Large aperture telescope commonly features segment mirrors and a coarse phasing step is needed
to bring these individual segments into the fine phasing capture range. Dispersed Fringe Sensing
(DFS) is a powerful coarse phasing technique and its alteration is currently being used for JWST.
An Advanced Dispersed Fringe Sensing (ADFS) algorithm is recently developed to improve the
performance and robustness of previous DFS algorithms with better accuracy and unique
solution. The first part of the paper introduces the basic ideas and the essential features of the
ADFS algorithm and presents the some algorithm sensitivity study results. The second part of the
paper describes the full details of algorithm validation process through the advanced wavefront
sensing and correction testbed (AWCT): first, the optimization of the DFS hardware of AWCT
to ensure the data accuracy and reliability is illustrated. Then, a few carefully designed algorithm
validation experiments are implemented, and the corresponding data analysis results are shown.
Finally the fiducial calibration using Range-Gate-Metrology technique is carried out and a
<10nm or <1% algorithm accuracy is demonstrated.
KEYWORDS: Calibration, Sensors, Monte Carlo methods, James Webb Space Telescope, Signal detection, Error analysis, Detection and tracking algorithms, Space telescopes, Wavefronts, Telescopes
Dispersed Fringe Sensing (DFS) is an elegant method of coarse phasing segmented mirrors. DFS performance
accuracy is dependent upon careful calibration of the system as well as other factors such as internal optical
alignment, system wavefront errors, and detector quality. Novel improvements to the algorithm have led to
substantial enhancements in DFS performance. In this paper, we present Advanced DFS, an advancement of
the DFS algorithm, which allows the overall method to be less sensitive to calibration errors. This is achieved
by correcting for calibration errors, which appear in the fitting equations as a signal phase term. This paper will
outline a brief analytical explanation of the improvements, results of advanced DFS processed simulations and
experimental advanced DFS results.
We have successfully demonstrated significant improvements in the high contrast detection limit of the Well-Corrected
Subaperture (WCS) using a number of steps aimed at reducing non-common path (NCP) wavefront errors, including the
Autonomous Phase Retrieval Calibration (APRC)1 software package developed at the Jet Propulsion Laboratory (JPL)
for the Palomar adaptive optics instrument (PALAO). APRC utilizes the Modified Gerchberg-Saxton (MGS) wavefront
sensing algorithm, also developed at JPL2. The WCS delivers such excellent correction of the atmosphere that NCP
wavefront errors not sensed by PALAO but present at the coronagraphic image plane begin to factor heavily as a limit to
contrast. The APRC program was implemented to reduce these NCP wavefront errors from 110 nm to 35 nm (rms) in
the lab, and now these exceptional results have been extended to targets on the sky for the first time, leading to a
significant suppression of speckle noise. Consequently we now report a contrast level of very nearly 1×10-4 at
separations of 2λ/D before the data is post processed, and 1×10-5 after post processing. We describe here the major
components of our instrument, the work done to improve the NCP wavefront errors, and the ensuing excellent on sky
results, including the detection of the three exoplanets orbiting the star HR8799.
Phase retrieval is an image-based wavefront sensing process, used to recover phase information from defocused
stellar images. Phase retrieval has proven to be useful for diagnosis of optical aberrations in space telescopes,
calibration of adaptive optics systems, and is intended for use in aligning and phasing the James Webb Space
Telescope. This paper describes a robust and accurate phase retrieval algorithm for wavefront sensing, which has
been successfully demonstrated on a variety of testbeds and telescopes. Key features, such as image preprocessing,
diversity adaptation, and prior phase nulling, are described and compared to other methods. Results demonstrate
high accuracy and high dynamic range wavefront sensing.
An autonomous wavefront sensing and control software suite (APRC) has been developed as a method to calibrate the
internal static errors in the Palomar Adaptive Optics system. An image-based wavefront sensing algorithm, Adaptive
Modified Gerchberg-Saxton Phase Retrieval (MGS), provides wavefront error knowledge upon which actuator command
voltages are calculated for iterative wavefront control corrections. This automated, precise calibration eliminates non-common
path error to significantly reduce AO system internal error to the controllable limit of existing hardware, or can
be commanded to prescribed polynomials to facilitate high contrast astronomy. System diagnostics may be performed
through analysis of the wavefront result generated by the phase retrieval software.
Image based wavefront sensing methods such as Adaptive Modified Gerchberg-Saxton Phase Retrieval1 (MGS) require
a matrix of a-priori phase knowledge to avoid high dynamic range "phase wrapping" during estimation. Previous
unwrapping methods have met with limited success or have required some degree of expert intervention. We have
succeeded in developing a method and algorithm for automatically unwrapping the phase estimate to generate "prior
phase knowledge". By utilizing first-round wavefront sensing results and image processing techniques, the algorithm is
able to create sufficient a-priori phase information to feed back to the phase retrieval software. The autonomous phase
unwrapping algorithm utilizes edge detection, morphological processing, and spatial filtering, and is able to perform well
on a variety of phase wrapping anomalies for both monolithic and segmented optical systems.
We describe the current performance of an adaptive optics testbed for free space optical communication. This adaptive optics system allows for simulation of night and day-time observing on a 1 meter telescope with a 97 actuator deformable mirror. In lab-generated seeing of 2.1 arcseconds (at 0.5μm) the system achieves a Strehl of 21% at 1.064μm (210nm RMS wavefront). Predictions of the system's performance based on real-time wavefront sensor telemetry data and analytical equations are shown to agree with the observed image performance. We present experimentally measured gains in communications performance of 2-4dB in the received signal power when AO correction is applied in the presence of high background and turbulence at an uncoded bit error rate of 0.1. The data source was a 100Mbps on-offkeyed signal detected with an IR-enhanced avalanche photodiode detector as the receiver.
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