DECEMBER 17, 2020

ACS STAN December 2020



1. Updated CALSPEC SEDs and Flux Calibration Changes for ACS

R. C. Bohlin, J. E. Ryon, and J. Anderson

Motivated by improvements in stellar atmosphere models, the CALSPEC models and observed spectral energy disributions (SEDs) for the primary spectrophotometric standard stars were updated in March 2020. In addition, a re-examination of Vega’s spectral flux resulted in a ~0.9% (grey) increase in its absolute flux.  The net result of the March 2020 changes is an increase in flux of up to 3% in the UV at wavelengths shorter than 0.15 microns, 2% increase between 0.15 and 0.4 microns, and an increase of 2% to 4% increase longward of 8 microns. From 0.4 to 8 microns, the net change is <1.5%.  Details are described in Bohlin, Hubeny and Rauch 2020, AJ, 160, 21. This update is designated as CALSPECv11.

CALSPEC is the database of the composite spectra of stars for HST and JWST flux calibration. Stellar atmosphere models are available for a large fraction of these flux standards. CALSPECv11 updated the standard star models and the SEDs based on re-calibrated STIS, WFC3, and/or NICMOS spectroscopy. CALSPECv11 files are available in the current CALSPEC directory. All versions of CALSPEC SEDs are available in the full database at  Further CALSPEC information is available at

The ACS Team prepared an updated, CALSPECv11-consistent, absolute flux calibration released in September 2020 for most of its filter complement.  ACS/SBC filters with revised photflam flux calibration values include F115LP, F122M, F125LP, F140LP, F150LP, and F165LP.  ACS/HRC filters with revisions include F220W, F250W, F330W, and F344N.  ACS/WFC and HRC filters with revised calibrations include F435W, F475W, F502N, F555W, F550M, F606W, F625W, F658N, F660N, F775W, F814W, F892N, and F850LP. For details, see Bohlin, Ryon, and Anderson (2020; ACS ISR 2020-08).

2. The Release of Improved ACS/WFC Geometric Distortion Reference Files with Respect to Gaia DR2

S. L. Hoffmann and V. Kozhurina-Platais

The HST ACS/WFC has a larger geometric distortion due to the optical assembly of the telescope and its off-axis configuration. In addition, the ACS/WFC linear distortion terms (Anderson 2007; ACS ISR 2007-08) have changed monotonically since ASC was installed in 2002. These changes are noticeable over 20 years, reaching about 25 mas (>0.5 WFC pixels) from the original 2002-based distortion solution. As a result, this introduces poor alignment (~0.5 pixels) into ACS/WFC images processed with HST software DrizzlePac.

Kozhurina-Platais et. al. (2015; ACS ISR 2015-06) developed an empirical correction for the time-dependent distortion which ensures high level accuracy in the global ACS/WFC astrometry. Recently, the ACS/WFC geometric distortion was assessed with respect to Gaia DR2 positions as discussed by Kozhurina-Platais (ACS ISR 2018-01). New geometric distortion reference files were derived (Hoffmann & Kozhurina-Platais, ACS ISR 2020-09) with respect to Gaia DR2 positions and revised WFC aperture location measurements in the Science Instrument Aperture File (J. Sahlmann et al. 2019; TEL ISR 2019).

The reference files were tested by comparing 20 years of 47 Tuc observations with Gaia DR2 positions. The results show significant improvement in the global astrometric accuracy of ACS/WFC compared to the International Celestial Reference System (ICRS) based on Gaia DR2. These improvements include a decrease in the absolute orientation from 0.029 degree to 0.008 degree, and close agreement in the plate scale. The empirical correction of the time-dependent distortion is accurate to the level of 0.02 WFC pixels (1 mas) until 2017-2018. The full description of the ACS/WFC geometric distortion based on Gaia DR2 geometric distortion calibration will be presented in an upcoming ISR by Kozhurina-Platais et. al. (in preparation).

3. Change in ACS/WFC Amplifier D Read Noise

T. D. Desjardins, M. C. McDonald, and S. L. Hoffmann

Following an anneal of the ACS/WFC detectors on July 20, 2020, the read noise in the readout amplifier D region of the WFC2 CCD rapidly increased from its previous value of 5.05 electrons up to 5.85 electrons. After a brief period, the read noise decreased slightly to 5.65 electrons, after which it held stable. Similar to analysis in Coe et al. (2013; ACS TIR 2013-01), observations of 47 Tuc were used to determine if the read noise jump could be explained by a change in the readout amplifier gain. Photometry of stars that were observed on both amplifier D and another amplifier yielded no measurable change in the gain, and subsequent analysis of observations from the ACS gain monitor program (HST proposal 16390; PI Hoffmann) confirmed that result. On September 2, 2020, a new CCDTAB reference file was delivered to the Calibration Reference Data System (CRDS) for use in calibrating ACS/WFC observations taken after July 24, 2020 using the updated read noise value of 5.65 electrons for readout amplifier D. The ACS Team will continue to monitor the noise properties of the four ACS/WFC readout amplifiers and update reference files in CRDS if necessary. For more information, users may request a copy of Desjardins & McDonald (2020; ACS TIR 2020-01) by contacting the ACS Help Desk via the HST Help Desk Portal.

4. Advice for Planning ACS Observations Document Now Available

R. A. Lucas, N. P. Hathi, N. A. Grogin, and the ACS Team

We wish to draw your attention to a new resource for those wishing to propose for ACS usage in Phase I proposals and to use it in accepted Phase II proposals. Linked web pages cover advice on a series of topics important for both the proposer and observer to consider when preparing Phase I proposals using ACS and also  Phase II observing programs using ACS which have been accepted by the TAC for execution. Although also released for reference in its initial version as ACS ISR 2019-07, the web page at is the best place to access this documentation in its most current and user-friendly form, as it is most likely to include any more recent major updates over time as opposed to the ISR which may be more static. We advise both proposers and observers to consult and make use of this resource as it draws on the collective operational and calibration expertise and findings of the ACS GTO Team, STScI ACS Group, ST-ECF, and other select studies over the years, with special emphasis on more recent aspects of its operation and calibration as it has aged, taking into consideration factors such as CTE effects.  

5. Recent Instrument Science Reports

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

  • ACS ISR 2019-10 "Bright Object Magnitude Limits for ACS/SBC and Color Corrections for All Three Channels" (Ryon et al.)
  • ACS ISR 2020-01 "Post-SM4 ACS/WFC L-Flats and Photometric Errors from Observations of Stellar Fields" (Cohen et al.)
  • ACS ISR 2020-02 "New and Improved Saturated Pixel Flagging for the ACS/WFC" (Cohen et al.)
  • ACS ISR 2020-03 "Irregularity in Ceiling of Analog-to-Digital Conversion for Post-SM4 ACS/WFC Images" (Cohen et al.)
  • ACS ISR 2020-04 "Accounting for Readout Dark in Superbiases II: Subarrays and Updated Readout Dark Measurement" (Ryon, Grogin, and Desjardins)
  • ACS ISR 2020-05 "Anneal Efficacy in the Advanced Camera for Surveys Wide Field Channel" (McDonald et al.)
  • ACS ISR 2020-06 "Validation of Reduced Operate Anneal Mode in the Advanced Camera for Surveys Wide Field Channel" (McDonald and Grogin)
  • ACS ISR 2020-07 "Unusual Horizontal Charge Overflow from Saturated CCD Pixels in the ACS WFC: Discovery and Remediation" (Cohen and Grogin)
  • ACS ISR 2020-08 "Update of the Photometric Calibration of the ACS CCD Cameras" (Bohlin, Ryon, and Anderson)
  • ACS ISR 2020-09 "Validation of the New ACS/WFC Geometric Distortion Reference Files Derived Using Gaia Data Release 2" (Hoffmann and Kozhurina-Platais)

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 2020-01 "ACS/WFC Readout Amplifier D Read Noise Anomaly" (Desjardins and McDonald)
  • ACS TIR 2020-02 "Eliminating EPER Light Leak, Patching Flight Software, and Fixing Shutter Keywords" (Ryon et al.)

Please Contact the HST Help Desk with any Questions