Alignment and performance verification of two-mirror focal corrector optics using computer-generated holograms for balloon-borne ultraviolet Telescope FIREBall-2

We describe the application of computer-generated holograms (CGHs) for simplifying the optical alignment of a two-mirror focal corrector (FC) assembly for the Faint Intergalactic-medium Redshifted Emission Balloon (FB-2). FB-2 is a joint NASA/Centre National D’Etudes Spatiales-funded suborbital balloon experiment designed to map faint ultraviolet (UV) emission from the circumgalactic medium around low red-shift galaxies. The first flight of FB-2, in September 2018, was interrupted due to a hole in the balloon and could not reach the minimum required altitude for science observations. The gondola landed with high impact, resulting in minor damage to the telescope mirrors and the two-mirror FC assembly. As a result, the FC, a high-conic optical assembly that maps the focal plane of the telescope to the slit mask of the multi-object spectrograph, was misaligned beyond tolerance. FC is a critical subsystem of the FB-2 optical chain as it corrects the aberration from the primary mirror over the wide field of view while preserving the magnification. It was essential to rework and realign FC to enable future FB-2 flights. However, this task is challenging due to the complex optical surfaces, the tight alignment tolerances (<30 μm lateral, <1 arcmin angular), and the contamination sensitivity of the UV-optimized mirror coatings. We discuss the optical alignment and performance verification scheme that uses CGHs to enable precise realignment of the FC optics while meeting the demanding flight schedule. Our scheme employs two custom CGH nulls combined with a Zygo (Fizeau) interferometer to verify the optical quality of the damaged FC mirror and align the two-mirror system. The successfully re-aligned FC was integrated into the FB-2 spectrograph in early 2023 for a flight in September 2023.