Flag use examples#

The flag use examples below include only a small set of flags that apply to most science analyses.

Note

These examples are not recipes for a “clean” sample (i.e., pure or complete). The correct application of flags depends on the science analysis.

General advice:

  • It is recommended to test queries with and without flag cuts to understand the selection effects and how they impact the science analysis.

  • When a measured quantity is used in an analysis, check also that the quantity’s general failure flag is false. For example, when using r_cModelFlux, check that the value of r_cModel_flag is false (or 0). In the snippets below, {band} stands for one of u, g, r, i, z, y.

Object table#

Guidance:

  • Use the {band} flag for each band that is necessary for the analysis.

  • In the example below, replace {band}_psfFlux_flag with the failure flag for the flux type used in the analysis (e.g., {band}_cModel_flag, {band}_free_psfFlux_flag).

  • For model photometry and shapes, require the matching general flag when using the quantity, e.g., {band}_cModel_flag = 0 for CModel fluxes, {band}_kronFlux_flag = 0 for Kron fluxes, or {band}_hsmShapeRegauss_flag = 0 for HSM shapes.

Selection effects: For a spatial query on the Object table near the edge of the COSMOS field, the example below reduces by about 10% the number of objects returned; with the two additional examples, the total reduction is about 23%. The fraction of objects cut will change depending on the observations obtained in the region being queried, and the filters included. It is important to consider which flag cuts are necessary for a given science analysis, and to characterize the selection effects.

Example:

WHERE {band}_inputCount > 0                        -- At least one input image at the position
  AND {band}_psfFlux_flag = 0                      -- Successful flux measurement (replace with flux type used)
  AND {band}_invalidPsfFlag = 0                    -- Valid PSF model
  AND {band}_pixelFlags_saturatedCenter = 0        -- No saturation at the center
  AND {band}_pixelFlags_interpolatedCenter = 0     -- No interpolated pixels at the center

Additional examples:

AND {band}_pixelFlags_crCenter = 0            -- No cosmic ray at center
AND {band}_pixelFlags_interpolated = 0        -- No interpolated pixels anywhere in the footprint

Extendedness#

Key points:

  • An “extendedness” parameter provides a measure of whether an astrophysical sources is point-like or extended. These parameters can help to distinguish between point-like stars and extended galaxies, but keep in mind that high-redshift objects can also appear point-like.

  • There are three measures of extendedness in the Object table, per band, plus one multi-band variant. Each differ with respect to what is measured, whether a companion failure flag exists, and how meaningful the numeric value is.

Guidance:

  • The extendedness parameters have not been characterized or validated as a star/galaxy separation parameter, and performance will vary across the DP2 fields due to their varying image depth and image quality. Any selection based on the extendedness parameters should be tested and validated for the science analysis.

  • The model_extendedness parameter is the most likely to have a finite value for a given object, and the griz_model_extendedness combines the four bands with the best signal. However, the drawback is that there is no associated flag column and the griz_model_extendedness tends to classify everything as a galaxy fainter than approximately i = 24 mag (see the discussion under Measurement).

  • See also the tutorial notebook on extendedness.

Columns

Range

Notes
{band}_extendedness
{band}_extendedness_flag
0 or 1
0 or 1

PSF-to-CModel flux ratio, thresholded by the pipeline.
refExtendedness
no flag

0 or 1

PSF-to-CModel flux ratio, in the refBand.
{band}_sizeExtendedness
{band}_sizeExtendedness_flag
0 to 1
0 or 1

Moments-based comparison of the source size to the local PSF.
refSizeExtendedness
no flag

0 to 1

Moments-based extendedness, in the refBand.
{band}_model_extendedness
no flag

0 to 1

Sersic model flux- and size-based, single band.
griz_model_extendedness
no flag

0 to 1

Sersic model flux- and size-based, combining the griz bands.

Source table#

Guidance:

  • Unlike the Object table, pixelFlags_bad, pixelFlags_edge, and pixelFlags_suspect are valid in the Source table.

  • When selecting or excluding calibration stars, use the calib_* flags (see Calibration flags).

Example:

WHERE centroid_flag = 0                 -- Centroid succeeded
  AND psfFlux_flag = 0                    -- (or the flag for whichever flux you use)
  AND pixelFlags_saturatedCenter = 0      -- No saturation at center
  AND pixelFlags_interpolatedCenter = 0   -- No interpolation at center

Additional examples:

AND pixelFlags_crCenter = 0       -- No cosmic ray at center
AND pixelFlags_edge = 0           -- Not on the exposure edge
AND pixelFlags_bad = 0            -- No known-bad pixels in footprint
AND pixelFlags_suspectCenter = 0  -- No suspect pixels at center

ForcedSource table#

Guidance:

  • When building light curves, apply these flags per measurement (row) so that poor epochs are dropped while good epochs for the same object are kept.

Example:

WHERE psfFlux_flag = 0                  -- Science-image PSF flux succeeded
  AND invalidPsfFlag = 0                 -- Valid PSF model
  AND pixelFlags_saturatedCenter = 0     -- No saturation at the forced position

If using the difference-image flux, add:

AND psfDiffFlux_flag = 0               -- Difference-image flux succeeded
AND diff_PixelFlags_nodataCenter = 0   -- Difference image has coverage at this position

DiaSource table#

Guidance:

  • No cut on the reliability column was applied before writing the DiaSource catalog (Reliability score is a machine-learned real/bogus score).

Example:

WHERE isDipole = 0                    -- Not a dipole subtraction artifact
  AND psfFlux_flag = 0                -- Difference-image flux succeeded
  AND pixelFlags_saturatedCenter = 0  -- No saturation at center

Additional examples:

AND centroid_flag = 0   -- Reliable position
AND pixelFlags_crCenter = 0  -- Not a cosmic-ray residual at center
AND glint_trail = 0     -- Exclude probable orbital-debris glint trails
AND isNegative = 0      -- Exclude flux-decrease detections (keep them for fading/disappearing sources)

ForcedSourceOnDiaObject table#

Guidance:

  • As with ForcedSource, filter per measurement (row) to remove bad epochs while keeping good ones.

Example when using difference-image flux:

WHERE psfDiffFlux_flag = 0              -- Difference-image flux succeeded
  AND diff_PixelFlags_nodataCenter = 0  -- Difference image has coverage
  AND invalidPsfFlag = 0                -- Valid PSF model
  AND pixelFlags_saturatedCenter = 0    -- No saturation at position

If using the science-image flux instead:

WHERE psfFlux_flag = 0                  -- Science-image flux succeeded
  AND invalidPsfFlag = 0                -- Valid PSF model
  AND pixelFlags_saturatedCenter = 0    -- No saturation at position

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