HST Focus Variations with Temperatures

Details HST Focus Variations with Temperatures HST Focus Variations with Temperatures

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007aas...21113513d&link_type=abstract

American Astronomical Society, AAS Meeting #211, #135.13; Bulletin of the American Astronomical Society, Vol. 39, p.974

Astronomy and Astrophysics

Astronomy

Scientific paper

One of the main advantage of space observatories is the stability of the point spread function that allows programs not feasible from the ground. However, when pushed to the limits, even an observatory like the Hubble Space Telescope may exhibit variations in the PSF that can be problematic for studies like weak lensing or identification of the host halos of bright quasars at high redshift. These variations are the result of small displacements in the focus of the telescope. Therefore, we characterize the variation of the focus position for HST in terms of the average temperature sensor values of the telescope. The focus is measured by means of phase retrieval fitting of point sources from selected ACS-WFC images. We perform our analysis first on a dataset made of one week contiguous observations of the same stellar field. This allows us to measure the focus position with an error smaller than 0.5 micron per exposure. A linear combination of 6 temperatures (measured at different positions along the telescope) plus the well known breathing model reproduces the focus variation within its observational error and is stable against a control dataset acquired in a different epoch. When tested on historic, low frequency, data starting from 2003, our temperature model continues to perform well, except for occasional step-like jumps observed in the focus, which are due to the less predictable shrinking of the spacecraft due to degassing. Therefore our model can be applied to improve the determination of the model point spread function for observations lacking point sources, where a direct measurement is not available.

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