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Image quality flags

The image quality of each SDSS field is tracked through a number of flags and other quantities. These values are stored in the photoField files on SAS as well as in the field table in the CAS database. When using SDSS images or catalogs, these values can be useful in determining whether to use data from a particular field. This page describes these flags, some of which are bitmasks and others of which are not. The most important quantity for each field is score, a combination of the image quality parameters, described below.

As noted below, some of these flags are relevant for both the images and the catalogs, whereas some are flagging problems with the catalogs alone (that is, the corrected frames may still be of good quality).

Note that there are some small nomenclature differences between SAS and CAS for these values; in particular, any underscores in the names below are omitted in the CAS naming convention.

As noted above, for using the catalogs, the score (and thus all these other flags are of importance and must be checked. However, if you are using corrected frames, not all of the above are relevant. In particular, the photo_status quantity can be ignored; typically that indicates a very bright object is in the field, but not that the data itself is intrinsically bad (though parts of it might be highly saturated).