ICESat-2 ATLAS telescope testing

Many lessons were learned in the comprehensive testing of the one meter Beryllium flight telescope for the ICESat-2 mission. This paper will focus on testing areas of encircled energy analysis, plate scale measurements, and boresight alignment for alignment of fiber optic cables. The Optical Development System Lab (ODSL) at NASA´s Goddard Space Flight Center (GSFC) was developed to build up experience using engineering test units. This experience was applied to testing the flight telescope. Several tests were able to be performed on the telescope itself, helping drive down risk, cost, and schedule during the integration phase of the telescope onto the instrument and box structure. The main ICESat-2 instrument is the Advanced Topographic Laser Altimeter System (ATLAS). It measures ice elevation by transmitting laser pulses, and collecting the reflection in a telescope. Because so few photons return from each pulse, the alignment of each receiver channel fiber is critical as well as minimizing the distortion. The lab consisted of a clean room with a one meter parabola collimator system with a point source fiber-coupled 532nm laser and a CCD detector. This was used to feed collimated light into the telescope that was recorded with a CCD detector in the telescope focal plane. A large one meter flat mirror was used to certify the collimator system. Fiber optic cables were also used to back-illuminate the telescope and image in the collimator focal plane. The telescope was mounted in a gimbal that allowed for three degrees of rotational freedom allowing the telescope to be steered to each respective science field point. The setup worked well for accomplishing the testing. Through well written procedures and prior experience, the testing was carried out according to plan and on schedule despite obstacles along the way such as late ground support equipment and tests that needed to be repeated. The objective of this paper is to share those lessons learned for optical align- ent of a receiver telescope assembly to promote future mission success.