HST shrinks and expands due to thermal fluctuations during orbit. This changes the distance between the primary and secondary mirrors, and thus the effective focus of the telescope. As it goes in and out of focus, the ACS PSF changes significantly. The PSF varies in a complex fashion over time, which is essentially impossible to predict in advance. However, we have used TinyTim to model the PSF and thus:
Pre-prepared FITS images, containing a dense grid of (noise-free) model stars are available at focus positions separated by 1 micron. These were created assuming light a single wavelength of 814nm, with 0.03" pixels, to match the oversampled version of the ACS COSMOS images. More details are available in Rhodes, Massey et al. (2006) and Rhodes, Massey et al. (2007).
The parameters of the PSF models can be easily changed, and new images can be manufactured, using TinyTim v6.3 and IDL wrappers. Instructions for use will follow shortly. In the mean time, start with tinytim_create.pro.
By matching a set of model PSFs to 10-12 stars in real images, it is possible to ascertain the effective focus position at the time of exposure to within 1 micron. We have mainly tested this procedure on the HST COSMOS survey. This is a uniform set of 589 ACS images taken with the F814W filter during HST cycles 12 and 13. We found the following results:
| Last updated by Richard Massey on 2nd May 2008. |