SExtractor mag_auto photometry of SDSS fields [Huan Lin, 04 Nov 2012]

SExtractor mag_auto photometry, from the firstcut standalone pipeline reductions, was compared to SDSS Stripe 82 coadd modelmag photometry for stars in SDSS fields, in order to check the zeropoints relative to the expected values given in Sig-R2. Values below are based on the median magnitude offsets for all stars overlapping with the SDSS coadd (and having SDSS modelmag < 20) across the full DECam focal plane.

Zpt (e-)/Expected
Filter Stripe 82 dither 20120924 SDSSJ0000+0000 20121101 SDSSJ0200+0000 20121101 SDSS J0320+0000 20121101
u 0.75 (airmass 1.5-1.6) 0.82 0.86
g 0.93 0.87 0.88 0.89
r 1.13 1.06 1.06 1.07
i 1.11 1.05 1.04 1.05
z 1.01 0.97 0.98 0.97

The values above show about a 5% drop in throughput for griz between 20120924 and 20121101, which I attribute to differences in transparency/extinction between the 2 nights. In u band the 20120924 observations were at higher airmass and should be affected by greater extinction (for the 20121101 data the airmass was about 1.2 in all filters, and for 20120924 it was about 1.2-1.3 for gri and 1.5-1.9 for z).

Aperture photometry of C26202 from 20121102 [Paul Martini, 11/3/2012]

Two datasets on C26202 were obtained on 20121102, the first at airmass 1.01 (g->Y in files 145966->145970) and the second at 1.11 (g->Y in files 146108->146112). The zeropoints are consistent to better than 0.05 mag, with the exception of r-band. The average of the two zeropoints correspond to:

Filter Zpt (mag) Zpt (e-) Zpt (e-)/Expected
g 25.296 591 1.00
r 25.374 635 1.15
i 25.379 638 1.11
z 25.064 477 1.11
Y 24.154 206 1.33

Zpt (mag) is the AB magnitude that corresponds to 1 DN/s. These assume the magnitudes for C26202 calculated by Douglas Tucker and listed in Sig-R2. Zpt (e-) is the number of e-/s that corresponds to AB=20 mag as assumes the gain is 4.5e-/DN. Zpt (e-)/Expected is the ratio of Zpt (e-) to the expected zeropoints calculated by Huan Lin for Sig-R2.

Other notes about the analysis:

1. No airmass correction was applied, although this should be minimal.
2. In all cases C26202 fell on Amp A of chip N4.
3. The astrometric solution in the headers are off by 3.8' and 3'.
4. While the last column indicates the performance exceed expectations, the specification is for airmass 1.3, while these data were obtained at about airmass 1.

Photometric signatures of glowing edge [Andres Plazas, 10/31/2012]

The first page of each file shows the mean astrometric residuals as a function of position in the CCD. The vector field shown is an average over all exposures and all CCDs of the focal plane.

For those with access to 'folio' at Penn, the data for producing the last PDF file are in /data4/DECAM/astrometry/test.wcscat. This is a match of 12 DECam exposures from the first week of observations. The matched catalog (with residuals) is the last extension. The others give information on the instruments, exposures, fields, etc.

In the same way, the data for the first PDF can be found in /data4/DECAM/astrometry/S82/s82g.wcscat. This is a catalog from a match of about 75 exposures with big dithers.

Finally, the data for the second PDF are in /data4/DECAM/astrometry/S82/s82g.r2.5.wcscat.