SAS Astrometry & Photometry Test (2010.03.10)

~ Eugene Magnier

I've done some work to test out the relative photometry and astrometry calibration system using the SAS data. I show here some example plots from that analysis, and the script used to run relastro and relphot the database. Hopefully this will provide more light than smoke...

The first figure shows the SAS field: black boxes are the outer bounds of the GPC1 focal plane, red is just the sky grid, blue is the region used to generate the rest of the plots.

I ran the attached script (run.relastro) which has a section for 'relastro' and a second one for 'relphot'. These analysis were used for the entire SAS DVO database. The dvo shell script (input) was used to generate the plots, for which I restricted the analysis to the blue box (partly to avoid edge effects and partly just for speed and memory footprint).

Some notes about the analysis: I did not tune any of the clipping parameters. The relative astrometry analysis only iterates over the two steps of [update the object positions, update the chip-level portion of the astrometry model]. I suspect the parameters for choosing the set of calibration stars (S/N limit, instrumental magnitudes) is poorly chosen because a number of these fields has surprisingly few reference stars (according to the log). Also, I suspect the choices do not do a good job of tossing out the saturated measurements (which are not all labeled with the SATURATED bit). There is also something very sub-optimal about the database level interactions for joining the detections to their images: relastro ran very slowly, and much of the time was spent in making associations which should be must faster (the indexes are still being ignored).

Aside from those issues, this analysis looks very encouraging. The plots above show the magnitude residuals for the bright, unsaturated portion of the data (15.5.-17.0 mags for griz, 12.5-15.0 for y-band). You will notice that the photometric residuals are all in the range 1.1 - 1.3%, which look really good. Note: no chip-to-chip color terms have been applied, though each chip has been allowed to float. This last point is interesting: in general, if the weather is photometric and we have solved for the flat-field correction, we should fix the chips since there is not enough overlap to constrain them very well. Only if we think the weather is non-photometric for some images should we let the chips float (or fit a higher-order model).

Note that the weather was good, except on part of one of the nights. We recover this last feature very well in the last plot (stormy.weather.png), where I show zero point offsets vs time.

As for astrometry, the residuals are OK (20-27 mas), but I suspect we can do better by tweaking the parameters. The nice result here is that we can get quite good measurements with almost no effort in tuning the analysis.

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