Understanding the image processing flags - Summary table

Introduction

For objects in the calibrated object lists (photoObj 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.

Objects in the catalog have several sets of flags:

The resolveStatus flag used to remove duplicate observations, described briefly below, and in full in the resolve algorithm documentation.
The calibStatus flags (one per band) used to identify well-calibrated objects, described briefly below and in full in the calibration algorithm documentation.
The object flags, with information about the success of measuring the object's location, flux, or morphology.

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Resolve status

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 (resolveStatus & 256) != 0 (see a description of bitmasks). An equivalent method included for simplicity and backward compatibility is to search for objects with mode=1.

A description of how resolve status is set is in the resolve documentation.

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Calibration status

The catalogs contain some field which were taken under non-photometric conditions, or at times where certain CCDs were misbehaving. The calibStatus flags provide for each band an indication of whether the data is photometric. For most applications, include only objects with (calibStatus[band] & 1) != 0.

A description of how calibration status is set is in the flux calibration documentation.

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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 flags stored as bit fields in two 32-bit table columns in the photoObj file. 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.

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Flags that affect the object's resolve status

These flags are used to reject duplicate catalog entries of the same object within each field. By using only objects with SURVEY_PRIMARY or RUN_PRIMARY resolve status (see the resolve documentation), you automatically account for these flags.

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.

Note also that there is a complementary description of most of these flags here.

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`				Flag internal to the pipeline code, `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.

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Flags that indicate problems with the raw data

These flags are mainly informational and important only for some objects and science applications.

Note also that there is a complementary description of most of these flags here and especially here.

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.

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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.

Note also that there is a complementary description of most of these flags here and here.

Flag	1/2	bit	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 radial profile bin (`r > 4`arcmin), 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

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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.

Note also that there is a complementary description of most of these flags here and here.

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 recognized 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` (also called `AMOMENT_FAINT`)	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

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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

Note also that there is a complementary description of most of these flags here and here.

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
`HAS_SATUR_DN`	2	27		object is saturated, but attempted to add counts from bleed trail back in
`DEBLEND_PEEPHOLE`	2	28		object was found in a second deblender pass attempting to catch special cases

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Further informational flags

Note also that there is a complementary description of most of these flags here and here.

Flag	1/2	bit	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.

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