We have defined a special set of imaging masks intended to define those areas in the imaging survey which one may wish to exclude from scientific analysis for a number of reasons. These masks are not themselves a complete description of the window function, which can be found in the sdssPolygons table generated in the resolve process. These also differ from thephoto imaging mask in thefpM files. Finally, they also differ from large scale structure masks used in the spectroscopic large scale structure samples. There are 5 different mask types.

Type
Meaning
0
BLEEDING
1
BRIGHT_STAR
2
TRAIL
3
HOLE
4
SEEING

These masks thus allow one to identify and exclude bright stars and their associated bleeding columns, trails caused by meteors and satellites, a small number of survey “holes” which have unacceptable data, as well as regions with bad seeing. These masks may of course be used in any science analysis to check the effects of excluding/including potentially problematic survey areas, but are likely to be most relevant and useful for studies of large-scale structure. Note that the masks are in the form of convex polygons, and they are generated separately for each of the 5 filters of each survey field. The masks are also solely contained within the PRIMARY survey area. Finally, for those photometric objects inside a BLEEDING, BRIGHT_STAR, HOLE or TRAIL mask, or an area of bad seeing (> 1.7 arcsec), they haveinsideMask set to a nonzero value.

## Accessing the special imaging masks

Within CAS, to identify individual SDSS photometric objects inside a mask, one can check the insideMask value in photoObjAll and its views. For objects inside a BLEEDING, BRIGHT_STAR, HOLE or TRAIL mask, or an area of bad seeing (> 1.7 arcsec), they haveinsideMask set to a nonzero value. The maskedObject table provides a one-to-many link from photometric objects to masks. The masks themselves are described in CAS by the mask table. This table contains a central location and bounding circle, as well as a long string describing the vertices of the convex polygon describing the mask.

There are flat file versions of the mask described in the data model. These have the same information as in the mask table, broken out into one file per photometric field.

  seeing = 0.663 * 0.400 * sqrt(neff)  arcsec