Object list returned after running astrometry

[Ben Cartwright]

Hi,

Does the object list return every single object detected in the image or is it only a subset of what's been seen/processed (ie perhaps in magnitude order)

I'm writing a tool to detect the lowest magnitude object in an image for photometry evaluation purposes on amateur astrophotography setups.  Basically I'm wondering to what level I need to work back through object catalogues to find the lowest magnitude object or if some of this information is guaranteed to be returned.

[Dustin Lang]

Hi,

The axy.fits file contains all sources detected in the image; the corr.fits file contains only "correspondences" -- likely matches between stars in the image and reference stars; this will be a subset of all the stars detected in the image.

By lowest magnitude, do you mean faintest, or brightest?  (Gotta love the magnitude system?)  Assuming you mean the faintest: our star detector is designed for robustness and simplicity rather than accurate photometry.  For example, an optimal star detector would apply a matched filter -- a model of the PSF -- to detect stars.  Instead, we use a fixed Gaussian (width sigma = 1 pixel), so this is a "mismatched filter" -- it's not as sensitive as possible.  Also, by default we use a rather high detection threshold of 8 sigma, vs a more typical 5 or 6.  Finally, the fluxes we report are not optimized -- we're just reading off the nearest-pixel approximate flux *if* our PSF model were correct (we're not trying very hard; it isn't).  Very much caveat emptor, in other words!

cheer,

--dustin

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