Focus and alignment of the Space Surveillance Telescope: procedures and year 2 performance results

The Space Surveillance Telescope (SST) is a three-mirror Mersenne-Schmidt telescope with a 3.5 m primary mirror that is designed for deep, wide-area sky surveys. The SST design incorporates a camera with charge-coupled devices (CCDs) on a curved substrate to match the telescope’s inherent field curvature, capturing a large field-of-view (6 square degree) with good optical performance across the focal surface. The unique design enables a compact mount construction for agile pointing, contributing to survey efficiency. However, the optical properties make the focus and alignment challenging due to an inherently small depth of focus and the additional degrees of freedom that result from having a powered tertiary mirror. Adding to the challenge, the optical focus and alignment of the mirrors must be accomplished without a dedicated wavefront sensor. Procedures created or adapted for use at the SST have enabled a successful campaign for focus and alignment, based on a five-step iterative process to (1) position the tertiary mirror along the optical axis to reduce defocus; (2) reduce spherical aberration by a coordinate move of the tertiary and secondary mirrors; (3) measure the higher order aberrations including astigmatism and coma; (4) associate the measured aberrations with the predictions of optical ray-tracing analysis; and (5) apply the mirror corrections and repeat steps 1-4 until optimal performance is achieved (Woods et al. 2013). A set of predicted mirror motions are used to maintain system performance across changes in telescope elevation pointing and in temperature conditions, both nightly and seasonally. This paper will provide an overview of the alignment procedure developed for the SST and will report on the focus performance through the telescope’s second year, including lessons learned over the course of operation.