The conventional adaptive-optics (AO) system configuration consisting of a Shack-Hartmann wavefront sensor
using the Fried geometry is prone to an unsensed waffle mode because of an inability to have discrete point
reconstruction of the phase at the actuator positions. Techniques that involve filtering and/or projecting out the
waffle mode in the reconstructor have been shown to be effective at not allowing the unwanted mode to occur,
but come at the cost of also omitting relevant high frequency content from the measured phase. This paper
analyzes a technique of sensing the waffle mode in the deformable mirror commands and applying a spatial filter
to those commands in order to mitigate for the waffle mode. Directly spatially filtering the deformable mirror
commands gives the benefit of maintaining the reconstruction of high frequency phase of interest while having
the ability to alleviate for the waffle pattern when it arises.
This paper is the 3rd in a series of papers discussing characterization of a Micro-Electrical-Mechanical-System (MEMS)
deformable mirror in adaptive optics. Here we present a comparison between a conventional adaptive optics system
using a Xinetics continuous face sheet deformable mirror with that of segmented MEMS deformable mirror. We
intentionally designed the optical layout to mimic that of a conventional adaptive optics system. We present this initial
optical layout for the MEMS adaptive optics system and discuss problems incurred with implementing such a layout;
also presented is an enhanced optical layout that partially addresses these problems. Closed loop Strehl highlighting the
two systems will be shown for each case as well. Finally the performances of both conventional adaptive optics and the
MEMS adaptive optics system is presented for a range of adaptive optics parameters pertinent to astronomical adaptive
optics leading to a discussion of the possible implication of introducing a MEMS adaptive optics system into the science
community.
The use of a laser guidestar (LGS) for the purpose of a beacon in an adaptive-optics (AO) system is prone to
perspective elongation effects on the spots of a Shack-Hartmann wavefront sensor. The elongated spots can
vary in size over the subapertures and affect the gradient sensitivity of the sensor. The Air Force Research
Laboratory (AFRL) has developed a LGS model that outputs gradient gains which represent the effects of an
extended beacon on the spots for a Shack-Hartmann wavefront sensor. This paper investigates the application
of these gains in an experimental setup in order to both analyze the effects of the variation in those gains due to
spot size elongation and to measure the impact on the performance of an AO system.
KEYWORDS: Actuators, Adaptive optics, Turbulence, Deformable mirrors, Control systems, Wavefront sensors, Signal to noise ratio, Wavefronts, Digital filtering, Cameras
The Air Force Research Laboratory's (AFRL) Sodium Guidestar Adaptive Optics for Space Situational Awareness
program (NGAS) sponsored research on spatially non-uniform gain for the servo-loop controller of an
adaptive optics (AO) system. The edge subapertures of a Shack-Hartmann wavefront sensor have lower signalto-
noise ratios and are more susceptible to measurement errors than fully illuminated center subapertures. These
measurement errors produce errant commands over the corresponding edge actuators and can induce instabilities
over these regions in strong turbulence conditions. The Non-uniform Gain Experiment (NUGE) concentrated
on the development and experimental analysis of spatially varying gain maps on the servo-loop controller of a
deformable mirror. The goal was to improve AO system performance and mitigate instabilities that can occur
over the edge actuators of a deformable mirror. A gain map with a ring of lower filter gains b over just the outer
actuators was experimentally shown to increase the overall Strehl ratio of the AO system in all of the tested
turbulence conditions. A Gaussian gain map was also shown to significantly reduce the overall residual phase
variance over the edge actuators thereby reducing the formation of the instabilities. Experiments were conducted
at the Starfire Optical Range (SOR), AFRL, Kirtland AFB.
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