<title>Segmented mirror figure control for a space-based far-IR astronomical telescope</title>

Abstract

For segmented large mirrors to be used effectively for astronomy they must be actively aligned and controlled to extreme levels of precision. We consider the figure control problem for a spaceborne far-IR telescope, the Precision Segmented Reflector Project Focus Moderate Mission Telescope. We propose a two-stage approach. A figure initialization controller is used to achieve initial phasing and alignment of the telescope using an imaging science detector. A figure maintenance controller keeps the telescope aligned during normal operation using a laser metrology optical truss sensor system. We show that performance of any figure control system is subject to limits on the controllability of the wavefront. Maintenance controllers are additionally limited by considerations of the observability of the wavefront from the maintenance sensors. We show preliminary results for the figure initialization controller. We present a "Wavefront Compensation" method for figure maintenance control that minimizes wavefront errors due to misalignment errors.

Model-based wavefront control for CCAT

“…The formally optimal way to do this is by Kalman Filter techniques: weighted least-squares, where the estimate weights the measurements and the prior knowledge using the covariance matrices associated with each term to find the most likely state estimate [14], [15].…”

“…The first is simply to use a single-step approach, as described in [15]. This approach uses a single-step form of Eqs.…”

Wavefront controls for a large submillimeter-wave observatory

“…The formally optimal way to do this is by Kalman Filter techniques: weighted least-squares, where the estimate weights the measurements and the prior knowledge using the covariance matrices associated with each term to find the most likely state estimate [8], [9].…”

Modeling a large submillimeter-wave observatory

“…JPL has many years of experience in wavefront and segment sensing and control for Far-IR, IR and optical telescopes, including extensive analysis and testbed work for the James Webb Space Telescope and Keck Telescope [4][5] [6] [9] [10]. We are leveraging our expertise in segmented telescope sensing and control, combined with recent JPL work in edge-sensor development, to develop a comprehensive strategy for both CCAT telescope initialization and wavefront maintenance control.…”

“…For CCAT wavefront maintenance, edge sensors are a cost-effective option for segments alignment, and JPL has developed an optical edge sensor, called the LAser Directed Displacement (LADD) sensor developed at JPL [11]. We have examined the sensitivity modes of LADD sensors for CCAT segments, and combined it with a wavefront control scheme proposed by Redding [4] to evaluate the effectiveness of segment control and robustness of sensing noise suppression of a wavefront control strategy designed for CCAT system maintenance. This paper is organized as follows: Section 2 discusses a strategy for CCAT telescope initial alignment, including a combined cross-gap and IR sensing option which appears to be robust with sensing noise.…”