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 1: difference_image dataset type DOI Logo https://doi.org/10.71929/rubin/2570312

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}, ExposureF)

  • Format: FITS

  • Number of Butler datasets: 15,972

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.

For DP1, the difference images are served and can be retrieved and displayed by users instantaneously, but the inputs and tools for their re-creation are not supplied.

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, similar to the SDSS bitmasks.

Metadata

The metadata for difference images retrieved from the Butler include information about the observation (e.g., pointing, weather), and the derived PSF, photometric calibration, and WCS.

Tutorials

See the 200-level notebook or 200-level portal tutorials demonstrating how to access the difference images.