Richard Holmes, Boeing LTS, Brett Sickmiller, Leidos, Nicholas Steinhoff, tOSC, Skip Williams, Air Force Research Laboratory, Andrew Whiting, Boeing LTS
Keywords: Focus Control, Wavefront Control, Telescopes
Abstract:
A ubiquitous problem in observations with large telescopes is focus control. Typical auto-focus algorithms used in commercial cameras are not effective for the astronomical application due to the long range to the typical objects and the random focus caused by atmospheric turbulence. This problem can be mitigated with an adaptive optics system. However, adaptive optics systems are typically complex and costly. This paper discusses alternative approaches that are relatively low in cost and complexity. These options include two tracker/imager based means for focus control, and a dedicated focus sensor approach. The dedicated focus sensor is a simplified form of a Hartmann sensor. The specific implementation of such a focus sensor will be shown to provide significant benefits for focus correction. The tracker/imager-based implementations have an intrinsic plus/minus focus ambiguity, due to the nature of the focus sensed on an image plane. However, this ambiguity can be overcome with careful algorithm design. Two options are considered for tracker/imager-based focus control: an auto-focus metric that has been preferred in commercial cameras, and a spot-width estimation algorithm. It is found that the spot-width estimation algorithm works as well as a dedicated focus sensor when the plus/minus ambiguity is resolved, and that this ambiguity can be resolved in most cases. In addition to performance, cost and implementation issues are also considered for generic telescope systems with apertures greater than 0.5 meters.
Date of Conference: September 15-18, 2015
Track: Poster