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?

Can you fix charge-transfer inefficiency without a theory-driven model?

The Gaia mission needs to centroid stars with accuracies at the 10^-3-pixel level. At the same time, the detector will be affected by charge-transfer inefficiency degradation as the instrument is battered by cosmic radiation; this causes significant magnitude-dependent centroid shifts. The team has been showing that with reasonable models of charge-transfer inefficiency, they can reach their scientific goals. One question I am interested in—a boring but very important question—is whether it is possible to figure out and fix the CTI issues without a good model up-front. (I am anticipating that the model won't be accurate, although the team is analyzing lab CCDs subject to sensible, realistic damage.) The shape and magnitude of the effects on the point-spread function and positional offsets will be a function of stellar magnitude (brightness) and position on the chip. They might also have something to do with what stars have crossed the chip in advance of the current star. The idea is to build a non-trivial fake data stream and then analyze it without knowing what was put in: Can you recover and model all the effects at sufficient precision after learning the time-evolving non-trivial model on the science data themselves? The answer—which I expect to be yes—has implications for Gaia and every precision experiment to follow.

In order to work on such subjects I built a one-dimensional (yes the sky is a circle, not a 2-sphere) Gaia simulator. It currently doesn't do what is needed, so fork it and start coding! Or build your own. Or get serious and make a full mission simulator. But my point is not Will Gaia work? it is Can we make Gaia analysis less dependent on mechanistic CCD models? In the process we might make it more precise overall. Enhanced goal: Analyze all of Gaia's mission choices with the model.

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.