Optical Performance of Roman Optical Telescope Assembly

The Roman Space Telescope is a three mirror anastigmat design with a 2.4 m primary mirror, which will provide science information on dark energy and exoplanets via two science instruments. The Optical Telescope Assembly (OTA) was aligned at ambient temperature in a vacuum chamber. Following a vibration test, the OTA’s optical performance, wavefront error, and pupil alignment, were tested at cold operational temperatures in a thermal vacuum chamber. Both test campaigns were performed in double-pass configuration, employing interferometry, Focus Diverse Phase Retrieval, and Shack-Hartman Wavefront Sensing test techniques. Alignment adjustments during the tests were made to the actuated Secondary Mirror, and Fold Mirror 1 in the Wide Field Instrument channel. Stability of the optical telescope assembly (OTA) is a critical need for Roman. For the TVAC test campaign, L3Harris and NASA developed a novel test with the objective of measuring the optical response of key parts of the OTA in the presence of small amplitude thermal stimuli; the Sinusoidal Thermoelastic Distortion Test (STDT). The basic concept is to use the flight hardware’s own thermal control system to generate repeated thermal disturbances while continuously monitoring the optical response.

The telescope’s predicted On-Orbit, End of Life quasi static optical wave front error performance is well within the required 67.3/74.2 nm RMS for the WFI/CGI channel.