Pointing control for the SPIDER balloon-borne telescope

TL;DR

This paper details the development of a precise pointing control system for the SPIDER balloon-borne telescope, enabling accurate scanning of a heavy payload for cosmic microwave background observations.

Contribution

It introduces a novel pointing control system combining reaction wheels, motorized pivots, and high-frequency feedback for balloon-borne telescopes.

Findings

Achieved azimuth control with < 0.02 deg/s RMS error.

Successfully scanned 5000 lb payload sinusoidally at 6 deg/s.

Maintained sub-arcminute pointing accuracy during operation.

Abstract

We present the technology and control methods developed for the pointing system of the SPIDER experiment. SPIDER is a balloon-borne polarimeter designed to detect the imprint of primordial gravitational waves in the polarization of the Cosmic Microwave Background radiation. We describe the two main components of the telescope's azimuth drive: the reaction wheel and the motorized pivot. A 13 kHz PI control loop runs on a digital signal processor, with feedback from fibre optic rate gyroscopes. This system can control azimuthal speed with < 0.02 deg/s RMS error. To control elevation, SPIDER uses stepper-motor-driven linear actuators to rotate the cryostat, which houses the optical instruments, relative to the outer frame. With the velocity in each axis controlled in this way, higher-level control loops on the onboard flight computers can implement the pointing and scanning observation modes required for the experiment. We have accomplished the non-trivial task of scanning a 5000 lb payload sinusoidally in azimuth at a peak acceleration of 0.8 deg/s$^2$, and a peak speed of 6 deg/s. We can do so while reliably achieving sub-arcminute pointing control accuracy.