The transmitted wavefronts of optical filters for the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) are characterized using the Wildly and Openly Modified Broadband Achromatic Twyman Green (WOMBAT) Interferometer developed in the NASA/GSFC Optics Branch's Diffraction Grating Evaluation Facility (DGEF). Because only four of thirty-three of ACS's optical bandpass filters transmit the 633 nm light of commercial interferometers, a broadband interferometer is required to verify specified transmitted wavefront of ACS filters. WOMBAT's design is a hybrid of the BAT1 interferometer developed by JPL used for HST Wide Field and Planetary Camera 2 (WFPC-2) filters and the DGEF's WYKO2 400 phase shifting interferometer. It includes a broadband light source, monochromator, off-axis, parabolic collimating and camera mirrors, an aluminum-coated fused silica beam splitter, flat retroreflecting mirrors for the test and reference arms, and a UV-sensitive CCD camera. An outboarded, piezo-electric phase shifter from WYKO holds the flat mirror in the interferometer's reference arm. The interferometer is calibrated through interaction between the WYKO system's software and WOMBAT hardware for the test wavelength of light entering the beam splitter. Phase-shifted interferograms of the filter mounted in the test arm are analyzed using WYKO's Vision (r) software. Filters as large as 90 mm in diameter have been measured over a wavelength range from 200 to 1100 nm with a sensitivity of (/200 rms at ( = 633 nm. Results of transmitted wavefront measurements are shown for ACS fixed band pass and spatially-variable bandpass filters at several different positions.
Keywords: Interferometry, Optical filters, Wavefront, Advanced Camera for Surveys
Brief Biography, Principal Author:
Presently: Rene A. Boucarut, Optical Engineer in the Optics Branch at Goddard Space Flight Center, Greenbelt, MD; Research areas include testing of flight and research optics and development of vacuum ultraviolet optical technology; B.S. Physics, Polytechnic Institue of New York, Brooklyn, NY, 1979; M.S. Physics, Cornell University, Ithaca, NY, 1982
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