In-flight calibration and performance verification for space instruments

In-flight calibrations play a crucial role in ensuring the reliability of scientific data obtained by space optical instruments, such as telescopes or cameras. One effective method for assessing the optical performance of these instruments consists of analyzing the images acquired during the commissioning phase: flat-field, dark, and star images, by comparing them to on-ground measurements. Throughout this PhD work, the activity has focused on devising methods and realizing appropriate codes to study the performance and calibration of space instruments. This work presents a comprehensive overview of the concept, input requirements, and simulations conducted to: predict the stars visible in the field of view (FoV) of specific space instruments, and the behavior of sources in front of their apertures. In particular, this work applies to two specific space instruments: the Metis coronagraph on board Solar Orbiter and the STC stereo camera on board the Bepi-Colombo mission. For both instruments, I simulated stars within their respective FoV, compared the analysis with in-flight star images, and extracted information related to the Point Spread Function (PSF). Real images of the stars are also compared with those simulated with point sources. This permitted to validate the effectiveness of our calibration methodologies, and to gain valuable insights into the instrument’s expected performance. For Metis, the behavior of sunlight reflected by the aperture door is studied, confirming that the signal intensity follows the 1/r2 law for door illumination and is proportional to exposure time, but infirming the idea that the door’s retroreflection could not serve as a suitable homogeneous illumination source. The passage of a star through Metis’ field of view is also studied and has a dual purpose: assessing the evolution of the vignetting function during the mission, and evaluating its potential for detecting optical element degradation. The results demonstrated good agreement between on-ground, in-flight, and simulated vignetting functions, as expected for a recent mission. For STC, emphasis was put on the analysis of dark images acquired during the commissioning phase. The detection of bad pixels for different integration times was made, leading to the discovery of patterns in the Pan L and Pan H filters. Their evolution was studied in detail, and further research to understand their origin is proposed. For both instruments, these analysis enabled us to set the basis for the monitoring of variations in instrument performance and sensitivity, primarily attributed to degradation or misalignment of optical components. Indeed, the instruments under consideration operate in a challenging environment, they are working close to the Sun and in orbit around Mercury, which subjects them to experience high temperatures and significant temperature variations.