Changes between Version 3 and Version 4 of PS1_MD04_RefStack

Timestamp:

Jun 20, 2010, 11:58:17 AM (16 years ago)

Author:

eugene

Comment:

--

PS1_MD04_RefStack

@@ -85 +85 @@
 }}}
 
+==== Photometric Calibration ====
+
+The relative photometry analysis does not tie the photometry to any specific system.  The overall zero point is a free parameter which we can independently set.  I measured the zero point for this field relative to SDSS for all bands, including y-band as discussed below.  To do this, I created a copy of the reference database and then ingested SDSS tsObj files for the regions into the database.  I only included a fraction of the SDSS fields, but enough to cover most of the area.  The commands to perform the ingest are:
+
+{{{
+foreach f (md04.sdss/tsObj-*.fit)
+  addstar -D CAMERA sdssmosaic -D CATDIR $catdir -update $f
+end
+}}}
+
+Next, I extract the sdss measurements and the average magnitudes in the equivalent average photcode to determine the zero point offsets between the initial calibration and the SDSS magnitude system.  Here is the mextract command (cycle photcode value: g,r,u,z,y = 1056,7,8,9,60).
+{{{
+mextract ra, dec, mag, mag:ave where (photcode == 1056)
+}}}
+
+I then supplied this offset to the database by editing the dvo photcode table, extracted with {{{photcode-table -export (file) -D CATDIR (catdir)}}} and imported with {{{photcode-table -import (file) -D CATDIR (catdir)}}}.  Changing the photcode table immediately changes the interpretation of the measurement magnitudes, since the calibrated values are calculated from the instrumental magnitudes on the fly.  For the average magnitudes, however, it is necessary to recalculate the averages.  There is not currently a tool to update the average magnitudes and the photcode table in a single consistent way (though this is a reasonable tool to create).  It should have been possible to supply this offset to the high-quality photometry database and update the averages based on only that subset of the data.  However, to be sure everything was consistent, I simply updated the full-scale databases, and then recreated the high-quality ones from the original databases.
+