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

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 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.