Understanding the image processing flags - Summary table

For objects in the calibrated object lists (tsObj files and the tables with photometric data in the Catalog Archive Server database), the photometric pipeline sets a number of flags that indicate the status of each object, warn of possible problems with the image itself, and warn of possible problems in the measurement of various quantities associated with the object. They are briefly described here, with links to a companion page with details about the flags. For yet more details, refer to Robert Lupton's flags document.

Possible problems associated with individual pixels in the reduced images ("corrected frames") are traced in the image masks.

• The status flags in the tsObj table, with information needed to discount duplicate detections of the same object in the catalog.
• The object flags, with information about the success of measuring the object's location, flux, or morphology.

The "status" of an object

The catalogs contain multiple detections of objects from overlapping CCD frames. For most applications, remove duplicate detections of the same objects by considering only those which have the "primary" flag set in the status entry of the tsObj file.

A description of status is provided on the details page. The details of determining primary status and of the remaining flags stored in status are found on the algorithms page describing the resolution of overlaps (resolve).

Object "flags"

The photometric pipeline's flags describe how certain measurements were performed for each object, and which measurements are considered unreliable or have failed altogether. You must interpret the flags correctly to obtain meaningful results.

• flags and flags2 as arrays of five flags, one each for u, g, r, i, z.
• objc_flags and objc_flags2 are scalars. These are a combination of the per-filter flags appropriate for the whole object.

Recommendations

Clean sample of point sources

In a given band, first select objects with PRIMARY status and apply the SDSS star-galaxy separation. Then, define the following meta-flags:

 DEBLEND_PROBLEMS = PEAKCENTER || NOTCHECKED || (DEBLEND_NOPEAK && psfErr>0.2)
 INTERP_PROBLEMS = PSF_FLUX_INTERP || BAD_COUNTS_ERROR ||
(INTERP_CENTER && CR)

 BINNED1 && !BRIGHT && !SATURATED && !EDGE && (!BLENDED || NODEBLEND) &&
 !NOPROFILE && !INTERP_PROBLEMS && !DEBLEND_PROBLEMS 

If you are very picky, you probably will want not to include the NODEBLEND objects. Note that selecting PRIMARY objects implies !BRIGHT && (!BLENDED || NODEBLEND || nchild == 0)

These are used in the SDSS quasar target selection code which is quite sensitive to outliers in the stellar locus. If you want to select very rare outliers in color space, especially single-band detections, add cuts to MAYBE_CR and MAYBE_EGHOST to the above list.

Clean sample of galaxies

As for point sources, but don't cut on EDGE (large galaxies often run into the edge). Also, you may not need to worry about the INTERP problems. The BRIGHTEST_GALAXY_CHILD may be useful if you are looking at bright galaxies; it needs further testing.

If you want to select (or reject against) moving objects (asteroids), cut on the DEBLENDED_AS_MOVING flag, and then cut on the motion itself. See the the SDSS Moving Objects Catalog for more details. An interesting experiment is to remove the restriction on the DEBLENDED_AS_MOVING flag to find objects with very small proper motion (i.e., those beyond Saturn).

Descriptions of all flags

Flags that affect the object's status

These flags must be considered to reject duplicate catalog entries of the same object. By using only objects with PRIMARY status (see above), you automatically account for the most common cases: those objects which are BRIGHT, or which have been deblended (decomposed) into one or more child objects which are listed individually.

In the tables, Flag names link to detailed descriptions. The objc column indicates that this flag will be set in objc_flags or objc_flags2 if this flag is set in any of the filters. "bit" is the number of the bit; to check whether bit n in flags1 is set, compute flags1 & 2**n where & is the bitwise AND operator.

Flags that indicate problems with the raw data

Flags that indicate problems with the image

Problems associated with specific quantities

All flags so far indicate some problem or failure of a measurement. The following flags provide information about the processing, but do not indicate a severe problem or failure.

Informational flags related to deblending

Further informational flags