Ultrafast time-of-flight imaging with SPAD and picosecond laser for validation of the stray light rejection in an optical calibration facility

Abstract

Stray light characterization using ultrafast time of flight imaging was demonstrated recently for the testing of refractive telescopes, using a streak tube with a femtosecond laser. It was shown that individual contributors such as ghost reflections and scattering could be measured individually and identified through their optical path length with a time-resolved measurement. This allows unprecedented understanding of stray light properties in optical instruments, especially for high-end space telescopes. In this paper, we demonstrate the extension of this method for the validation and improvement of stray light rejection in an optical calibration facility for large space instruments. Here, the stray light paths to be characterized have long range, up to 20 meters. Therefore, the ultrafast sensor considered is a single photon avalanche diode (SPAD) and the illumination is achieved by a picosecond laser. We demonstrate that we are able to measure the individual stray light contributors, for example the scattering on the optical surfaces or the multiple scattering events occurring between baffles and vanes. As each contributor can be identified, the time-of-flight results are used to determine how to improve the optical calibration facility. Moreover, while the measurement is affected also by the detection system intrinsic contribution, this effect can be removed so that the final result only shows the performance of the optical facility. This new approach is extremely useful as it allows pushing further the achievable performances of space telescope characterization, where conventional methods were currently reaching a plateau.