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10 August 2010 OGSE telescope WFE testing at 30K
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The James Webb Space Telescope (JWST) Optical Telescope Element (OTE) gathers the light from astronomical objects and provides it to four scientific instruments and the observatory guider. The Canadian contribution to JWST, the Fine Guidance Sensor (FGS), includes both the FGS-Guider and one of the science instruments, the Tunable Filter Imager (FGS-TFI); both are packaged together and are functionally independent. The FGS OGSE (Optical Ground Support Equipment) is used to simulate the image from the OTE and verify the optical performance of the FGS Guider and TFI during instrument level testing. The OGSE consists of 25 separate telescopes, each of which simulates a point source at a different field location. The OGSE must maintain alignment and image quality at the cryogenic (30-40K) operating temperature of the FGS. This paper presents WFE (wavefront error) testing for one of the telescopes over a temperature range from ambient to cryogenic operating temperatures (30 K). This test made use of a Zygo interferometer with the standard Zygo transmission sphere replaced by a custom-made transmission sphere located in the cryo vacuum chamber. Meanwhile, image position displacements (focus) during cooling down with respect to ambient are also obtained by tracking the position of the transmission sphere. The results show that the WFE degrades from 19 nm (RMS) at ambient to 42 nm (RMS) at 30 K, while the image displaces about 5.6 mm at 30 K with respect to ambient temperature. The reason for the focus displacement is discussed.
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Hua Lin, Sandra Delamer, Clinton Evans, Ashley McColgan, Shenghai Zheng, Peter Klimas, Pascal Marais, and Frédéric Grandmont "OGSE telescope WFE testing at 30K", Proc. SPIE 7731, Space Telescopes and Instrumentation 2010: Optical, Infrared, and Millimeter Wave, 77313O (10 August 2010);

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