Simulation of Stray Light Contamination on CHEOPS Detector

TL;DR

This paper models and quantifies Earth stray light contamination on the CHEOPS satellite detector, analyzing how it varies with orbit, season, and other factors to optimize observation scheduling and noise mitigation.

Contribution

It introduces a software suite for simulating stray light contamination on CHEOPS, including methods for sky visibility, flux calculation, and observation planning.

Findings

Seasonal and altitude variations affect stray light levels.

South Atlantic Anomaly significantly impacts higher orbits.

Optimal observation regions identified based on orbit and sky geometry.

Abstract

The aim of this work is to quantify the amount of Earth stray light that reaches the CHEOPS (CHaracterising ExOPlanets Satellite) detector. It will carry out follow-up measurements on transiting planets. This requires exquisite data that can be acquired only by a space-borne observatory and by well understood and mitigated sources of noise. Earth stray light is one of them which becomes the most prominent noise for faint stars. A software suite was developed to evaluate the contamination by the stray light. As the satellite will be launched in late 2017, the year 2018 is analysed for three different altitudes. Given an visible region at any time, the stray light contamination is simulated at the entrance of the telescope. The amount that reaches the detector is, however, much lower, as it is reduced by the point source transmittance function. Information about the faintest star visible in any direction in the sky is therefore available and is compared to a potential list of targets. The influence of both the visibility region and the unavoidable South Atlantic Anomaly are also studied as well as the effect of a changing optical assembly. A methodology to compute the visible region of the sky and the stray light flux is described. Techniques to prepare the scheduling of the observation and a possible way of calibrating the dark current and the map of hot pixels are presented. The simulations show that there are seasonal variations on the amount of flux received and on the altitude. However, the South Atlantic Anomaly impacts more direly higher orbits. This high radiation region demand the interruption of the science operations. Even if the viewing zone at low altitude is smaller, the availability of instrument is greater. There exist two favoured regions for the observations. The field of view is the widest then as the plane of the orbit and of the terminator merge.