Straylight (SL) characterization using ultrafast time of flight imaging (ToF) has been demonstrated for the testing of refractive telescopes, using a streak tube with a femtosecond laser. It was shown that individual SL contributors such as different ghost reflections and scattering features can be measured individually and identified by temporal discrimination due to the specific optical path length of each of them. This allows to analyze them individually for a better understanding of straylight properties in instruments. Recently, we have used the ToF approach to characterize a testing facility that was then used in the frame of the calibration campaign for the Narrow Angle Camera (NAC) of the Earth Return Orbiter mission. The facility itself could generate its own SL that has to be retrieved from that coming from the instrument. Due to the large facility dimensions, optical path lengths can be discriminated by using a low temporal resolution that is enabled by picosecond lasers associated to a SPAD detector. At the end, the SL coming from the facility can be reverse engineered to find its origin and either removed by facility adaptation or by processing.
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
Space telescopes require always better stray light control. However conventional characterization methods rely on the same principles since several decades and we are reaching the limits of their capabilities. We have developed a disruptive approach which solves these limitations: stray light characterization by ultrafast time of flight imaging. By using a pulsed laser source and an ultrafast sensor, we are able to discriminate and identify individual stray light contributors, thus providing the ultimate understanding of the stray light origins in an instrument. In this paper, we present our approach and its potential for the future of space telescopes. We will present the experimental characterization of a refractive telescope with this method, using a streak camera and a fs laser. We will present how we use this approach was used to experimentally characterize, validate and improve a stray light measurement facility, using a SPAD sensor and a ps laser.
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.
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