Richard Tansey (Advanced Technology Center, Lockheed Martin), Adam Phenis (Advanced Technology Center, Lockheed Martin), Ker-Li Shu (Advanced Technology Center, Lockheed Martin)
Keywords: Adaptive Optics
Abstract:
A radial shear interferometer (rsi) is produced by the interference of two different sized images of the test wavefront. When the center of curvature of the wavefronts are at the same location, they produce a shear in the radial direction. The rsi has a unique attribute which distinguishes it from other wavefront shear interferometers. For sufficiently large shears S, where S = R1/R2, R being the radius of inner or outer beam, the interferometric fringe pattern is almost identical to a Michelson amplitude splitting interferometer. The usual conversion of measured phase tilt to wavefront phase, as required in other shear interferometers, is avoided. The resultant wavefront reconstruction is eliminated, and the radial shear interferogram can be treated as a direct phase measurement.
Developed for optical testing in prelaser days, the radial shear interferometer was a prime candidate for our use with the black fringe wavefront sensor (bfwfs) described in a companion presentation at this conference.1 We also considered a point diffraction interferometer (pdi) as a self reference interferometer, which led to a testing program to analyze the attributes of each of these optical designs for their later incorporation into an adaptive optics control system with the bfwfs.
The rsi has several advantages over the pdi, including more efficient use of the input light from the test wavefront and insensitivity to vibration and environmental disturbances, due to the common path nature of the coincident wavefronts. However, the radial shear interferometer is hampered by at least two issues in its implementation: the measured wavefront is only an approximation to the true input wavefront (based on the shear ratio), and it cannot be used with a centrally obscured telescope because of the radial shear.
The remainder of the presentation will summarize an experimental investigation in which various wavefront aberrations are introduced at the input of a radial shear interferometer and the resultant wavefront error map obtained. This phase map will be compared with that obtained when similar aberrations are input to a point diffraction interferometer.
There is a common theme to the recent successful applications of curvature sensing, image sharpness algorithms, and stochastic optimization techniques in adaptive optics.
All aberrations do not need to be corrected to obtain either better images or corrected outgoing wavefronts, and averaging techniques can be a viable alternative for adaptive optics correction in severely degraded atmospherically generated or high flow conditions. The use of a radial shear interferometer, combined with a black fringe wavefront sensor is finally described as an alternative method to correct for many of these aberrations, and, to the best of our knowledge, represents the first use of an rsi for atmospheric correction.
%Z Tansey,R.J., Chan,H., Hokam, A., The Black fringe Wavefront Sensor, Amos Technical Conference, Sept 10-14, 2006
Date of Conference: September 10-14, 2006
Track: Adaptive Optics