Project

General

Profile

Year 1 star flats » History » Version 2

Gary Bernstein, 09/06/2013 06:12 PM

1 1 Gary Bernstein
h1. Year 1 star flats
2 1 Gary Bernstein
3 1 Gary Bernstein
_(6 Sep 2013 Gary Bernstein)_
4 1 Gary Bernstein
5 1 Gary Bernstein
Star flat observations (and dome flats) were taken during the engineering nights 15 Aug and 29 Aug 2013, for purposes of generating a star flat / illumination frame to be installed in the DESDM pipeline for the start of Year 1 processing.  This page describes the comparison of the dome and stellar response of these two dates to each other and to the SV-season response.
6 1 Gary Bernstein
7 1 Gary Bernstein
Terminology to keep clear:
8 1 Gary Bernstein
* _Dome flat_ is simply the (relative) counts across the array in the dome flats for a given filter.
9 1 Gary Bernstein
* _Star flat_ is the correction (in mags) that must be applied, as a function of array position, to instrumental magnitudes from dome-flattened images, in order to make the mag of a star be independent of where it is imaged on the array.  It is the ratio of response to focussed starlight vs diffuse dome light, and is basically a map of stray (unfocussed) light falling on the array.
10 1 Gary Bernstein
* _Sky response_ is the product of the dome and star flats, which gives the response of a given pixel to focussed light from the sky.  It is ultimately the thing we care about.
11 1 Gary Bernstein
12 1 Gary Bernstein
*Conclusion:* The Aug 15 dome and sky response patterns are both different at >2% level (p-p) from the SV season flats.  But this change appears largely reversed on the Aug 29 flats, suggesting that something was different on 15 August and we should not use those flats.  The _dome_ response on Aug 29 shows up to +-1% changes from January SV data, however the _sky = dome*star flat_ response is very similar to the Jan/Feb data, with only ~2 mmag RMS difference.  This suggests that the dome illumination is a little different than last season, but the response to the sky is (as expected) pretty close.  
13 1 Gary Bernstein
14 1 Gary Bernstein
My recommendation would therefore be that we adopt the Aug 29 star flats for the initial processing of Year 1 data.  The seasonal change in dome illumination is not a problem.  We should try to figure out why the dome and sky response would have differed on 15 Aug; the PF cage was flipped to f/8 and back both before and after this run, perhaps that is an issue.
15 1 Gary Bernstein
16 1 Gary Bernstein
*Details:*  I retrieved the combined dome flats from these DESDM runs per instructions from Margaret Gelman:
17 1 Gary Bernstein
18 1 Gary Bernstein
* 15 Aug: 20130819150900_20130815
19 1 Gary Bernstein
* 29 Aug: 20130903172904_20130829
20 1 Gary Bernstein
21 1 Gary Bernstein
The star flat observations' exposure numbers, and the DESDM runs in which they were reduced, are:
22 1 Gary Bernstein
23 1 Gary Bernstein
* 15 Aug: Exposures 226510-226621, DESDM runs 20130820071917_20130815, 20130823084219_20130815, 20130821081604_20130815, 20130823111122_20130815, 20130823084101_20130815
24 1 Gary Bernstein
* 29 Aug:   228544-228685, DESDM run 20130903224504_20130829.  This includes u-band data which I will not address here.
25 1 Gary Bernstein
26 1 Gary Bernstein
First we can examine the dome flats.  These three plots show the ratio (expressed as mags) of the dome flats for 15 Aug and 29 Aug to each other and to superflats made from late-Jan 2013 data (or Nov 2013, which is very similar)
27 1 Gary Bernstein
28 2 Gary Bernstein
!AugMinusNovDomes.png! !AugMinusSepDome.png! !SepMinusJanDomes.png!
29 2 Gary Bernstein
30 2 Gary Bernstein
You can see a substantial change (2% shift along the left and top edges of the flats) from last season to the 15 Aug flats.  Then it changes back for 29 Aug.  The last plot is the ratio of 29 Aug to last season, and milder changes are apparent.  These milder changes are filter-dependent whereas the deviations on 15 Aug show a common pattern in all filter.  The variation of individual CCDs seen in Y band was observed during SV to occur whenever the focal-plane temperatures change, and so this type of change is expected to have occurred between seasons.
31 2 Gary Bernstein
32 2 Gary Bernstein
Next we examine the sky response = product of dome and star flats.  Here again are intercomparisons between 15 Aug, 29 Aug, and Feb 2013 star flat run results.