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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.

Objects in the catalog have two major sets of flags:
  • 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.

For each object, there are 59 flags stored as bit fields in two 32-bit table columns in the tsObj file, flags and flags2. There are two sets of these flag variables for each object:
  • 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.
Here we describe which flags should be checked for which measurements, including whether you need to look at the flag in each filter, or at the objc_flag.

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)
Then include only objects that satisfy the following in the band in question:

 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 OR operator.

Flag 1/2 bit objc Description
BINNED1 1 28 detected at >=5 sigma in original imaging frame
BINNED2 1 29 detected in 2x2 binned frame; often outskirts of bright galaxies, scattered light, low surface brightness galaxies
BINNED4 1 30 detected in 4x4 binned frame; few are genuine astrophysical objects
DETECTED 1 32 BINNED1 | BINNED2 | BINNED4
BRIGHT 1 1 X duplicate detection of > 200 sigma objects, discard.
BLENDED 1 3 X Object has more than one peak, there was an attempt to deblend it into several CHILD objects. Discard unless NODEBLEND is set.
NODEBLEND 1 6 X Object is a blend, but was not deblended because it is:
  • too close to an edge (EDGE already set),
  • too large (TOO_LARGE), or
  • a child overlaps an edge (EDGE will be set).
CHILD 1 4 X Object is part of a BLENDED "parent" object. May be BLENDED itself.

Flags that indicate problems with the raw data

These flags are mainly informational and important only for some objects and science applications.
Flag 1/2 bit objc Description
SATURATED 1 18 X contains saturated pixels; affects star-galaxy separation
SATURATED_CENTER 2 11 as SATURATED, affected pixels close to the center
EDGE 1 2 object was too close to edge of frame to be measured; should not affect point sources
LOCAL_EDGE 2 7 like EDGE, but for rare cases when one-half of a CCD failed
DEBLENDED_AT_EDGE 2 13 object is near EDGE, but so large that it was deblended anyway. Otherwise, it might have been missed.
INTERP 1 17 object contains interpolated-over pixels (bad columns, cosmic rays, bleed trails); should not affect photometry for single bad column or cosmic ray
INTERP_CENTER 2 12 interpolated pixel(s) within 3 pix of the center. Photometry may be affected.
PSF_FLUX_INTERP 2 15 more than 20% of PSF flux is interpolated over. May cause outliers in color-color plots, e.g.
BAD_COUNTS_ERROR 2 8 interpolation affected many pixels; PSF flux error is inaccurate and likely underestimated.
COSMIC_RAY (CR) 1 12 object contains cosmic rays which have been interpolated over; should not affect photometry
MAYBE_CR 2 24 object may be a cosmic ray; not interpolated over. Useful in searches for single-filter detections.
MAYBE_EGHOST 2 25 object may be an electronics ghost of a bright star. Be suspicious about faint single-filter detections.

Flags that indicate problems with the image

These flags may be hints that an object may not be real or that a measurement on the object failed.
Flag 1/2 bit objc Description
CANONICAL_CENTER 1 0 could not determine a centroid in this band; used centroid in CANONICAL_BAND instead
PEAKCENTER 1 5 used brightest pixel as centroid; hint that an object may not be real
DEBLEND_NOPEAK 2 14 object is a CHILD of a DEBLEND but has no peak; hint that an object may not be real
NOPROFILE 1 7 only 0 or 1 entries for the radial flux profile; photometric quantities derived from profile are suspect
NOTCHECKED 1 19 object contains pixels which were not checked for peaks by deblender; deblending may be unreliable
NOTCHECKED_CENTER 2 26 as NOTCHECKED, but affected pixels are near object's center
TOO_LARGE 1 24 object is larger than outermost radiale profile bin (r > 4arcmin), or a CHILD in a deblend is > 1/2 frame. Very large object, poorly determined sky, or bad deblend. Photometry questionable.
BADSKY 1 22 local sky measurement failed, object photometry is meaningless

Problems associated with specific quantities

Some simply say that the quantity in question could not be measured. Others indicate more subtle aspects of the measurements, particular of Petrosian quantities.
Flag 1/2 bit objc Description
NOSTOKES 1 21 Stokes Q and U (isophotal shape parameters) undetermined
ELLIPFAINT 1 27 no isophotal fits performed
PETROFAINT 1 23 Petrosian radius measured at very low surface brightness. Petrosian magnitude still usable.
NOPETRO 1 8 no Petrosian radius could be determined. Petrosian magnitude still usable.
NOPETRO_BIG 1 10 Petrosian radius larger than extracted radial profile. Happens for noisy sky or low S/N objects.
MANYPETRO 1 9 more than 1 value was found for the Petrosian radius.
MANY_R50 / MANY_R90 1 13/14 object's radial profile dips below 0 and more than one radius was found enclosing 50%/90% of the light. Rare.
INCOMPLETE_PROFILE 1 16 Petrosian radius hits edge of frame. Petrosian quantities should still be reasonable.
DEBLENDED_AS_MOVING 2 0 object recognised to be moving between different filters. For most purposes, consider only this flag to find moving objects.
MOVED 1 31 candidate for moving object. Does not mean it did move - consider DEBLENDED_AS_MOVING instead! Not useful.
NODEBLEND_MOVING 2 1 X candidate moving object (MOVED) but was not deblended as moving
TOO_FEW_DETECTIONS 2 2 object detected in too few bands for motion determination
TOO_FEW_GOOD_DETECTIONS 2 16 even though detected, no good centroid found in enough bands for motion determination
STATIONARY 2 4 A "moving" object's velocity is consistent with zero.
BAD_MOVING_FIT 2 3 motion inconsistent with straight line, not deblended as moving
BAD_MOVING_FIT_CHILD 2 9 in a complicated blend, child's motion was inconsistent with straight line and parent was not deblended as moving
CENTER_OFF_AIMAGE 2 17 nominal motion moves object off atlas image in this band
AMOMENT_UNWEIGHTED 2 21 'adaptive' moment are actually unweighted for this object. NB: to find out if a moment measurement failed entirely, check the error field.
AMOMENT_SHIFT 2 22 centroid shifted too far during calculation of moments, moment calculation failed and M_e1,M_e2 give the value of the shift
AMOMENT_MAXITER 2 23 moment calculation did not converge
AMOMENT_UNWEIGHTED_PSF 2 27 PSF moments are unweighted.

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

Flag 1/2 bit objc Description
DEBLEND_TOO_MANY_PEAKS 1 11 object has more than 25 peaks; only first 25 were deblended and contain all of the parent's flux
DEBLEND_UNASSIGNED_FLUX 2 10 X more than 5% of the parent's Petrosian flux was initially not assigned to children; all this flux has been redistributed among children
DEBLEND_PRUNED 1 26 parent containing peaks which were not deblended
PEAKS_TOO_CLOSE 2 5 some peaks were too close to be deblended
DEBLEND_DEGENERATE 2 18 some peaks had degenerate templates
BRIGHTEST_GALAXY_CHILD 2 19 brightest child among one parent's children
DEBLENDED_AS_PSF 1 25 child is unresolved

Further informational flags

Flag 1/2 bit objc Description
BAD_RADIAL 1 15 last bin in radial profile < 0; usually can be ignored
CANONICAL_BAND 2 20 object is undetected in r-band; this band was used to determine Petrosian and Model radii
SUBTRACTED 1 20 object is part of extended wing of a bright star
BINNED_CENTER 2 6 object was extended and centroid was determined on 2x2 binned frame. Avoid for astrometric work, e.g.

Last modified: Sun Mar 14 22:08:24 CST 2004