Calibrations#

Calibration products (e.g., combined bias, dark, and flat frames).

Citation: NSF-DOE Vera C. Rubin Observatory (2025); Legacy Survey of Space and Time Data Preview 1: calibrations dataset type DOI Logo https://doi.org/10.71929/rubin/2570309 [BibTeX]

Access#

The calibration products are accessible via the Butler.

Unlike other data products, most calibrations are associated with a “validity range” as well as a data ID (e.g. detector and physical_filter) in the butler, since different calibrations can be valid for different observations. This is usually best provided by also passing an exposure or visit ID when looking up a calibration in addition to the calibration’s regular data ID dimensions, since both exposure and visit are associated with a timespan in the butler database.

Butler#

Number of Butler datasets: 496

Examples of dataset types:

  • ('bias', {instrument, detector}, ExposureF, isCalibration=True)

  • ('dark', {instrument, detector}, ExposureF, isCalibration=True)

  • ('flat', {band, instrument, detector, physical_filter}, ExposureF, isCalibration=True)

Other calibrations dataset types include ptc for Photon Transfer Curve and bfk for brighter-fatter kernel.

Description#

The process of Instrument Signature Removal (ISR; also called “image reduction”) uses calibration products such as bias, dark, and flat field calibration frames as part of the process to transform raw images into visit images.

Bias images: An exposure obtained with zero exposure time to measure the pedestal level of counts applied during readout.

Dark frames: An exposure obtained with a nonzero exposure time but with no illumination (shutter closed) to measure the detector’s response to the thermal energy in the camera.

Flat fields: An exposure taken with even illumination across the field to measure pixel response variations.

Processing#

The calibration products are created by processing calibration images and used in the Instrument signature removal (ISR) pipeline.

Pixel data#

ExposureF calibration products such as the combined bias, dark, and flat have an image plane and a variance plane.

The pixel data units are ADU (analog-to-digital units).

Metadata#

ExposureF calibration frames have a header and bounding box but don’t have a World Coordinate System (WCS) object.

Tutorials#

See the 200-level notebook tutorial demonstrating how to access the calibration products.