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ACS PSF Information

The point-spread function (PSF) of an instrument describes how light from a point source is distributed on the detector. Many science cases require accurate knowledge of the PSF to perform their analysis. This page details several versions of PSFs that have been generated by members of the ACS team for the user community. A standard set of PSFs are available for all three detectors (WFC, SBC, and HRC), and additional “focus-diverse” PSFs that account for changes in HST’s focus are provided for ACS/WFC. All of these PSFs are provided directly in the original, distorted FLT/FLC image space.

Standard PSFs (STDPSFs)

The Standard PSF (STDPSF) format provides 4x-supersampled empirical PSF models. The STDPSF format is built to be compatible with the hst1pass routine.

For ACS/WFC, these products are provided as a 9x10 array to cover variations across the field-of-view. For ACS/SBC, these products are available in 1x1 or 3x3 formats (the latter format is not available for the F115LP filter). For ACS/HRC, the only format available is 1x1.

Additionally, for ACS/WFC, separate products are available to best-fit the PSFs before and after Servicing Mission 4 (SM4). Note that the SM4 STDPSFs were part of an effort to address changes in the WFC PSF before the focus-diverse STDPBFs (below) were developed, and are thus are only available in F606W and F814W.

More details can be found in ACS ISR 2022-02 and ACS ISR 2026-02.

Focus-Diverse PSFs (STDPBFs)

The STDPBF format is similar to the STDPSF format described above, with the addition of another dimension that accounts for variations in the PSF due to changes in telescope focus, or “breathing”. As with the STDPSFs, these are 4x-supersampled models that are readily compatible with the hst1pass routine. These products are not available for the HRC or SBC. More details can be found in ACS ISR 2018-08 and ACS ISR 2022-02.

Additionally, we provide an online webtool and corresponding API that allows users to automatically download best-fit focus-diverse effective PSFs on a per-image basis. A companion tutorial Jupyter Notebook is also provided. More details can be found in ACS ISR 2018-08, ACS ISR 2022-02, and ACS ISR 2023-06.

PSF Information and Downloads

Last Updated: 05/20/2026

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