Magnitudes in the Image Coadd Using New PSF Code
April 12, 2005
RHL developed new code to calculate the PSF based on
the first run of the code on the coadd, and then using the
fpObjc information to calculate a new PSF.
The residuals are much, much better. One can compare
the new residuals of the image level coadd minus the catalog level
coadd residuals to those from the
single run minus the catalog level coadd residuals for
The PSF residials have an rms of 0.004, much better then the old version
of photo, and better even then the single run, which had 0.003 mags,
and the flatness of bright-dim is better.
We should run much bigger data sets.
Magnitudes in the Image Coadd
March 04, 2005
Subject: mags from image coadd and catalog coadd
From: James Annis
Submitted: Fri, 04 Mar 2005 09:03:07 -0600 (CST)
We have run photo and target on one column of the
coadd and compared these against catalog level coadds.
The residuals show structure at the 2% level for psf
magnitudes, lower than that for model magnitudes,
and substantially lower for both psf and model colors.
The behavior of the psf magnitudes is interesting/worrisome,
indicating investigations to be done before one
wants to perform precision stellar/quasar work.
The model magnitudes and colors are stable and lack
interesting behavior, so galaxy work should be able to proceed.
The state of affairs
We have two Southern coadds, a depth optmized coadd and a weak
lensing optimized coadd.
We have run 1 column of the depth optimized coadd through photo,
of which ~90% of the fields successfully ran. This version of
photo bypasses ssc/psp and makes coadded psField files (coadded
PSFs) from single run psField files.
We have calibrated these using standard target technology.
At the end, we now have 680 or so tsObj files for
camcol 3 of run 100001, the north strip of the depth
Check of the data quality
We matched up the tsObj with the catalog level coadds
of Scranton and Johnston, sdss-gneral#3030, demanding
matches in both catalogs within 1" and constructed
The answers are:
We didn't work cmodel magnitudes as the catalog level
coadd didn't have psf_frac.
- psf mags are squirelly
- petrosian mags are noisy
- model mags are stable
The plots take an explanation.
shows for the first 300 fields psf<2>_im - psf<2>_cat.
The green dots are the medians, the extrema of the bars contain
95% of the residuals, and the white space inside the bars contains
63% of the residuals. These are for magnitudes 16-20. The
red line shows the medians of the 16-19 range minus the medians
of the 19-22 range, looking for magnitude dependent systematics.
The plots of the other filters, say
show that the trend in 6) goes away in the blue and gets worse
in the red. This suggests a sky subtraction issue.
- an offset of -0.057 mags, probably calibration
- rms residuals of 0.019, pretty good... 2% photometry
- bright-dim of 0.01, consistet with 0, good
- how squirelly psf magnitudes are
- bigger spreads to neg, i.e. image coadds tend to produce
- a worrisome trend towards the neg after field 200,
which shows up in the bright-dim as a pos trend
both of which are consistent with image coadd
objects having more light, increasingly so at
- even more worrisome, near field 100, the 1-sigma scatter
to pos is much greater than the 1-sigma scatter to neg
The other filters are similar.
- an offset of -0.025 mags, why is this different from psf?
- bright-dim of 0.08
- how noisy petrosian magnitudes are
- the -very- large scatter towards the neg, so
image coadd objects tend to be brighter
The last comment holds for regions where the psf mags are doing
something funny. The other filters are slightly less well behaved,
with z showing after field 250 an offset of the medians from the
center of the white space. Still, model magnitudes look
very good in this comparison.
- an offset of -0.022 mags, consistent with Petrosian
- bright-dim of 0.02+- 0.01, very nice
- how stable model magnitudes are
- also large scatter to the neg, im coadds tend to the brighter
- very little structure with commenting on.
These plots are built in the same way as the magnitude plots,
except we are looking at colors and over a slightly smaller
magnitude range, 16-19.
This is nice behavior, except for 0.05 mag level of wandering
of the residuals about 0. The other colors are similarly
- an offset of 0.004 mags+-0.020, consistent with 0
- bright-dim of 0
- well behaved quartile distributions
This is very nice behavior, and the other colors are similar.
u-g is noisy, and i-z is perhaps the worst behaved.
- an offset of -0.002 mags+-0.006, very nice
- bright-dim of 0.014+- 0.014, very nice
- a noisy quartile distribution
To the extent that we understand how the catalog level
coadds were made, these plots show that the modified verion
of photo we used to run on the coadds works reasonably well.
Galaxy work that uses cmodel magnitudes and model colors
should be in good shape. There is little indication of
systematics with magnitudes or colors. I do not understand
the behavior of the psf magnitudes: they are showing with
precision -something- and workers who want to use them in
detail will want to make plots of residuals versus sky brightness,
psf size, and number of image in the coadd.
For the basic characterization of the galaxy content,
the data look to be in good shape. We could always
use investigations of the star galaxy separation and the
There are a large number of these quartile residuals plots at
There are averages of the residuals over 100 fields as a function
of column in
this directory. In these plots, the plot for the
g band model-mags for cols 400-500 is typical,
g psf-mags for cols 30-100 is perhaps the worst. The means
for fields 30-100 are always bad.
There are a pair of quartile residuals plots of run 3384 versus the
catalog level coadd in
psf magnitudes and the
model magnitudes show much reduced scatter in the median residuals.
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