Mask planes#

In the LSST Science Pipelines, each processed image includes not only the measured flux values but also a companion bit mask image that records the condition of every pixel. These mask planes encode information about detector defects, cosmic rays, saturation, missing data, and other effects that influence data quality. Each named mask plane corresponds to a specific bit flag that can be set independently or in combination with others on a given pixel. Bit values are assigned dynamically and may change.

Visit and difference images#

Visit and Difference Image Mask Planes

Deep and template coadds images#

Deep and Template Coadd Mask Planes

Summary of key planes#

The following table provides a summary.

Mask Plane	Image Type	Description (DP1-specific)
BAD	visit + coadd	Permanently bad pixels, including entire bad amplifiers. Excluded from science use.
SAT	visit + coadd	The flux in this pixel was too high to be accurately recorded (its value exceeded the Photon Transfer Curve (PTC) turnoff point). In coadds, it indicates that saturation affected at least part of the stack at this location.
INTRP	visit + coadd	Pixel’s value was replaced via interpolation (usually because it was flagged `BAD`, `SAT`, or `CR`). For coadds, the value was interpolated during stacking, or all inputs had this pixel interpolated.
CR	visit + coadd	Pixels hit by a cosmic ray; these pixels are interpolated. For coadds, one or more input visits flagged this pixel as a cosmic ray.
EDGE	visit + coadd	Region unprocessed due to the convolution kernel footprint extending beyond the image edge.
DETECTED	visit + coadd	Pixel footprint belongs to a detected source above threshold. For coadds, the pixel was detected as part of a source footprint on the coadd itself.
DETECTED_NEGATIVE	difference	Negative source detection in difference images.
SUSPECT	visit + coadd	Pixel above the PTC turnoff but not fully saturated. Not dilated like `SAT`. Propagates to coadd if a configurable fraction of input visits flagged it as `SUSPECT`.
NO_DATA	visit + coadd	No valid data (chip gap, missing coverage, or failed amplifier).
VIGNETTED	visit + coadd	Pixel vignetted by optics; low-weight or low-quality. For coadds, vignetted in all contributing visits.
STREAK	difference	Linear artifact (satellite trail, diffraction spike).
CLIPPED	coadd only	At least one input image contributing to this pixel was identified as an artifact and excluded.
CROSSTALK	visit + coadd	Pixel affected by electronic crosstalk from a bright source in another amplifier. For coadds, one or more inputs flagged this pixel as affected by crosstalk.
INEXACT_PSF	coadd only	The PSF at this pixel is ill-defined or varies significantly across inputs. When set, this flag is accompanied by at least one of `SENSOR_EDGE`, `CLIPPED`, or `REJECTED`.
ITL_DIP	visit + coadd	“ITL dip” artifact: dark vertical trails from bright sources on ITL CCDs. For coadds, one or more input images flagged this pixel as affected.
NOT_DEBLENDED	visit + coadd	Pixel in a source footprint that was not deblended.
REJECTED	coadd only	Pixel where a contributing image was masked and not used.
SENSOR_EDGE	coadd only	Pixel lies within a margin near the edge of at least one contributing input image.
UNMASKEDNAN	visit + coadd	Pixel contains NaN without other masks; indicates invalid data.
INJECTED	visit only	Pixels with synthetic sources injected for testing or validation.
INJECTED_TEMPLATE	difference	Pixels with synthetic sources injected into template images (for difference image analysis).
SAT_TEMPLATE	difference	Pixel saturated in the template used for difference imaging.