Nazim Bharmal, Durham University
Keywords: laser, wavefront sensing, beam wander, tip/tilt
Abstract:
The optical characterisation of orbiting objects, be they debris (so poorly known a priori) or catalogued satellites (better knowledge) not only improves the current awareness of objects in their orbital plane but also permits the parameters of the object to be more robustly understood. The latter can then permit several applications such as lifetime assessment and in-orbit servicing to benefit from such knowledge enhancement. Techniques for characterisation using passive imaging include imaging, spectroscopy, and polarimetry. This paper is instead concerned with active (laser-based) illumination from the ground, which then opens new avenues for modifying the illumination in time, for example. The first step is to improve the knowledge of beam wander from laser propagation through the atmosphere, and then be able to perform beam wander compensation. Here I discuss the application of astronomical tomographic wavefront sensing for G-tilt measurement of the linear phase aberrations causing beam wander. I use a realistic example telescope situation to show that for moderate seeing, the probability of reducing wander from 1 arcsecond by 50—90% using 1 guide-star (NGS) ranges from 20—33%, and for 2 NGS 1—20%. For these non-negligible probabilities, the 1-sigma motion at 1000km would be reduced from ca.5m to 0.5—2.5m and therefore a potential beam-power reduction by at least 50% in terms of photon noise-limited S.N.R. I end by discussing the modest technical requirements and limitations, together with how improved knowledge of beam wander can be used with laser illumination for improved characterisation possibilities including at GEO.
Date of Conference: September 14-17, 2021
Track: Optical Systems & Instrumentation