Advances in stray light characterization by ultrafast time-of-flight imaging

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 features can be measured individually and identified, allowing unprecedented understanding of stray light properties in telescopes. This opens the door to the development of higher performing instruments, with stray light properties significantly reduced compared to the state of the art. In this paper, we will present the latest advances in the domain of stray light characterization by ultrafast time-of-flight imaging. This includes the characterization of imaging instruments, and the use of the time-of-flight measurements for reverse engineering instruments properties. In addition of using the time-of-flight approach for characterizing instruments, we will show that this method can be used to validate and improve conventional stray light measurement devices and facilities. In the case of large facilities, the typical optical path lengths is of the order of several centimeters to tens of meters. Therefore, in that case streak cameras can be replaced by a less expensive alternative, namely SPAD detectors. We will present the dedicated SPAD detector that we developed and the results obtain in the validation and improvement of the stray light facility for the FLEX Earth observation instrument. This system will be also used in the near future also for the NAC instrument in the ERO mission
to Mars.