Difference image#
The result of subtracting a template coadd from a visit image.
Citation: NSF-DOE Vera C. Rubin Observatory (2025); Legacy Survey of Space and Time Data Preview 2: difference_image dataset type 
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Access#
The difference images are accessible via the Butler, SIA, and TAP services.
Butler#
Dataset type: (‘difference_image’, {band, instrument, day_obs, detector, physical_filter, visit}, DifferenceImage)
Format: FITS
Number of Butler datasets:
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SIA and TAP#
Schema: ObsCore table
IVOA calibration level: 3
Dataproduct subtype: lsst.difference_image
Description#
Difference images are created by taking template coadd images, then resizing, warping, PSF-matching, and scaling them to a given visit image, and subtracting the result from the visit image. Each individual difference image contains data from one of the camera’s detectors.
Difference images contain sources of residual flux from the subtraction, both positive and negative. Real astrophyisical transients, variables, and moving objects appear as sources with a flux that is equal to the difference between their flux in the template and visit images.
For future data releases, to save space, the difference images will not be stored. Instead, a tool for on-the-fly difference image recreation will be provided to users.
Processing#
The difference images are the result of Difference image analysis (DIA).
Pixel data#
The difference images have three planes of pixel data.
Image: sky pixel data in flux units of nJy.
Variance: uncertainty (noise) in the flux in units of nJy^2.
Mask: an integer bitmask of representative flag values that indicate processing status or issues. See Visit and difference image mask planes.
Metadata#
The metadata for difference images retrieved from the Butler include the derived PSF and the WCS.
Tutorials#
See the 200-level catalog Tutorials for a notebook on difference images.