Class lsst::afw::image::PhotoCalib

class PhotoCalib : public lsst::afw::table::io::PersistableFacade<PhotoCalib>, public Storable

The photometric calibration of an exposure.

A PhotoCalib is a BoundedField (a function with a specified domain) that converts from post-ISR counts-on-chip (ADU) to flux and magnitude. It is defined such that a calibration of 1 means one count is equal to one nanojansky (nJy, 10^-35 W/m^2/Hz in SI units). The nJy was chosen because it represents a linear flux unit with values in a convenient range (e.g. LSST’s single image depth of 24.5 is 575 nJy). See more detailed discussion in: https://pstn-001.lsst.io/

PhotoCalib is immutable.

The spatially varying flux calibration has units of nJy/ADU, and is defined such that, at a position (x,y) in the domain of the boundedField calibration and for a given measured source instFlux:

\[ instFlux*calibration(x,y) = flux [nJy] \]

while the errors (constant on the domain) are defined as:

\[ sqrt((instFluxErr/instFlux)^2 + (calibrationErr/calibration)^2)*flux = fluxErr [nJy] \]

This implies that the conversions from instFlux and instFlux error to magnitude and magnitude error are as follows:

\[ -2.5*log_{10}(instFlux*calibration(x,y)*1e-9/referenceFlux) = magnitude \]

where referenceFlux is the AB Magnitude reference flux from Oke & Gunn 1983 (first equation),

\[ referenceFlux = 1e23 * 10^{(48.6/-2.5)} \]

and

\[ 2.5/log(10)*sqrt((instFluxErr/instFlux)^2 + (calibrationErr/calibration)^2) = magnitudeErr \]

Note that this is independent of referenceFlux.