summaryQA

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. 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.

• An EXCELLENT run is one in which all four median principal colors for all six camera columns are less than 0.01 magnitudes.
• A SUPERB run has all four median principal colors for all six camera columns less than 0.02 magnitudes.
• A GOOD run has all four median principal colors for all six camera columns less than 0.03 magnitudes.
• An ACCEPTABLE run has all four median principal colors for at least one camera column less than 0.03 magnitudes.
• A BAD run is one which satisfies none of these criteria.

The following table shows the distribution of field quality values in DR4.

To within the accuracy given here, the fractional distribution of field quality is 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 table XXX WHICH TABLE???XXXalso 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.

Note that it is a bug that only the s color is used in the above; this will be fixed in a later release.