summaryQA
Over to single-run QA - Up to Photometry QA
Summary QA quantities
The SDSS imaging data are taken as a series of runs
(typically indicated by a four-digit number). Each run is made up of
six parallel scanlines, and each scanline is divided up further
into 10'x13' frames. Global photometric quality assurance is
done both on individual frames, and on each run. These global
quantities are based on the detailed quality assurance tests carried
out in single-run QA.
Summary Run quality assurance
This is summarized in the third table in a
summary of the SDSS run quality assurance results (the DRsupplemental
list is available here). This includes
links to the single-run QA for each of the imaging runs. The
following is a description of the method used to assign an overall
quality to the run.
Each field has four principal colors determined
from the locus of stars in color-color space; these principal colors
are normalized to average to zero over the survey. The median value
of the principal colors in each camera column gives a measure of the
quality of the photometric calibration of that camera column. The
following table shows how the run quality is determined.
Quality |
All four abs(median color) are at most... |
...in at least this many camera columns |
SUPERB |
0.01 |
6 |
EXCELLENT |
0.02 |
6 |
GOOD |
0.03 |
6 |
ACCEPTABLE |
0.03 |
1 |
To give an impression of how common each value is, he following
table shows the distribution of field quality values in DR4.
QUALITY |
# of fields |
% |
# of fields this good or better |
% |
EXCELLENT |
147001 |
58 |
147001 |
58 |
GOOD |
67196 |
26 |
214197 |
84 |
ACCEPTABLE |
32980 |
13 |
247177 |
97 |
BAD |
7990 |
3 |
255167 |
100 |
To within the accuracy given here, the fractional distribution of
field quality was identical in DR3 and DR4 (with about 201,000 and
255,000 fields, respectively).
There are separate criteria on the quality of each field based on
the seeing and the quality of the PSF determination, as explained in
detail in the quality section of
the single-run QA page; the summary
table also indicates the fraction of fields in each run of different
quality by these criteria.
Summary Field quality assurance
At a finer level, the quality of each of the hundreds of thousands
of fields in the SDSS imaging data is assessed, and is summarized in a
very
large ascii file. The following description of the assignment of
quality flag is taken straight from the SDSS
DR3 paper.
This flag is based on five attributes:
- The seeing in the r band
- As a diagnostic of errors in the determination of the Point
Spread Function (PSF) magnitude, the difference between aperture and
PSF magnitude for bright stars on the
frame, as described in the PSF
section of the single-run QA page
- Systematic offsets of the stellar locus in color space, and/or
increased width of the stellar locus, using the principal colors
- Finally, the initial processing of the data itself can indicate
problems: the photometric pipeline can time-out in processing, the
telescope tracking can be poor, etc.
Based on these quantities, we assign a field quality for each field
as follows:
- By default, the field in question is assigned quality
ACCEPTABLE.
- If the absolute value of the median PSF-aperture difference is
greater than 0.05 magnitudes in any of the five bands, or if the
absolute value of the s color median is larger than 0.05
magnitudes, or if the s color distribution width is 2.5 times
larger than its median value for the whole run, or if the r-band
seeing is worse than 3 arcseconds, or if the initial processing
indicates severe problems with the data, the field in question is
downgraded to BAD.
- If the absolute median PSF-aperture difference is smaller than 0.03
magnitudes in all five bands, and if the absolute value
of the s color median is smaller than 0.03 mag, and if the
s color distribution width is smaller than twice
its median value for the whole run, the field in question is
upgraded to GOOD.
- If the median PSF-aperture difference is smaller than 0.02
magnitudes in all five bands, and if the absolute value
of the s color median is smaller than 0.02 mag, and if the
s color distribution width is smaller than 1.5 times
its median value for the whole run, and the $r$-band seeing
is better than 2 arcseconds, the field in question is
upgraded to EXCELLENT.
Last modified: Wed Jun 21 18:43:25 BST 2006
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