Method and apparatus for measuring the stability of large lateral transfer retroreflectors through a single optical port.

Achieving precise alignment between spatially separated optical axes is a challenging problem in optical applications such as boresighting, rangefinding, laser tracking, and optical beam delivery. The Advanced Topographic Laser Altimeter System (ATLAS) developed for NASA's Ice, Cloud and Land Elevation Satellite-II (ICESat-2) mission launched in 2018 was one such application where the alignment of the transmitter laser beam to the receiver telescope is critical for the operation of the instrument. This paper discusses a novel approach for verifying the stability of high-precision beam pointing in the conditions of the Thermal Vacuum (TVAC) test, developed for ATLAS testing at NASA Goddard Space Flight Center. It includes a method and an optical assembly that enables measuring tiny variations in the pointing of the beams of a large lateral transfer retroreflector through a single optical access port such as a TVAC chamber window. The method can be applied for verifying the alignment of optical beams separated by extended distances of tens of meters in vacuum.