Kepler as an insanely expensive thermometer!

The Kepler spacecraft is taking incredibly precise photometric data on tens of thousands of stars for the purpose of detecting exoplanets. For many reasons, the lightcurves it returns are sensitive to the temperature of the spacecraft: The focus and astrometric map (camera calibration) of the camera changes with temperature, and the detector noise properties might be evolving too. This wouldn't be a problem (it's a space mission) but the spacecraft changes its sun angle abruptly to perform high-gain data downlink about once per month, and the temperature recovery profile depends on the orientation of the spacecraft post-downlink. Instead, there are sub-percent-level traces of the temperature history imprinted on every lightcurve. Each lightcurve responds to temperature differently, but each is sensitive.

Of course the spacecraft keeps housekeeping data with temperature information, but it hasn't been extremely useful for calibration purposes. Why not? The onboard temperature sensors are low in signal-to-noise or dynamic range, whereas the lightcurves are good (sometimes) at the part-in-hundred-thousand level. That is, there is far more temperature information in the lightcurves than in the direct temperature data! Here's the project:

Treat the housekeeping data about temperature as providing noisy labels on the lightcurve data. Find the properties of each lightcurve that best predicts those labels. Combine information from many lightcurves to produce an extremely high signal-to-noise and precise temperature history for the spacecraft. Bonus points for constraining not just the temperature history but a thermal model too.

limits to ground-based photometry?

A conversation with Nick Suntzeff (TAMU) in Lawrence, KS, brought up the great idea (Nick's, not mine) to figure out why ground-based photometry of stars never gets better than a few milli-mags in precision. Seriously people, Kepler is at the part-per-million or better level. Why can't we do the same from the ground? Why not at least part-per-hundred-thousand? Is it something about the scintillation, the transparency, the point-spread function, the detector temperature, scattered light, sky emission, sky lines, what? Not sure how to proceed, but the project could make the next generation of projects orders of magnitude less expensive. I guess I would start by taking images of a star field with many different (very different) exposure times and at different twilight levels (Suntzeff's idea again). Could it be that all we need is better software?

Find more of the GD-1 stream

The GD-1 stream spans many tens of degrees in the SDSS data. The stellar density in the stream is inhomogeneous, but the stream appears to be terminated by the survey boundary, and not before. So we should be able to find much more of it! And more stream means better constraints on the mass model for the Milky Way and the formation of cold streams. A few years back we made a model of GD-1, so we can predict where the stream will be on the unobserved parts of the sky and at what heliocentric distance. These properties of the stream set the parameters for a simple (say) three-color ground-based imaging survey to recover the stream in the Southern Hemisphere. Before you go get the observing time, I would recommend looking in the various data archives; there might already be sufficient data out there to map parts of the stream right now.

get SDSS colors and magnitudes for very bright stars

The SDSS saturates around 14th magnitude. However, (a) the gains are set such that the CCD pixels saturate before the analog-to-digial read-out saturates, and (b) the bleeding of charge on the CCD is essentially charge-conserving. Also, when very bright stars cross the readout register in the CCD, they leave a thin 2048-pixel line across the full camera column. And also also, the stars have well-defined diffraction spikes that are visible to large angular radii.

No-one says this is easy; this is a blog of good ideas not easy ideas: For one, the detector may become weakly nonlinear shortly before CCD pixel saturation; that is, the effective gain may be lower at brighter magnitudes; any project would have to look carefully into this, and you don't have variable exposure times to use (all of SDSS was taken at 55-second exposure time for very important reasons). For another, the shape and size of the diffraction spikes might be a strong function of position in the focal plane. However, I have hope, because the charge bleeds are so very very beautiful when inspected in detail.

Some prior work on this has been done by myself and Doug Finkbeiner (Harvard). It would be worth checking in with Fink before embarking.