In the usual form of astronomical self-calibration—like what we did to get the flats in SDSS or what I wrote up with Holmes and Rix—you use repeated observations of the same stars at different focal-plane positions to determine the sensitivity as a function of focal-plane position. Today at computer-vision-meets-astronomy group meeting, Fadely showed that he has the potential to calibrate the sensitivity of a device using only the statistical properties of observed image patches. He doesn't need to even identify sources, let alone determine when the same source has been re-observed. It is still early days, so we don't know the final precision or how it depends on the number of images, number of sources in those images, PSF sampling, and so on, but it looks very promising. In particular, if we pair it with the more traditional self-calibration it could be extremely precise. Our goal: Obviate the taking of (always questionable) flat-field images. This is also something I enjoyed discussing two weeks ago with Suntzeff (TAMU) when I was out in Kansas. (He thought I was talking crazy.)
David, not crazy! but more like, what the huh?? You have much more control over a flat field than constructing it from the data - sort of. Sky flats (which some people use) are really flats often made using night time emission lines. These lines will fringe in many CCDs and the resulting flat may be quite incorrect for broad-band phot. I would also worry about the slow seasonal change of the night sky SED, and the color differences between summer and winter background light due to the effects of the Milky Way. As a computational problem, it is totally cool. But a normal flat gives a much more stable calibration throughout the year.
ReplyDeleteWhat is more important to me is how to engineer the flat field to make the best calibration. Why do we have white circles on a black screen (to reduce scattered light?). What is the appropriate SED for the FF? Is there an "inverse FF" that just isolates scattered light? That would be a big help. Chris Stubbs, like Jim Gunn before, has begun to rethink flat fielding. Maybe we need a workshop on flat fielding. (I can imagine it will be *very* small). cheers nick
Agreed on all points! One of the super-cool things about a flat made from science data is that the flat information from the stars would be illuminating the detector *exactly* the way stars illuminate the detector. So in principle the flat could be perfect.
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