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 RESOLVE_STATUS flag used to remove duplicate observations, described briefly below, and in full in the resolve algorithm documentation.
• The CALIB_STATUS 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.

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.

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.

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 ugriz.
• 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 global flags.

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 '1/2' column indicates which group of flags this flag is in (i.e. FLAGS or FLAGS2). 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.

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.

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.

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.

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

Informational Flags Related to Deblending

Flags in this list provide information about processing associated with an object; they do not indicate that an error has occurred.

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

Further Informational Flags

Flags in this list provide information about processing associated with an object; they do not indicate that an error has occurred.

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