27 July 2016 Collimating the Gemini telescopes using a peripheral wavefront sensor
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Abstract
A new technique has been developed to collimate the Gemini telescopes using the Peripheral Wavefront Sensors (PWFS) to measure focal plane offset and tilt. For several years prior to 2014, observers at Gemini North noticed a variation in the focus Zernike term of about ±30 μm when guiding with the PWFS. It was speculated that variation was due to a tilt of the PWFS rotary table. Further testing revealed that it was actually due to an incorrect tilt of the secondary mirror (M2), causing the focal plane to be offset and tilted relative to the PWFS axis. Due to the Ritchey- Chrétien design of the telescopes there is no Seidel comatic field pattern typical of an aligned telescope. Instead a constant comatic field pattern occurs from either tilt or decenter of M2, and patterns arising from tilt can be eliminated with the appropriate decenter. For the Gemini telescopes, proper alignment is not guaranteed from a zero-coma condition. The new technique measures PWFS focus variation around the periphery of the imaging field, 6 arcminutes off-axis, by programming the telescope pointing to move in a circle while PWFS tracks a guide star, completing a full circle. The measured focus variation is then used to calculate M2 tilt. The tilt and decenter offset are then adjusted to zero both focus variation and coma and achieve collimation. The technique permits correction of the erroneous M2 tilt to <~ 30 arcseconds, corresponding to a wavefront error <~ 3 μm, but is limited by short-period focus variations.
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T. Schneider, T. Schneider, T. L. Hayward, T. L. Hayward, A. Stephens, A. Stephens, K. Roth, K. Roth, } "Collimating the Gemini telescopes using a peripheral wavefront sensor", Proc. SPIE 9906, Ground-based and Airborne Telescopes VI, 99063Y (27 July 2016); doi: 10.1117/12.2233797; https://doi.org/10.1117/12.2233797
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