Welcome to the ACS/WFC focus-diverse, ePSF webtool!
This tool allows easy access to focus-diverse, effective point-spread functions (ePSFs) for the Wide Field Channel on the ACS instrument. Uneven heating of the telescope's assembly is known to cause variations in the shape of the PSF, and the empirical ePSF models provided here take this into account. They are shown to be an improvement over the standard library PSFs. We strongly recommend you first read ACS ISR 2018-08 by A. Bellini et al. and ACS ISR 2023-06 by G. Anand et al. for more details on these ePSF models before using this tool.
To use this tool, simply enter the rootname of your file in the IPPPSSOOT format and press the "Retrieve ePSFs" button. Your browser will download a multi-extension FITS file which is best matched to your input observation. The FITS file has 90 extensions, corresponding to a 9x5 spatial array for each of the two WFC CCDs.
Please note that only IPPPSSOOT formats will work (e.g. jds408jsq), and the tool does not support inputs in the form of association IDs or product names (e.g. jds408010 or jds408011).
Notably, these ePSFs work well with the hst1pass software package, available for retrieval from Jay Anderson's webpage.
If you are interested in performing a batch retrieval of focus-diverse ePSFs for multiple input files, please see the focus_diverse_epsfs module within acstools. The same python module also includes a function for obtaining the best ePSF model at a particular (x,y) position on the CCD. Please see this Python notebook for worked examples of the above.
The retrieved file has its name the format %ROOTNAME-STDPBF_ACSWFC_%FILTER_%SM_%F.fits, where the variables are:
%ROOTNAME– This is the image rootname that you passed to the webtool, which is appended to allow for easier bookkeeping of retrieved files.
%FILTER– This is the ACS/WFC filter that was used in the observation.
%SM– Short for "Servicing Mission", SM3 or SM4, depending on when the observation was taken.
%F– Best fit focus group that the individual exposures lies within.
For user-support of this tool, please contact the HST Help Desk.
November 13th, 2023 (Version 1.0): Initial launch.
June 7th, 2023 (Version 0.1): Not yet launched.