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Kepler Guest Observer Program

Frequently Asked Questions

WHITE PAPER CALL FOR ALTERNATIVE SCIENCE INVESTIGATIONS WITH KEPLER

  1. Where can I find the white paper call?
  2. Given the estimated Kepler pointing drift (maximum of 1.4 deg in 4 days), will it be possible to a specify a target in celestial coordinates in such a way that the aperture on that target follows the drift of the target across the Kepler focal plane?
  3. Which spacecraft pointings provide the most stable photometry?
  4. How will the spacecraft pointing behave if science returns to the original Kepler field of view?
  5. How can I write a white paper without a broader understanding of photometric precision?
  6. What is the longevity of a two-wheel mission?
  7. What is the upper limit on the duration of spacecraft operations during a two-wheel mission, if science data were collected only a small fraction of the time?
  8. Would it be wise to assume that Kepler will lose another reaction wheel soon? If so, any ideas of what range we should plan for?
  9. Is there a way to use Kepler that critically changes the number of short-cadence (SC) targets from 512 to many more?
  10. Is a spacecraft attitude possible that allows for the spacecraft to leave the high-gain antenna pointed at Earth while collecting data?
  11. Is it possible for Kepler to operate in a Target of Opportunity mode? If so, what constraints does the project envision on such a mode of operations?
  12. Can target uploads be more frequent than once a quarter? If so, how often?
  13. What are the limitations on using really big apertures to do modestly wide field imaging with the current flight software?
  14. Is there a possibility of significantly increasing the number of pixels download by implementing improved compression?
  15. How many full frame images could be collected before the spacecraft would need to point towards Earth to download the data?
  16. How long does it take to downlink 42 FFIs (i.e. A full recorder's worth)? Please include estimated overheads to repoint to Earth etc.
  17. What are the possible integration times for Full Frame Images?
  18. Can the mission provide existing coarse point attitude files for the purpose of calculating two-wheel photometric precision?
  19. What are the body axis vectors of the four Kepler reaction wheels and which of these failed?
  20. What are the design performance specs of the star trackers? (Accuracy and update rate, initial acquisition solution, etc.)
  21. What are the boresight body axis vectors of the two star trackers?

THE EXOPLANET ARCHIVE AT NExScI

  1. When the Kepler mission delivers a new KOI to the Exoplanet Archive does this mean that the Kepler mission is delivering a new planet candidate?
  2. If the Kepler Project hasn't finished the analysis of KOIs, why are they releasing this information now? It seems rather preliminary.
  3. What guidance is there for the scientific community about using data within activity tables that are still 'open'?
  4. With previous Kepler data releases, the term 'KOI' was synonymous with planet candidate. Can you explain what has changed?
  5. How are KOIs found within the list of TCEs?
  6. Is every TCE that passes triage automatically promoted to KOI status?
  7. What does it mean to "disposition" a KOI?
  8. Does the Kepler Project reanalyze old KOIs?
  9. Will the Kepler Project redisposition all the old KOIs within each activity table at the archive as well as disposition the new KOIs?
  10. In the Q1-12 data set there are a surprising number of KOIs with orbital periods near one Earth-year. Do Earth-size planets tend to prefer Earth-like periods?
  11. Why weren’t false-positive one Earth-year KOIs seen in earlier data releases?
  12. Are there other reasons for an increased fraction of false positives over time?
  13. Are KOIs held back from the archive for any reason?
  14. Why are some columns in the TCE or KOI tables blank?
  15. What are the rules for displaying the precision of values and their errors in the TCE and KOI tables?

THE KEPLER DATA ARCHIVE AT MAST

  • See the FAQ maintained at the MAST archive.

Where can I find the white paper call?

Given the estimated Kepler pointing drift (maximum of 1.4 deg of roll in 4 days), will it be possible to a specify a target in celestial coordinates in such a way that the aperture on that target follows the drift of the target across the Kepler focal plane?

At present, we cannot make apertures that follow the targets. Thus, either you have to define a long narrow aperture to cover the entire time period (i.e., 1.4 deg of roll in 4 days or a smaller part thereof) or this requirement will need a rewrite of flight software.

Which spacecraft pointings provide the most stable photometry?

Two reaction wheels can maintain stable pointing except for a roll component about the pointing direction. Targets will trace an arc on the sky. This arc is reduced to zero if the spacecraft is pointed along the ecliptic, i.e. the orbital plane. Reaction wheel momentum must be reset at least once every four days, but targets can be re-acquired after the reset at their original pixel location in the ecliptic, avoiding the systematic effects of e.g., inter-pixel sensitivities, detector gain, stray light, ghosts and detector imperfections. At the ecliptic, pointing precision will be dominated by the the precision of the star trackers and noise incrued by the rotation of the two wheels. Both properites requires operational testing. Before operational testing, it is also not clear how accurately the boreight can be pointed. The closer to the ecliptic we can point, the smaller the drift in boresight roll. There are sun angle constraints for the solar panels. A target in the ecliptic can only be oberved for 6 months of any year - 3 months around the direction of the orbital velocity vector and 3 months around the opposite direction to the velocity vector. More details can be found in the initial two-wheel pointing study.

How will the spacecraft pointing behave if science returns to the original Kepler field of view?

Over the duration of 24 hours, a target will roll around the boresight by a few arcminutes. Note the difference between arcmin of boresight roll and arcmin of celestial sky angle. This translates to 3-4 detector pixels in the corners of the field of view. If momentum in the wheel is reset once per day, the sun angle has moved a degree and the boresight has to be rotated by a degree in order to maintain the roll drift at a manageable few arcmin per day. This will result in a target falling on a different part of the detector, a degree away, after each momentum dump. Such data will have significantly larger systematic artifacts than data collected from the ecliptic. More details can be found in the initial two-wheel pointing study.

How can I write a white paper without a broader understanding of photometric precision?

Obtaining a more detailed understanding of photometric precision during a two-wheel mission requires sky tests. The answer will be a function related to e.g. source crowding, wheel jitter and the rate of boresift roll. Very preliminary simulations of an ecliptic field undergoing ~1 arcsec jitter suggests photometric S/N over exoplanet transit timescales of a 12th mag target will be on the order of hundreds of parts per million using simple aperture summation. There are prospects for optimizing S/N further using PSF or optimial aperture algorithms. Tests are being scheduled for September and into the fall. It should be noted that the white papers requested are not "proposals" and the call is not a competition. The idea is to collect potential scientific incentives for operating a two wheel mission. A zeroth-order description of what S/N your science would require in order for observations to be a success would be very valuable to the project but is not a requirement of the call.

What is the longevity of a two-wheel mission?

Other than the reaction wheel, there are no moving parts on the spacecraft. The two remaining wheels show no signs of failure. Fuel reserves are expected to last 4 years or more. We have no current funding for an extended 2-wheel mission but plan to build a case for such over the next 4-6 months.

What is the upper limit on the duration of spacecraft operations during a two-wheel mission, if science data were collected only a small fraction of the time?

If the spacecraft were to remain in Point Rest State, minimizing the fuel usage, then the available fuel may last another 5-7 years, with some significant uncertainty.

Would it be wise to assume that Kepler will lose another reaction wheel soon? If so, any ideas of what range we should plan for?

The remaining reaction wheels have shown no signs of degradation, hence their predicted life is largely uncertain. Because the repetitive saturation/desaturation profile in fine-point operations will not be available, it is also uncertain whether there will be the kind of warning of impending wheel failure as there was with Wheel 4. An assumption that the remaining wheels will function for 1-3 years would seem reasonable. Note that while in Point Rest State, the wheels are not spinning.

Is there a way to use Kepler that critically changes the number of short-cadence (SC) targets from 512 to many more?

To increase the number of simultaneous SC targets would require changes to the flight software and would place constraints on the focal plane readout and data downlink which might be hard to accommodate.

Is a spacecraft attitude possible that allows for the spacecraft to leave the high-gain antenna pointed at Earth while collecting data?

Functionally, the spacecraft can point the High Gain Antenna to the Earth and take data simultaneously. However, such an attitude is strongly influenced by the solar torque and would generate drift about the boresight, perhaps as much as half a degree per hour. Therefore it is unlikely to be a useful observing attitude.

Is it possible for Kepler to operate in a Target of Opportunity mode? If so, what constraints does the project envision on such a mode of operations?

ToO observing modes are viable. Refer to the next answer on target upload frequency.

Can target uploads be more frequent than once a quarter? If so, how often?

Quarters do not have to be a driving mechanism in a 2-wheel mission. Target uploads can occur more often than once per quarter. The practical limits are driven by the effort required to define the pixels to be collected after each upload and how often the project can negotiate contact times between the spacecraft and the Deep Space Network. The more complex the target procedures, the more cost they will accrue in engineering and services.

What are the limitations on using really big apertures to do modestly wide field imaging with the current flight software?

The fundamental pixel limit is a 5.44 Megapix total (170,000*32), There is a maximum of 1,024 aperture definitions (shapes), and a maximum of 32,767 (2^15) pixels in any given aperture. There is no problem tiling a single CCD channel with these large apertures. You could collect approximately 1 full module (4 x 1,100 x 1,024 pixels = 4.5 Megapix). The data download rate is determined by how many pixels you are using and whether compression is on or not. One of our questions for Ball is to tell us the transfer rate as a function of pixels.

Is there a possibility of significantly increasing the number of pixels download by implementing improved compression?

Compression was nominally done for cadence data during the prime mission, but not for FFIs. It is doubtful that cadence data compression efficiency will increase in a 2-wheel mission. Compression of FFI data may be feasible, but may require a Flight Software Update. The impact of such a change is currently unknown. However, more pixels may be downloaded through more frequent downlinks from the spacecraft at the price of reduced observing efficiency.

How many full frame images could be collected before the spacecraft would need to point towards Earth to download the data?

42.

How long does it take to downlink 42 FFIs (i.e. A full recorder's worth)? Please include estimated overheads to repoint to Earth etc.

Previously, we have only used about 50% of the recorder space during the nominal mission, and currently that takes about 20-25 hours to download, including overheads. That time will increase as the spacecraft continues to drift further from Earth over time. Kepler can store a maximum of 38 FFIs in the science buffer. A downlink of each one at the highest rate (4.33 Mbps) took 15 min, however that rate is no longer achievable. At the lowest downlink rate expected at the end of 8 years of flight (~1 Mbps) it would take 65 minutes to downlink one FFI. A full solid state recorder would take almost 2 days to downlink at that time - plus there will be inefficiencies in Deep Space Network scheduling which will add unknown overhead. These estimates assume coarse point performance, a capability which has not yet been tested. The maneuver to Earth-point and overhead time to lock up on Ka-Band would be approximately 2 hours. If fine point science is achievable then the time for the system to thermally settle after slewing adds another 10 hours approximately.

What are the possible integration times for Full Frame Images?

FFI exposure times are governed by the same rules as long- and short-cadence science data. The rules are described on page 8 of the call for white papers.

Can the mission provide existing coarse point attitude files for the purpose of calculating two-wheel photometric precision?

The nominal four-wheel mission course point is not a representative course point situation of what we expect during the possible two-wheel mission. Thus, it would not provide the proper photometric values. The best current information is listed in the call and is a zeroth order estimate.

What are the body axis vectors of the four Kepler reaction wheels and which of these failed?

Reaction wheels 2 and 4 have failed.

Wheel 1Wheel 2Wheel 3Wheel 4
X0.573526-0.5735260.573526-0.573526
Y0.4846840.4846840.4846840.484684
Z0.6603280.660328-0.660328-0.660328

What are the design performance specs of the star trackers? (Accuracy and update rate, initial acquisition solution, etc.)

The Kepler spacecraft is equipped with two BATC CT-603 633 star trackers. These star trackers output a measurement quaternion, as well as the star centroids used to synthesize that measurement. Each tracker outputs a measurement at 5 Hz, has an 18° x 18° FOV, and tracks up to five stars between visual magnitudes of -1 and 5.3. Each tracker contains a star catalog containing 2039 stars for attitude determination across most of the celestial sphere. The CT-633 trackers can be commanded search locations to seek out specific stars in their catalog. This is referred to as the “Directed Search” command, and has been used with great success for transitions to and from the Fine Guidance Sensor. Ground processing tools exist to synthesize directed-search commands for the trackers at any sky attitude.

What are the boresight body axis vectors of the two star trackers?

The tracker boresight vectors and body-to-tracker-measurement quaternions are shown below.

Boresight vector:
ST1
Qb2tr:
ST1
Boresight vector:
ST2
Qb2tr:
ST2
X0.10576000000.38749833210.10576072990.2049812991
Y-0.8782959100-0.3416259713-0.8782959142-0.5400903415
Z0.46627347600.63896168970.46627347650.2876506229
Z0.56996903180.7638993448

Some documentation suggests that the Kepler spacecraft has two inertial measurement units. Is there any information about these?

Kepler has 2 Inertial Measurement Units: Vendor: Northrop Grumman (formerly Litton): Model: LN-200S

The three stages of vetting Kepler Transit Threshold Crossing Events (TCEs) for Planet Candidates

Figure 1: The three stages of vetting Kepler Transit Threshold Crossing Events (TCEs) for Planet Candidates.

When the Kepler mission delivers a new KOI to the Exoplanet Archive does this mean that the Kepler mission is delivering a new planet candidate?

No. When KOIs are first delivered their nature has not been analyzed completely. The term KOI means exactly what the name implies – Kepler has declared these to be “objects of interest,” not planetary candidates. By promoting these transit-like signatures to KOI status, the Kepler Project is saying is that their light curves contain interesting patterns of repetitive dips that might indicate the presence of a transiting planet. However, there are several other ways to produce similar looking transit-like patterns. For example, the dips could be due to stellar variability, excess detector noise, other transient events associated with the spacecraft, or a background star occulting a second background star (i.e., a background eclipsing binary). We use the term “false positive” to describe those KOIs that are explainable by means other than the planetary hypothesis. We know that with further analysis, many of these new KOIs will become false positives.

If the Kepler Project hasn't finished the analysis of KOIs, why are they releasing this information now? It seems rather preliminary.

he analysis is indeed preliminary, but it also represents a significant body of work and contains valuable information for the scientific community. To summarize the process of identifying KOIs. the Kepler project started with the light curves of nearly 200,000 stars that were observed for some or all processed quarters. That’s a lot of data to plow through. When the Project begins searching data they typically identify 10,000-20,000 threshold-crossing events (TCEs). These TCEs had to pass a series of tests, each with a threshold, that were designed to identify the events that look transit-like. This list of TCEs and their accompanying diagnostic reports (i.e., data validation reports and one-page summaries) are also released to the public in tables through the NASA Exoplanet Archive. The criteria required to pass this first set of tests are intentionally lenient. The Project prefers to include many non-transit-like events at this early stage of analysis, rather than miss small, Earth-size candidates in long orbits – the hardest candidates to find).

What guidance is there for the scientific community about using data within activity tables that are still 'open'?

The value of the different stages of product delivery greatly depends on your scientific objectives. If you are looking for interesting KOIs to study or for follow-up observations then providing results in a sequential fashion is a big help. If you are trying to understand the statistical population of small planets in the galaxy, this delivery isn’t going to hand you what you need until the activity table is closed, published and the nature of the population, detection algorithms and vetting procedures are well understood.

With previous Kepler data releases, the term 'KOI' was synonymous with planet candidate. Can you explain what has changed?

This is a common misconception. Actually, the definition of KOI has not changed; but the Kepler Project's reporting philosophy has. In the past, the Kepler mission published lists of KOIs that were deemed to be planet candidates; and separately posted the KOIs that were declared false positives at MAST (Mikulski Archive for Space Telescopes). This may have given some the mistaken impression that all KOIs are planet candidates, but this has never been the case. For example, four of the first ten KOIs identified using the first month of data are currently marked as false positives in the cumulative activity table at the NASA Exoplanet Archive. The reporting philosophy has been modified so that all KOIs can be archived in one place. This makes it much easier to change the status of a KOI from ‘planet candidate’ to ‘false positive,’ and vice versa. In addition, the new format enables more rapid release of incremental information as progress is made.

How are KOIs found within the list of TCEs?

The Kepler Project evaluates each TCE using objective criteria that are difficult to code into a computer algorithm. This exercise is called “triage” because it is a relatively quick assessment that eliminates the obvious false positives, while retaining anything that looks remotely transit-like for further assessment. During this exercise, most of the events produced by spacecraft transients and stellar variability are discarded. This is process step 1 in Figure 1 which summarizes the process using the Q1-12 TCEs and Q1-12 KOI activity table as examples.

Is every TCE that passes triage automatically promoted to KOI status?

No. If at least two scientists determine that a TCE looks transit-like, then the light curve is fit with an analytic model of a transiting planet (Mandel and Agol 2002). If the model fit looks reasonable, then the TCE is promoted to KOI status. If the model fit is poor, then the TCE is ignored and receives no further analysis. As shown in Figure 1, slightly more than half of the TCEs that pass triage are typically promoted to KOI status. Moreover, many of the KOIs found among a TCE sample are old ones that were discovered and cataloged during previous transit searches. After triage, detailed attention is most typically spent focusing on new KOIs, found for the first time in the current sample. Many of the new KOIs will eventually become false positives, but the Kepler Project can afford the additional analysis because the number of light curves that require in-depth assessment has been reduced by a factor of 100.

What does it mean to "disposition" a KOI?

Dispositioning is the final step of vetting KOIs. Dispositioning is the action of using a variety of analytical tests to label a KOI as a planet candidate or a false positive. The most common tests are documented within the one-page DV summary reports and full DV reports archived with the TCE tables. Dispositions are the current opinion of the Kepler Project regarding the nature of a KOI, although it must be noted that there are gray cases and users have freedom to disagree with the disposition using the statistics provided at the archive. Since there is a set of possible reasons for dispositioning a KOI as a false positive, there are future plans to flag false positives with an extra layer of detail that provides specific reasons for the decision made.

Does the Kepler Project reanalyze old KOIs?

Yes. With more quarters of data and sequential improvements in diagnostic tools, some old KOIs will change status when the Kepler Project dispositions them again. (See process step 3 in Figure 1). Some planet candidates will become false positives and some false positives will become planet candidates.

Will the Kepler Project redisposition all the old KOIs within each activity table at the archive as well as disposition the new KOIs?

That is the long-term plan. However, it is not possible to complete all this work before search results from the next run of the pipeline become available. Hence, the Kepler Project plans to disposition all of the new KOIs within each activity table, but a complete redisposition old KOIs is a future activity for longer baselines of data and even better diagnostic tools that are currently under development.

In the Q1-12 data set there are a surprising number of KOIs with orbital periods near one Earth-year. Do Earth-size planets tend to prefer Earth-like periods?

Remember that the Kepler spacecraft orbits the sun every 371 days. Given its extremely stable environment, some noise sources associated with the local detector electronics exhibit repetitive behavior with this periodicity. Since these electronics read-out the charge-coupled devices (CCDs), this noise is intertwined with the astronomical signals in such a way that the two are almost impossible to disentangle. Hence, this repetitive noise can mimic the signature of a transiting planet. Fortunately, we can identify these noise-produced TCEs and distinguish them from true planet candidates in one Earth-year orbits, but it requires a lot of effort. Although most of these bogus TCEs were ignored at the triage or model fitting stage, a small fraction of them have crept into the KOI population and still need to be identified and declared false positives. So, don’t get too excited by the pile-up of KOIs with one Earth-year periods, or the smaller, associated pile-up at 180 days. Most of these are probably not real planet candidates; then again, there may be some real gems there. This explanation is documented more formally in the Q1-12 TCE Release Notes at the archive.

Why weren’t false-positive one Earth-year KOIs seen in earlier data releases?

The Kepler spacecraft is in a 371-day orbit (i.e., just over one Earth-year). Three transits are required to define a TCE (and therefore a KOI). Hence, the Kepler Project has just begun to see these bogus events now because the Project is searching three years (i.e., 12 quarters) of data for the first time.

Are there other reasons for an increased fraction of false positives over time?

Yes. In the past the Kepler Project tossed out the eclipsing binaries (EBs) as soon as they were identified, so many of them have never been made into KOIs. This means that every time the Project search a dataset for transits, they were finding and re-evaluating the EBs again. The decision was made from the Q1-Q12 activity onwards to pass EBs through triage, fit models to them, and turn them into KOIs. Now they are documented as false positives, providing a lasting record of past decisions that help to minimize the amount of work going forward.

Are KOIs held back from the archive for any reason?

Yes, users of the archive will notice from time to time gaps in the sequential KOI numbers (i.e., some KOIs are missing). The missing KOIs are troublesome cases that require manual processing. For example, some KOIs are found but their properties calculated by the Kepler pipeline can be incorrect. For these KOIs, the Kepler Project need to recompute their properties before they can be delivered. By staging the deliveries as described, the best information is delivered to the community in a timely fashion rather than waiting for a complete analysis of all KOIs.

Why are some columns in the TCE or KOI tables blank?

The propagation of information from the Kepler Project to the Exoplanet Archive remains a relatively new process. There remain wrinkles to iron out regarding some of the data. Empty fields will not remain so for long.

What are the rules for displaying the precision of values and their errors in the TCE and KOI tables?

The number of significant digits for a property delivered to the archive by the Kepler Project is identical to the number of significant digits in the error. Large calculated uncertainties from, for example, a bad model fit, can cause less-than desired precision being reported for some archived values. Some values have too many significant figures because either the error bar was reported as zero, or the formal fit gave a very small error. The default behavior of the KOI table is to display +/-0 for values delivered as 'NULL'. This behavior is under consideration.


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NASA Official: Jessie Dotson
Last Updated: Jan 11, 2013
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