Design and performance of the BBRISON UV-VIS fine pointing system

Balloon-borne telescopes have the potential to provide space-quality imaging. However, long exposures require pointing stability on the order of the optical point spread function (PSF). Consequently, one of the main goals of the BRRISON (Balloon Rapid Response for ISON) payload is the characterization of a fine pointing system. The BRRISON telescope is stabilized at the 5 arcsecond (RMS) level by the gondola's coarse control system. The job of the fine pointing system is to improve the pointing by another two orders of magnitude, from ±5" to better than ±0.05". Due to a flight anomaly, this paper discusses the design and ground test performance of the two main components of the fine pointing system: the fine guidance system (FGS) and the fast steering mirror (FSM). The FGS was designed with the goal of providing a 20 Hz optical reference signal with 0.05" positional acuity. The FSM is specified to provide 100 nRad control of the mirror, translating to 0.0024" on-sky. We report on measured performance and observed FWHMs during preflight ground testing. During a successful re-flight we expect ~100X better performance than was achieved in ground testing.