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hst1pass: One-Pass PSF Photometry for HST Detectors

Presentation #206.01 in the session New Mission Concepts — iPoster Session.

Published onJun 29, 2022
hst1pass: One-Pass PSF Photometry for HST Detectors

We are making public and documenting hst1pass: a software routine that has been used for years to find and measure stars in HST images. The hst1pass routine makes use of empirical models of HST’s effective PSF in order to identify and measure relatively isolated stars in images taken with HST’s primary imaging detectors: ACS/WFC, WFC3/UVIS, WFC3/IR, ACS/HRC — and even WFPC2. The user can specify the finding parameters used to search the images as well as the specific measured parameters to be output for each source. The time-averaged “library” PSFs are generally accurate enough for most projects, but the routine has the ability to use focus-diverse PSFs to tailor the PSF to the particular focus of the exposure. Alternatively, the routine can perturb the PSF in an empirical way to improve the photometry. For S/N ~100 stars, the errors in the measured positions are typically 0.01-0.02 pixel and the photometry is generally good to 0.01-0.02 magnitude, and point-source/resolved-source discrimination is quite accurate. The routine has a flexible output format in order to facilitate subsequent collation of outputs from multiple dithered exposures. Future improvements will depend on demand, but could include PSF-convolved Gaussian or 2-source fitting, or even point-source-plus Gaussian fitting.

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