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ACS STAN March 2026

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March 31, 2026
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Table of Contents

  1. Empirical PSFs Constructed for SBC & hst1pass Enabled for SBC Imaging Data

  2. Recent Instrument Science Reports

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1. Empirical PSFs Constructed for SBC & hst1pass Enabled for SBC Imaging Data

Jay Anderson

The ACS team has created empirical “effective” PSFs for the SBC’s five long-pass filters: F115LP, F125LP, F140LP, F150LP, and F165LP. The globular cluster M70 is observed annually for SBC calibration purposes. In each of the 80 archival images of this cluster for each long-pass filter, there are about 50 UV-bright stars with high signal-to-noise, resulting in about 4,000 star images from which to construct empirical PSFs. Figure 1 shows contour plots of the central regions of the empirical PSFs. The F115LP PSF is sharp and round, whereas the other PSFs are elongated in the -70° direction, and two of them (F140LP and F165LP) exhibit two distinct cores. Such structure in the PSF could easily be mistaken for binarity, so it is critical to model stars with accurate PSFs.

SBC_contour_plots.png
Figure 1: Contour plots of the central 4x4 pixels of the SBC PSFs for each long-pass filter. The inner heavy black line indicates the pixels in the PSF contain 10% of the star’s total light in each pixel.  The outer heavy black line is where the pixels contain 1% of the star’s total light. The SBC native pixel scale is about 32x34 milli-arcseconds.

The SBC PSFs exhibit a small amount of variation with position around the detector. They also show some variation over time; around 80% of the SBC images of M70 have nominal PSFs, and about 20% have PSFs that are broader than nominal. 

The empirical SBC PSFs are provided in the STDPSF format. See WFC3 ISR 2016-12 for more on the STDPSF format. Average PSFs for the entire SBC detector are provided as well as PSFs that include spatial variations across the detector in a 3×3 grid. The hst1pass software routine (ACS ISR 2022-02) has been enabled to run PSF-fitting photometry on SBC images utilizing these new PSFs. Additionally, a new geometric distortion solution in preparation for the SBC has been converted into the STDGDC format needed by hst1pass to convert detector positions into an absolute astrometric frame. Full details can be found in ACS ISR 2026-02.

2. Recent Instrument Science Reports

For a complete list with abstracts, please visit the ACS ISR webpage.

 

 

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