A calibration cover for multi-spectral space instruments

The study of many astrophysical objects relies on repeated measurements of specific areas of the sky to allow, for example, the identification of transient phenomena, or of targets of interest extracted automatically from a large set of observations. A key aspect coming to play in these cases, is the stability of the measurement system that allows data to be traceable and comparable over time, which in turn leads to the necessity of a reliable and stable calibration. Observations from space allows for an intrinsically more stable environment due to the absence of ground-generated disturbances, leading to a less complex calibration system to correct for effects such as ageing of optics/detectors or temperature variations in the instrument. In this context, making use of external sources reduces further the need for internal devices, minimizing the impact of calibration in the design of a spacecraft. In this paper we present the performance of a device that takes advantage of these aspects and makes use of the sun to passively generate calibration scenes at visible and infrared wavelengths, with a single part. It consists on a movable cover placed at the entrance pupil of a telescope, with pinhole inserts and a black-coated internal surface. Sunlight passing through the pinholes is used to generate a flat field at visible wavelengths, while by heating the inner surface an infrared uniform scene can be formed. We apply this concept to a case-study for a small-sized satellite (e.g. 6U CubeSat) showing a good relative stability with such system in both IR and VIS bands, over the course of a 2-3 years mission.