Flight Performance Of The Attitude Control System Of The Balloon Experimental Twin Telescope For Infrared Interferometry (Bettii)

The Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII) is an 8-meter baseline far-infrared interferometer designed to fly on a high altitude balloon. BETTII uses a double-Fourier Michelson interferometer to simultaneously obtain spatial and spectral information on science targets; the long baseline provides subarcsecond angular resolution, a capability unmatched by any other far-infrared facilities. BETTII had its first successful engineering flight in June 2017. The pointing loop on BETTII is based on an Extended Kalman Filter, which uses different sensors and actuators to keep the telescope pointed at the desired target star. In order to achieve high precision pointing, we use an embedded Field-programmable gate array (FPGA) that combines the gyroscope and star cameras information to generate a pointing solution every 10 milliseconds.The BETTII control system serves a critical function in making interferometric observations possible. This paper discusses the design and implementation of the BETTII control system and presents engineering data of the attitude control system from our pre-flight tests at the Columbia Scientific Balloon Facility (CSBF) and data from our first 12-hour flight from Palestine, TX. This includes pointing performance of the Kalman Filter estimator in the RA, DEC and ROLL Equatorial Coordinate System as well as the payload's attitude behavior when switching between the different modes we implemented: Safe, Brake, Slew, Track and Acquire. These modes are part of the procedure to point the telescope to a desired target. We discuss the performance of the payload's control system in each of these modes and present data showing how the azimuth actuators follow the position and velocity profiles calculated by the flight computers.