Determination of the Photometric Calibration and Large-Scale Flatfield of the STEREO Heliospheric Imagers: I. HI-1

Abstract

The aim of this paper is to calculate an accurate large-scale flatfield for the STEREO HI-1 instruments. This is done by analysing the variation in intensity of stars in the background starfield as they pass across the CCD. In order to use the background starfield, a photometric calibration is performed which defines a HI magnitude scale and a conversion between this scale and measured intensity. The photometric calibration uses stellar spectra folded through the instrument response to make initial intensity predictions. However, a secondary prediction method based on the photometric calibration, which blends the R-, V- and B-magnitudes of a star, is derived for stars with no spectral information.
To perform this calibration, an initial analysis of the shape of the point spread function was required. This indicated that the PSF for the HI-1s is well approximated by a Gaussian function and does not vary substantially from the centre of the field-of-view to the corners and is essentially constant over time.
In addition, a solar spectrum folded through the instrument response is used to determine conversion factors to convert from HI intensity units into mean solar brightness, S10 and SI units, for diffuse or extended sources.