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Quality of DR3 coverage (including 'holes')
The following is a reference note on the quality of fields, including hole fields in the DR3 data set.
The DR3 CAS has a field quality table, called RunQA, which provides information relevant to the average data quality for the objects in each field of DR3. In the CAS, a SQL query such as
select top 10 * from RunQAwill show sample entries for this table. This table may be joined to rest of your query to sub-select only fields of the very highest quality, for instance, with seeing below a certain threshold, if that is desired.
If you want just one number which describes the overall quality of a field, we recommend you look at the FieldQall number (0=bad, 1=acceptable, 2=good, 3=excellent). For example, the following query finds fields of GOOD or EXCELLENT quality:
This overall quality determination, FieldQAll, is based on the (dereddened) principal colors of all the stars in the field vs. the Galactic value of this quantity, the PSF quality, the difference between Aperture and PSF magnitudes for the same stars in the field, and the seeing.
Principal colors of stars are linear combinations of the SDSS colors u-g, g-r, r-i, i-z which align perpendicular to the stellar locus of stars with F thru K spectral types. The position of the locus in color space of the Milky Way's stars (suitablely dereddened), does not appear to change by more than 1% or so toward most high latitude lines of sight, and so, assuming this locus to be fixed, the value of the principal colors is then an excellent measure of the absolute photometric calibration of the data.
A shift of more than about 1% from the cosmic (Milky Way) values of the colors perpendicular to the stellar locus indicates a possible mis-calibration of the data.
The width of the stellar locus in principal value colors is also a good indication of the quality of the data, in particular of the relative photometry of stars.
The individual principal colors (averaged over the field, along with rms widths of the distribution), are available in this RunQA table, along with Aperture-PSF magnitudes and widths.