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December 30, 2022

About This Article

1. Updated calacs, acstools, and ACS/WFC photometric CTE calculator

J. E. Ryon, M. De La Peña, M. Chiaberge, and G. Anand.

Summary of updates to CALACS since December 2020

The most recent version of hstcal, version 2.7.4, contains CALACS version 10.3.5.
There have been a number of important updates to CALACS since December 2020, the date of the last ACS STAN:

  • DARKTIME header keyword in WFC images now includes a commanding overhead specific to the observing mode found in CCDTAB
  • WFC dark correction step now uses DARKTIME keyword to account for time that dark current accumulates
  • WFC Analog-to-digital saturation threshold is no longer constant, it is read from CCDTAB
  • WFC and HRC full-well saturation thresholds are now two-dimensional maps of the full well level (SATUFILE)
  • Fixed a bug in the computation of FLASHDUR and DARKTIME header keywords in CRJ/CRC images - they are now the sum of the individual input images
  • Cosmic ray rejection algorithm now performs error propagation of input ERR extensions to produce output CRJ/CRC ERR extensions rather than computing them from a model

We recommend users check for new versions of hstcal periodically on conda-forge (conda) or on github at
For more information on calibration of ACS data, see the ACS Data Handbook.

Release of acstools 3.5.0

The latest version of acstools (3.5.0) was released on 06 December 2022.
There have been several notable updates since December 2020, the date of the last ACS STAN:

  • New acsphotcte module, a programmatic interface for the ACS/WFC Photometric CTE webtool
  • acs_destripe_plus can now keep intermediate files
  • New crrej_plus function to CR-reject and combine (ACSREJ), and further process (ACS2D) destriped subarrays
  • New polarization_tools module, a toolkit for analyzing WFC and HRC polarization data
  • New AWS instance of the ACS Zeropoints Calculator is queried with acszpt module
  • New AWS instance of the ACS Photometric CTE Calculator is queried with the acsphotcte module
  • Requires Python ≥3.8

We recommend users check for new versions of acstools periodically at preferred package managers PyPI (pip) and conda-forge (conda) or on github.

See the acstools documentation for more information.

Improved Algorithm for the ACS/WFC Photometric CTE Calculator

The ACS Team recently published the Instrument Science Report "ACS/WFC CTE Photometric Correction: Improved Model for Bright Point Sources" (Chiaberge and Ryon; ACS ISR 2022-06). This report presents improvements to the CTE photometric correction model to improve the accuracy for bright sources, based on analyses of the latest ACS CAL monitoring programs.  The prior CTE correction formula was seen to underpredict the flux losses for the brightest stars, particularly in low backgrounds. This is due to the linear nature of the prior model and the lack of data for bright stars in that regime, which introduces a systematic bias towards fainter sources.  A new curvilinear model was derived that corrects this bias and yields better results for bright sources, while maintaining the accuracy of the fit to faint stars.  The online ACS CTE calculator and the acsphotcte module in acstools have been updated with the new model and its parameter coefficients.

2. ACS/WFC Sky Backgrounds Might Under-run ETC Estimates

G. Anand

The ACS Instrument Team has recently published an Instrument Science Report, "Revisiting ACS/WFC Sky Backgrounds" (Anand et al.; ACS ISR 2022-01). This report compares the observed values of the ACS/WFC sky background with those predicted by the Exposure Time Calculator (ETC). The analysis of the full set of ACS/WFC broadband filters shows that the ETC slightly overestimates the sky background, with ~10-30% of images having backgrounds lower than the ETC low-background preset. For F850LP in particular, ~50% of the images fall below the ETC estimate for the low-background preset.

Due to the intrinsically low background levels of filters F435W and F850LP, as well as other broadband filters if the exposure times are shorter than ~200s (and also medium and narrowband filters at longer exposure times), ACS/WFC observers should pay close attention to their anticipated background levels. At least 20 electrons of combined dark plus sky background per pixel is currently recommended to avoid severe charge-trailing during ACS/WFC CCD readout due to poor charge-transfer efficiency.

3. Correction to ACS/WFC Pre-defined Dither Pattern ACS-WFC-DITHER-BOX

N. Grogin and J. Anderson

In the course of the ACS Team's development of prime/parallel optimized dithering with WFC3 detectors, the team discovered that the workhorse pattern ACS-WFC-DITHER-BOX was significantly missing the intended target subpixel dither placements, at half-pixel offsets. Detailed astrometric analysis of archival GO data using ACS-WFC-DITHER-BOX has shown that the X-dithers are approximately correct, while the Y-dithers are incorrect by 0.1-0.25 pixel. The ACS Team has revised the APT-level specifications of the ACS-WFC-DITHER-BOX so that future use of this pattern should properly attain the intended subpixel spacing. 

In detail, the changes to ACS-WFC-DITHER-BOX (APT versions 2022.5.1 and later) are:

  • Point Spacing: changed from 0.262 arcsec to 0.2637 arcsec
  • Line Spacing: changed from 0.192 arcsec to 0.1856 arcsec
  • Pattern Orientation: changed from 18.39 deg to 20.70 deg
  • Angle Between Sides: changed from 68.14 deg to 69.02 deg
The corrected ACS-WFC-DITHER-BOX pattern, expressed as X,Y POS TARG values, is now:


All the above changes are also made to the ACS-WFC-DITHER-BOX documentation.


This correction to ACS-WFC-DITHER-BOX will not be applied retroactively to any as-yet unexecuted visits using this pattern, given that some programs may already have executed visits with the prior geometry. The ACS Team generally recommends that ongoing-program GOs resubmit their Phase 2 APT files with the updated dither pattern, available within APT versions 2022.5.1 and later, if not harmful to their science objectives.

4. Replacement of Astroconda by stenv, for STScI Software Distribution

M. Rafelski, J. Carlberg, and N. Dencheva

The Space Telescope Science Institute has released "stenv" to supersede Astroconda for its software distribution. stenv provides a common environment for both the Hubble Space Telescope (HST) and the James Webb Space Telescope (JWST) pipelines and includes most of the packages in Astroconda.  While Astroconda has served our community well for many years, more recently the vast number of versions with an ever increasing number of software packages would often result in Astroconda taking a very long time to resolve dependencies. Moreover, some Astroconda dependencies are fundamentally incompatible with Python versions >3.7. To better support our users, stenv employs pre-determined Conda environments that are verified compatible with Python versions from 3.8 to 3.11.  Additionally, while Astroconda primarily uses Conda recipes to build and serve packages, which need to be updated separately from PyPI releases, stenv draws most of its packages directly from PyPI with pip (though it still requires use of a Conda environment for hstcal and fitsverify, which are provided by conda-forge).

Support for Astroconda will end on 01 February 2023. 

Instructions for installing and using stenv can be found at Questions can be directed to the HST Help Desk Portal  

5. Recent Instrument Science Reports

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

  • ACS ISR 2022-08 "Improved Identification of Satellite Trails in ACS/WFC Imaging Using a Modified Radon Transform" (Stark et al.)
  • ACS ISR 2022-07 "Fading Hot Pixels in ACS/WFC" (Ryon et al.)
  • ACS ISR 2022-06 "ACS/WFC CTE photometric correction: improved model for bright point sources (Chiaberge and Ryon)
  • ACS ISR 2022-05  "Update of the STIS CTE Correction Formula for Spectra" (Bohlin and Lockwood)
  • ACS ISR 2022-04 "The Impact of CTE on Faint Sources in ACS" (Anderson)
  • ACS ISR 2022-03 "Improved Absolute Astrometry for ACS and WFC3 Data Products" (Mack)
  • ACS ISR 2022-02 "One-Pass HST Photometry with hst1pass" (Anderson)
  • ACS ISR 2022-01 "Revisiting ACS/WFC Sky Backgrounds" (Anand et al.)
  • ACS ISR 2021-02 "Long-term Monitoring of the ACS Tungsten Lamp Brightness" (Cohen and Grogin)
  • ACS ISR 2021-01 "Systematic Effects of Pixel-based CTE Correction on the Accuracy of ACS/WFC Point Source Polarimetry" (Desjardins et al.)

6. Recent Technical Instrument Reports

To request a copy of these reports, please contact the ACS Help Desk via the HST Help Desk Portal.

  • ACS TIR 2022-01 "Python Build of the IDC Table Generator for ACS/WFC" (Hoffmann et al.)
  • ACS TIR 2021-02 "Subarray Superbias Generation for Pre- and Post-SM4 ACS/WFC Observations" (McDonald and Ryon)
  • ACS TIR 2021-01 "An Exploration of Reduced Exposure Time and Post-Flash Duration of ACS/WFC Calibration Darks" (Ryon et al.)


HST Help Desk 

Please contact the HST Help Desk with any questions.