Phase Retrieval to Monitor HST Focus: II. Results Post-Servicing Mission 4

Details Phase Retrieval to Monitor HST Focus: II. Results Post-Servicing Mission 4 Phase Retrieval to Monitor HST Focus: II. Results Post-Servicing Mission 4

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010tel..rept....3s&link_type=abstract

Instrument Science Report TEL 2010-03, 16 pages

Physics

Hubble Space Telescope, Hst, Space Telescope Science Institute, Acs, Wfc3, Advanced Camera For Surveys, Wide Field Camera 3

Scientific paper

Hubble Space Telescope (HST) focus has been monitored throughout the Observatory's life primarily using high-resolution imaging cameras. The preferred method to determine the focus position is a Phase Retrieval technique. It solves for certain Zernike polynomials such as focus, coma and astigmatism, by fitting a model Point Spread Function interactively adjusting the aberration parameters to observed data. In this report, we discuss results of the monthly focus monitoring program since the latest mirror move in July 2009. Since the primary purpose for this monitoring is to support accurate focus maintenance, we present a picture of the current focus state of the HST. We discuss focus measurements done with both the Advanced Camera for Surveys (ACS) and Wide Field Camera 3 (WFC3) and draw conclusions about their confocality. We also predict when the Observatory is going to be in the best focus. The spread in these predictions is large and arises from uncertainties, such as orbital thermal variations (breathing) and long-term trends, which are difficult to model. Our best estimate, based on the long-term historical focus trend, implies that ACS (and all science instruments confocal to it) is close to the best focus at the time of writing. There is tentative evidence that the best focus of WFC3 UVIS is ˜ 0.5 ± 0.2 μm below that of ACS.

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