Changes between Version 1 and Version 2 of IPP_Progress_Report_20091026

Timestamp:

Nov 2, 2009, 8:18:27 AM (17 years ago)

Author:

eugene

Comment:

--

IPP_Progress_Report_20091026

@@ -5 +5 @@
 
 === Eugene Magnier ===
+
+I've been digging into the photometry on difference images.  There were some minor bugs related to the psphot implementation used by ppSub.  However, more importantly, there is an inconsistency in the case of convolved images between the errors (and the resulting chi-square values) for the fits to source flux models and the per-pixel errors reported for the image.  This is yet another effect of working on images with correlated errors.  We believe we understand the per-pixel errors as illustrated by the histogram of signal-to-noise values in the background of the images.  We believe we understand the impact of the correlated noise on the photometry measured in apertures, based on the experiments performed by Paul when we first started tracking the covariance matrix for the convolutions.  This effect shows that there is a difference in the scaling between the covariance as used by aperture photometry and by PSF (or other model) fitting.  In order to test this effect and the photometry analysis within ppSub, I have found it necessary to create some additional IPP programs.
+
+{{{psphotForced}}} -- this does forced photometry on a collection of
+positions, which may either supplied as a cmf file or as a text file
+(currently only X,Y chip coordinates are allowed, but it would be easy
+to make the load apply astrometry to R,D positions).  This program is
+a small modification relative to psphot, and makes some assumptions:
+the psf model must be supplied (we can possibly make this optional in
+the future), a text list of X,Y corresponds to positions in just the
+single processed chip (it is also allowed to send a list of lists, in
+which case there must be one for each of the chips being processed as
+a group).  This program performs the background modelling, loads the
+sources and psf model, and measure photometry of the sources; it does
+not attempt to measure moments of the sources (is this desired?).  I
+have tested it a bit against simtest images, and the forced photometry
+of sources using the positions from an earlier normal psphot analysis
+result in magnitudes that agree well, but not perfectly at the <0.01
+mag level.  There are a few issues that I need to double check related
+to how the aperture correction is being performed (if at all) and what
+size aperture is used for each object.  these could certainly explain
+the observed errors.  This program is needed to finish the 'forced photometry' analysis stage, so this effort served multiple goals.
+
+{{{psphotMakePSF}}} -- this program simply generates a psf model from an
+image, either based on a specified collection of sources or based on
+newly detected sources in the image.
+
+{{{ppSmooth}}} -- a stand-along smoothing program.  I am finding that the
+chi-square of photometry of objects in convolved images is not quite
+what I expect, even if I take the covariance into account as I
+understand it (I'm probably getting it wrong).  To make a more
+complete test suite, I decided it would be useful to have a
+stand-alone program that could smooth an image, and do the 'right
+thing' with the covariance determined from an earlier smoothing or
+convolution.  This program does not build with the rest of the suite
+via psbuild; I'll add it in after it is a bit better tested.
+
 
 === Heather Flewelling ===