteristics of stars derived from the KIC reliable?
Often not. Physical characteristics like the effective temperature are most trustworthy for cool stars. It is well known that this quantity is poorly calibrated for stars earlier in type than about F0. An recent published presentation by Batalha et al.(2010ApJ…713L.109B) shows that cool giants and dwarfs are statistically well separated. Unpublished information from the Project suggests that metallicity is not well determined. information on these topics has become available in the Brown et al.(2011AJ….142..112B) paper.Information from the GO office takes precedence over this response.
- What are the exposure times of short, long cadence time series data?
Following the terminology given in the Kepler Instrument Handbook, the interval between reads of a given pixel on a CCD is called a “frame” (equivalently an integration time). The integration time consists of the “exposure time” (accumulated time of flux from a celestial source on the pixel), 6.02 seconds, plus a fixed read out time of 0.52 seconds. The default exposure time for a short cadence is thus 6.02 x 9 or 54.2 seconds. The cadence rate or integration time, is (6.02 + 0.52) x 9, or 58.9 seconds. The default exposure time for long cadences is thirty times 54.2 , or 1626 seconds. The cadence rate between starts of consecutive integrations is thirty times 58.9, or 1766 seconds.
- What kind of time series (light curve) data product is the Kepler Project planning to release?
The light curve is produced using simple aperture photometry (SAP). The light curve files contain two light curves. The SAP light curve is the flux in units of electrons per second contained in the optimal aperture pixels collected by the spacecraft. This light curve is the output of the PA module in the SOC pipeline. The PDCSAP light curve is the flux contained in the optimal aperture in electrons per second after the PDC module has applied its detrending algorithm to the PA light curve. Readers should also consult the target pixel file FAQ.Information from the GO office takes precedence over this response.
- In what units are the time series light curves given?
The time units are seconds (SI). The project has revised the initial units of “flux” from electrons per cadence” to electrons per second. These quantitites may be thought of as an instrumental flux unit closely approximating a linearized unit (for unsaturated pixels). There are no plans for the Project to produce a product in a calibrated absolute magnitude system.Information from the GO office takes precedence over this response.
- UTC, barycentric time, MJDs, etc. – What does Kepler use?
Kepler provides time in UTC and TDB (barycentric dynamical time, without the relativistic correction), formatted as MJD or (depending on where you are looking) reduced JD. There are several FITS keywords in the Kepler data headers that relate to time. For light curves and target pixel files, the relevant time keywords are in the extension header (BINTABLE). For FFIs, the time keywords are in the primary header. In the discussion below, the time is taken at the mid-point of the first and last cadences unless otherwise noted. Times expressed in UTC or MJD are geocentric, that is they are not corrected to the Kepler – Solar System barycenter. In the data, time is specified as a reduced form of TDB, expressed in Julian Data (so Barycentric JD or BJD). For light curves and target pixel files, this is BJD-2454833.0. See the Release Notes, given as the DATA_REL keyword in the header, specific to your data for more information.For light curves and target pixel files:
The LC_START and LC_END keyword values are the UTC, given in MJD format, of the first and last cadence in the light curve or target pixel file. These keyword values are intended for the use of the DMC.The BJDREFI and BJDREFF keyword values are the integer and fractional part of the BJD reference date for that file.The TSTART and TSTOP keyword values are the exposure start and stop time, in BJD – BJDREF format. These times are TDB, expressed in JD format. This is referred to as the Barycentric Kepler Julian Date (BKJD) in the Data Characteristics Handbook.
The DATE-OBS and DATE-END keywords give the TSTART and TEND values in UTC.
For FFI files:
The STARTIME and END_TIME keyword values are the UTC start and end times, in MJD format. They are found only in the FFI primary header. Since the FFI is a single readout (cadence), these times are the not mid-point values.
The BSTRTIME and BSTPTIME keyword values are the TDB start and end times, in MJD format. They are found only in the FFI primary header. Since the FFI is a single readout (cadence), these times are the not mid-point values.
- What is the reference time for Kepler data?
For Kepler, the reference time for a cadence is the midpoint observation time. For a light curve or target pixel file, which is made from many cadences, the start time is the mid-point of the first cadence and the end time is the mid-point of the last cadence. The Data Characteristics Handbook, Section 6.2, contains a discussion of the Kepler times as does the Kepler Archive Manual, Section 2.1.2.The readout time for each cadence, which is recorded as the Vehicle Time Code (VTC), is produced within 4 ms of the readout of the last pixel of the last frame of the last time slice. The VTC is converted from spacecraft time to UTC (i.e., leap seconds accounted for) at the Mission Operations Center and the Data Management Center. The conversion to Barycentric Dynamical Time (TDB) is done at the Science Operations Center. In the headers, TDB is expressed as a reduced Barycentric Julian Date (BJD), with the offset from TDB given in the BJDREFI and BJDREFF header keywords.The Kepler Instrument Handbook, Section 7.3, warns users against assuming a precision of better than 9.0 seconds in the absolute time. Users who require temporal accuracy better than 1 minutes should carefully read the Kepler Data Characteristics Handbook (Section 6) and the associated Data Release Notes.For further details consult the Kepler Instrument Handbook.
- What are target pixel files? Are they the same as cadence files? When will I need them?
Target pixel files contain the pixels used to create the light curves. The data are packaged as a time series of images in a binary table, where each image is a single cadence. The intent of these files is to provide the data necessary to perform photometry on the raw or calibrated data when needed (or desired) to understand (or improve) the automated results of the Kepler pipeline.See the Kepler GO web site for tools that may be used to extract data from the target pixel files.Note that target pixel files are not the same as cadence data. Target pixel files are target-specific, while cadence data contain the pixels for every target.
- Which FITS header keyword tells me the actual exposure time?
There is no single keyword in the light curve headers that gives the actual exposure time contained in the light curve. The SOC processing may reject individual cadences, so users who require the actual exposure time must inspect the data to determine how many, if any, cadences were rejected. There are a number of keywords that describe the time spent acquiring the data. These are listed below.The Kepler integration time is composed of a fixed exposure time plus a fixed readout time. The exposure time for science data is 6.02 seconds, while the read out time is 0.520 seconds, for an integration time of 6.54 seconds per pixel. Individual reads of each pixel are summed on board before being written to the recorder. Long cadence data are summed for 30 minutes, (270 integrations) while the short cadence data are 1 minute sums (9 integrations), yielding total per pixel exposure times of 1625.4 seconds and 54.18 seconds,respectively.TSTART = observation start time in BJD-BJDREFTSTOP = observation stop time in BJD-BJDREF
LC_START= mid point of first cadence in MJD
LC_END = mid point of last cadence in MJD
TELPASE = TSTOP – START
LIVETIME= TELAPSE multiplied by DEADC
EXPOSURE= time on source
DEADC = deadtime correction
- Which FITS header keyword tells me how many pixels were used to determine the aperture for observing an object? Which tells me how many we actually used?
There are no keywords in the light curve or target pixel file headers that provide this information. However, the target pixel file has an aperture extension containing a single image that describes which pixels were collected by the spacecraft and which pixels are contained in the optimal aperture. The FITS standard requires a rectangular bounding box even though many target apertures are not rectangles. Therefore the image contains null pixels that were not collected (i.e., the image includes the extra pixels necessary to create a rectangular image). It is clear which pixels are which.
- Where do I go to find the most recent report on data quirks, peculiarities, flaws, etc.?
The Project expects to puts out at least one set of Data Release Notes each quarter. These reports are on the left gutter of the MAST/Kepler home page. The notes detail many of the vagaries of the data, including the latest information on noise sources, “argabrightening,” loss of detectors, and so on. The notes are public even though almost all the data they refer to are proprietary. Users interested in attributes of data for a given quarter should start with the corresponding Release Notes version. However, it is useful to consult higher versions that may contain updates on tests on data properties. In February, 2011 the Project delivered the Kepler Data Characteristics Handbook, which is meant to be the definitive “static” document describing nonstandard properties of Kepler data.
- Have any Kepler detectors been lost since the start of mission operations?
Yes: All 4 channels of Module 3 (1 of 21) failed on Jan. 9, 2010 (MJD55205.745) during Quarter 4, and the Project has declared it permanently nonoperational. The impact on science observations is that about 20% of Kepler’s field of view suffers a one-quarter loss of data per year. Further details of this announcement can be found in Section 6 of the Project’s Data Release Notes #6 and in Section 4 of the Kepler Data Characteristics Handbook.
- Are the flux contaminations given in the Target Search page reliable?
The revised Characteristics Table (delivered June, 2011) has better contamination values for objects with given Kepler magnitudes than those used in the old CT. The latter values were single season estimates, whereas the new ones are better estimates based on season and default aperture. This means that they are specific from season to season and will repeat every four quarters). We expect that FITS files of light curves delivered from the Project in late 2011/early 2012 will contain keywords computed by the pipeline.
N.B.: Those KIC objects for which no Kepler magnitudes exist do not have these newer and better contamination values. Users can check on the origin of the contaminations in their list selections simply by noting which objects have Kepler magnitudes on the results page of the Target Selection form.
- Which of the available light curves should I use?
Quarter 8 light curves and Quarters 0-7 light curves reprocessed in July, 2011 contain two types of light curves now called SAP_FLUX (formerly “raw”) and PDCSAP_FLUX. The Project will soon provide tools via MAST that will enable users to apply corrections to the SAP_FLUX vectors to remove certain instrumental trends. These trends have been determined statistically by Principal Component Analysis. The PDCSAP_FLUX light curves will initially have some but not all of these trends removed. Future processing under Pipeline “Build 8” in early 2012 will include a best-attempt removal of the remaining trends. Thus, for the short term (late 2011-early 2012) researchers should consider using the provided the tools to remove these trends themselves to the SAP_FLUX vector in case the PDFSAP_FLUX light curves do not do an adequate job. Users should stay tuned to the MAST News page to learn where to upload these tools. The Kepler project has posted documentation on the available tools on its website titled Astrophysics with Kepler. Further details on the use of these tools will be found in the Archive Manual to be released in the summer of 2011.This subject is currently in a state of high flux. Questions should therefore be directed to the Project’s GO or Science Office. Their replies should take precedence over this FAQ.
- What information in the MAST Target Search form is different for targets having no Kepler magnitude entries?
The delivery of a new Characteristics Table in mid-2011 excluded some 122,000 (usually faint) objects that have no Kepler or griz magnitudes. In order to permit users to propose for these targets. MAST has restored the data associated with these entries from the (frozen) KIC. However, this means that some columns found on the Target Search page now have different meanings for objects with and without Kepler magnitudes, or there may be no given values at all.
One difference concerns “edge to CCD” values. For KIC targets with”null” Kepler magnitudes, this parameter refers to the distance of central pixel of its image to the nearest edge of the CCD it resides on, while for targets with Kepler magnitudes the edge value refers to the outer edge of the photometric extraction area computed for the image; the number Seasons_on_CCD is also based on this metric. Note well that these contamination values are predicted and are not measured.
The contamination, crowding, fractional flux, and signal-to-noise ration (SNR) values are given only for targets with Kepler magnitudes. For targets without Kepler magnitudes the values provided by MAST on the Target Search form and results pages may be less accurate.
Finally, objects included by catalogs other than the KIC (from the UKIRT, INT/KIS, and UBV surveys) have computed distances corresponding to their nearest KIC neighbors. However, targets are omitted if their positions do lie not at least 11 pixels from the detector edges. Surviving non-KIC entries are therefore guaranteed to lie on the detector (at least one season).Users should take care if targets not having Kepler magnitude are very close to the edge of a CCD. When in doubt users are encouraged to direct such questions to the GO Office.
- How robust is the calibration of Teff values from KIC colors?
The photometric calibration and temperature scale of stars in the KIC have been published by Brown et al. (2011) (2011AJ….142..112B) . The catalog and calibrations established a photometric color-derived scale for effective temperatures for cool stars and enabled a differentiation between giants and dwarfs. It has been succesful for these purposes, particularly in the range 4500K < Teff < 6500K. In an analysis of color calibrations determined from M67 stars, 2MASS colors, the Yale Rotating Evolutionary Code, and high resolution spectroscopy, Pinsonneault et al.(2012ApJS..199…30P) found that for the range 4000K-6500K Teff’s of KIC field dwarfs are about 215K too cool relative to temperatures found from calibration of infrared fluxes. This deviation is perhaps not surprising since Brown et al. estimate systematic errors in the KIC could be this large. Although the ultimate source of the systematics is still unknown, according to these authors, a significant component could arise from photometric errors and inclusion of other objects (including binary companions) in the photometric extraction apertures. Metallicity differences and extinction errors may play a secondary role.Pinsonneault et al. note similar systematic errors in warmer (mid A to F-type) stars. According to spectroscopic calibrations of a comparative handful of B stars (see (Mamejek 2010) and Lehmann et al. (2011) (2011A&A…526A.124L), the discrepancy in this calibration grows with increasing Teff.MAST offers no judgment on these claims. Users should go to these papers and follow-up literature to pursue this further. The new infrared, ultraviolet, and near-ultraviolet magnitudes that MAST is adding in its Enhanced Target Search page can enable users to test these calibrations with extended spectral energy distributions of KIC stars.
- How do I determine the best masses for stars that are candidates for hosting planets?
The Kepler Project will not compute stellar masses for inclusion in any of their Kepler Results Catalog tables. MAST and the Project recommend that users compute stellar masses directly from the radii (s_Rad) and log(g) in the Planetary Candidate table.(Do not use data provided in the KIC for this purpose.). Note that the Candidates table is part of the Kepler Results Catalog. An updated Planetary Candidates Table will be sent to NExScI in the fall of 2012, and from there to MAST. An updated Kepler Results Catalog, including the Candidates table, is expected in January of 2013. This FAQ was drafted in consultation with the Kepler GO office.
- Did the Sloan (SDSS) project survey the Kepler field?
No and yes. The original Sloan Survey (SDSS/DR7) covered 1/4 of the sky outside the Galactic plane and included very little of the Kepler field. However, to simulate the results of the Sloan survey the Kepler SCP team observed KIC objects with a set of Sloan-like filters. Note carefully from Pinsonneault et al. (2012ApJS..199…30P) that it is now known that there are both zeropoint and color effects affecting comparisons of results from the SDSS and KIC observations (even though the latter used a copy of the SDSS filters). The KIC catalog is comprised in large part from data taken from this program. Note that the Sloan photometric survey was undertaken in five filters, ugriz. The SCP team undertook a similar effort with its filter copies. However, because the u magnitude observations required relatively long integrations, these observations were abandoned after the first observing season. The project has delivered u-magnitudes of relatively few KIC objects (2997 of those appearing on the Kepler detectors). These are included as results of the Kepler Target search tool.In late 2012 MAST will add new u, g, r, i, and z magnitudes and potentially new targets from the extended part of the SDSS Project, Data Release 9. These data will be announced and will appear in MAST’s Kepler Target Search form and CasJobs tool. Please stay tuned.
- Has the GALEX project surveyed the Kepler field in the near-UV and far-UV?
In the summers of 2009-2011 the GALEX Project surveyed much of the Kepler field. These survey products, including a cross-matched catalog of KIC objects observed by GALEX were added to the GALEX GR6 (General Release 6) in 2010, and a final supplement in 2012. MAST has made available two cross-match catalogs as a MAST-style interface form and for in-depth use in a “CasJobs” implementation. See the MAST news corner item, dated October 28, 2010.
- Are there other UV-optical-IR missions that have surveyed the Kepler field?
The 2MASS Survey, a survey of 70% of the full sky at the 3 infrared J, H, K bands (1.25, 1.65, and 2.17 microns), includes nearly all bright KIC targets, and is fairly complete down to KEPMAG 16 or 17. For all but faint KIC objects the 2MASS_ID often provides a good (though imperfect) substitute for Kepler_ID values in queries on the MAST search pages, particularly for cross-referencing to external catalogs.Users should be also aware of objects observed from the ground in the Kepler All Sky Automated Survey. This is a photometric survey dedicated to the ground-based study of optical variables in the whole observable sky (both hemispheres), including the Kepler field.Users should watch the MAST/Kepler News corner for developments on ingests of new ground-based photometric observations on the Kepler field.
- Does the Digitized Sky Survey cover the Kepler field?
Yes. Go to the Google Sky site and enter the following in the string: http://keplergo.arc.nasa.gov/tools/KeplerFOV.kml to see the location of the Kepler CCD “footprints” against a Digital Sky Survey map of the sky. (Courtesy of a tip on the Kepler GO website.)
- Is there a handbook that describes resources at MAST for astronomers to investigate or propose for Kepler data?
Yes this document is available. The Kepler Archive Manual provides a description of MAST and its resources.