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Kepler Science Center
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?

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 properties 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 observed 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 boresight 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 optimal 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

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Editor: Thomas Barclay
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Last Updated: May 29, 2014
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