Justin M. Knight, University of Arizona; Michael Hart, HartSCI LLC
Keywords: Shack-Hartmann wavefront sensor, extended scene imaging, wavefront sensing comparison analysis, remote sensing simulation
Abstract:
A design goal for Earth observing satellites is to launch a low-weight, large aperture primary mirror which provides high-resolution images of extended scenes of interest. In remote sensing, this translates to achieving a small ground sampling distance over a possibly large bandwidth on the imaging detector in the presence of noise sources as well as error inducing environmental factors such as repeated exposure to the sun in low-earth orbit, causing thermal fluctuations which distort the primary mirror shape and misalign optical elements present in the satellite ultimately manifesting as wavefront aberrations across the telescope pupil. A practiced method of accomplishing this goal is to equip such a satellite with an adaptive optics (AO) system to provide a means of measuring and compensating for wavefront errors through the combination of a wavefront sensor and a corrector element such as a fast-steering or deformable mirror. A standard technique employed for wavefront sensing is to reconstruct the wavefront across the telescope pupil from wavefront slope estimates calculated using Shack-Hartmann wavefront sensor (SHWFS) subaperture information; in extended scenes, this is usually performed via subimage correlations with reference images. While popular, Shack-Hartmann wavefront sensors are subject to performance limitations, many of which are addressed by developing correlation algorithms to overcome practical issues such as thresholding useless information across subapertures, for example, from cloudy scenes, poor wavefront estimates from both low signal-to-noise ratio across subapertures and using subimage boundary information, and so on. While other extended scene wavefront sensing techniques such as broadband phase diversity and plenoptic sensors have been studied with respect to adaptive correction in remote sensing, even being compared to the SHWFS directly, we seek to compare a suite of wavefront sensing techniques with respect to their performance in the limit of error and noise sources focusing on the latter in this study. To this end, we consider wavefront sensing techniques previously developed for use in traditional nighttime astronomy, solar astronomy, or optical metrology such as the aforementioned SHWFS for extended scenes, hereafter referred to as the Shack-Hartmann correlation tracker, and optical differentiation wavefront sensor. Common error sources under our consideration include the previously detailed thermal stresses in orbit which cause telescope pupil aberrations, as well as other various environmental exposures which result in a degradation of specular efficiency from the surfaces of optical elements, producing stray light, and detector pixel loss from radiation damage. Meanwhile noise sources include various forms of detector noise, of which the most fundamental to light-matter interactions is photon noise; as such, it is our primary focus. We present a photon-noise sensitivity parameter in the presence of a telescope pupil phase aberration for various wavefront sensor techniques; such a parameter demonstrates how well the Shack-Hartmann correlation tracker performs against less well-characterized wavefront sensing techniques in the realm of remote sensing under ideal imaging conditions. This allows us to consider the viability of wavefront sensing techniques for an AO system based on physics-imposed limitations before extending our analysis to include practical ones.
Date of Conference: September 14-17, 2021
Track: Optical Systems & Instrumentation