Photometric calibration#
Photometric calibration derives the conversion of raw “counts” recorded by the camera into fluxes calibrated onto a standard system. This includes correcting for effects in the detectors themselves (discussed in Instrument signature removal (ISR)), filter throughputs, and atmospheric transparency which varies with airmass and time. These components all contribute to the photometric uncertainties reported in the catalog flux measurements.
System characterization: Initial steps not requiring on-sky data to characterize the instrumental and overall system properties included:
Measuring the instrumental response as a function of wavelength and position on the focal plane (mostly measured in the lab prior to going on sky).
Measuring the throughput of the filters (also as a function of position; this came from measurements performed by the vendors who manufactured the filters).
Measuring the optical system throughput as a function of wavelength using a collimated beam projector (CBP) and a tunable laser.
Measuring the pixel-to-pixel response (flat-fielding) using multi-band images of an illuminated calibration screen in the dome.
Calibration using on-sky data: On-sky images are used to characterize the system, and of course must be calibrated themselves to measure atmospheric and other time-varying effects. After Instrument signature removal (ISR) has been performed on the images, additional photometric calibration steps include:
Initial calibration is performed by detecting sources, estimating the point-spread function (PSF), then fitting an initial solution to The Monster reference catalog reference catalog. The main purpose of this step is to provide a good enough photometric and astrometric calibration to enable associating multiple observations of stars.
The final global photometric calibration uses the Forward Global Calibration Method (FGCM; Burke et al. 2018). FGCM is used to calibrate the full DP2 dataset with a forward model that uses a parameterized model of the atmosphere as a function of airmass along with a model of the instrument throughput as a function of wavelength. Nightly variations in the atmosphere are modeled by minimizing the variance in repeated observations of stars with an SNR greater than 10. Additional constraints, particularly the overall throughput, are provided by a subset of stars from The Monster reference catalog.
Observations of overlapping spectrophotometric flux standard standard stars are used to evaluate the accuracy of the absolute system throughputs.
Aperture corrections#
Rubin processing uses aperture corrections to ensure that different photometry estimators produce consistent results on point sources. These corrections are measured by applying different algorithms to the same set of bright stars on each single-visit image and interpolating the ratio of each algorithm to a standard one (a background-compensated top-hat aperture flux), which is then used for all photometric calibration. All fluxes other than the standard algorithm’s are then multiplied by the interpolated flux ratio. Aperture corrections on coadds are computed by averaging the single-detector ratios with the same weights that were used to combine images.
See Known issues for known issues with this scheme.
Additional resources#
For a description of the photometric calibration steps and hardware to enable them, refer to the “Rubin Baseline Calibration Plan” (sitcomtn-086.lsst.io).