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We report here the results of on-orbit tests performed to measure short term focus variations caused by a movement of the secondary mirror of the Optical Telescope Assembly (OTA) of the Hubble Space Telescope (HST). The focus changes measured over about 3 orbits correspond to movements of the OTA secondary mirror of less than 6 microns. The cause of the mirror movement is not fully understood though there is a strong indication that it is thermally induced. The effect of these focus changes on image motion and point spread functions is examined for the present science instruments as well as for the refurbished HST.
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