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June 5, 2023

About This Article

1. Alpha Release of the Pandeia-based HST Exposure Time Calculator (ETC)

J. E. Ryon, S. Lockwood, & V. Laidler

STScI has released an alpha version of the Pandeia-based Exposure Time Calculator (ETC) for the Hubble Space Telescope (HST) at The alpha release is intended for users to familiarize themselves with the new interface and perform example calculations for supported observing modes. 

IMPORTANT: The official HST ETC for Cycle 31 proposal submissions is located at Please do not include results from the Pandeia-based HST ETC in Cycle 31 programs.

Pandeia is a pixel-based exposure time calculator paired with a modern graphical user interface. While Pandeia was developed for JWST, it is a general framework, data-driven ETC capable of supporting multiple missions, including HST. It includes advanced features that go beyond the capabilities of previous ETCs, such as algorithms that accurately model both data acquisition and post-processing of data, and it provides functionality for users to efficiently explore and compare a large volume of parameter space in their calculations.

The HST implementation of Pandeia supports many HST observing modes. Its graphical user interface allows users to create workbooks to manage related sets of calculations, create complex astronomical scenes with multiple sources, compare the results of multiple calculations, and share their workbooks with other users. Several HST observing modes are not yet available but will be made available in future releases.

A Migration Guide has been prepared to facilitate the transition to the Pandeia-based ETC for experienced users of the official ETC. Please start there if you are not already experienced with the JWST ETC.

We request feedback from the user community regarding the alpha release via the HST Help Desk particularly feedback related to the clarity of calculation results. An ETC representative will also attend the summer AAS meeting in Albuquerque to demo the Pandeia-based ETC and answer questions. 

A User's Guide for the Pandeia-based HST ETC is available on HDox, and Known Issues are available as a Knowledge Base article on the HST Help Desk.

2. SBC Dark Rate Monitoring

The ACS/SBC offers far-UV imaging with a low dark rate that historically varies from ~1E-5 to ~5E-5 cts/sec/pix during the course of a visit, as the detector temperature increases by ~10 degrees C for a visit spanning 5 orbits.  The dark rate has recently been elevated by a factor of ~2, even though the temperature profile has not changed.  This behavior is under investigation, but users considering the SBC for their observations should plan on shorter visits (1-2 orbits) to help mitigate this behavior if it persists. 

Users are reminded of the continued availability of the SBC-LODARK aperture. Near the reference pixel (175, 185) of this aperture, the dark rate of the detector remains at a nominal level even at high temperatures. This aperture is therefore recommended when a visit will be longer than ~2 orbits and the target is small enough that it will not be affected by the elevated dark rates in the rest of the detector. See ACS ISR 2018-07 for further details.

3. Release of acstools version 3.6.1

Acstools 3.6.1 was released on May 19, 2023. Since the last newsletter announcement of an acstools release, there have been a number of notable updates:

  • New findsat_mrt module for identifying satellite trails in ACS/WFC imaging with the median Radon transform (MRT)
  • Bugfix to satdet module for new version of scikit-image
  • acszpt module uses new AWS-based zeropoint calculator API
  • General documentation updates

A new jupyter notebook demonstrating satellite trail detection and mask creation with acstools.findsat_mrt is available from the acs-notebook github repository.

As always, 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.


HST Help Desk 

Please contact the HST Help Desk with any questions.