Dark Energy Survey » Science Verification

h1. Photometric stability

Here is a comparison of the relative response of the array to stars across the FOV during the three star flat trials: Nov 20 & 21 2012, Dec 12 2012, Feb 22 2013.

h2. Definitions

* All plots are showing the ratio of response in one epoch to response in another.  Units are magnitudes.  All of the plots below have color scale spanning +-3 mmag.  All plots have had the best-fit constants and linear slopes across the field of view removed, as these can be produced by changes in atmospheric extinction.

* The *dome flat* is the response to the dome lamps.  These come from the DESDM supercal runs done for SVA1.  All CCDs are given a _common normalization_ factor, i.e. I undo the individual CCD-to-CCD normalizations done in DESDM using the _SCALMEAN_ values in the superflat headers.

* The *star flat* is adjustment that must be made to stars to get correct magnitudes after applying dome flats to the images.  It is essentially the fraction of the dome illumination that is caused by scattered light instead of focused light.

* The *stellar response* is the product of star flat and dome flat: it measures the response of the array to focused light from celestial objects.

* The plots below use MAG_AUTO to derive the star flat.  Results from using fixed 8" diameter aperture are very insignificantly different.

h2. Summary

* *The stellar response is more stable than the dome flats!*  In other words, using a different dome flat each night or week is actually degrading the photometric stability because the domes are changing for reasons (maybe light in the dome, etc.) that do not actually change the camera's sensitivity to stars.

* The stellar response changes by *<1 mmag RMS in riz and ~1.5 mmag RMS in gY between epochs*.  The biggest changes are about +-3 mmag peak-to-peak.  [There are larger changes on the flaky amplifier of CCD #31].

* The dome flats had a change in mid-Feb of about +-10 mmag. and have ~+-3 mmag changes on other epochs.   _I suspect that close examination would find that there are single nights that have discrepant dome flats at ~1% level due to some change in procedure.  Worth further investigation!_

h2. Dome flat variations

Here are the changes in dome flats between 5 different epochs of superflats produced by DESDM for SVA1.  These plots show the differences between adjacent pairs of these flats (note I have not looked at all the epochs produced by DESDM):

* 20130716165341_20121117t1124

* 20130718144123_20130113t0131

* 20130712170315_20121209t1226

* 20130719192814_20130201t0212

* 20130725170742_20130217t0227

!domediff1.png!

!domediff2.png!

!domediff3.png!

!domediff4.png!