An insight into the flux calibration of Gaia G-band images and BP/RP spectrophotometry

TL;DR

This paper discusses the complex process of calibrating Gaia's G-band images and BP/RP spectrophotometry, addressing technical challenges like CCD effects and the calibration model involving standard stars.

Contribution

It provides a detailed analysis of Gaia's flux calibration process, highlighting the technical complexities and proposing calibration strategies using standard stars.

Findings

Calibration involves addressing CCD effects and LSF variations.

A calibration model uses ~200 standard stars with few percent precision.

The process is crucial for accurate Gaia photometry and spectrophotometry.

Abstract

The Gaia mission is described, focussing on those technical aspects that are necessary to understand the details of its external (absolute) flux calibration. On board of Gaia there will be two (spectro)photometers, the blue one (BP) and the red one (RP) covering the range 330-1050 nm, and the white light (G-band) imager dedicated to astrometry. Given the fact that the focal plane of Gaia will be constituted by 105 CCDs and the sources will cross the the focal plane at constant speed, at different positions in each of the foreseen passages (on average 70--80, but up to 350) in the mission lifetime, the ``simple" problem of calibrating the integrated BP/RP and G-band magnitudes and the low resolution BP/RP spectra flux turns into a very delicate and complicated issue, including CTI effects, LSF variations across the focal plane and with time, CCD gating to avoid saturation and the like. The calibration model requires a carefully selected set of $\simeq$200 SpectroPhotometric Standard Stars (SPSS) with a nominal precision of a few \%, with respect to Vega.