Changes between Initial Version and Version 1 of PS1_SAS_AstromPhotom_20100310

Timestamp:

Mar 11, 2010, 12:31:06 PM (16 years ago)

Author:

eugene

Comment:

--

PS1_SAS_AstromPhotom_20100310

@@ +1 @@
+== 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'm sending 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 show the RA and 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.