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May 2023 STAN

May 31, 2023

About This Article

In this STAN we announce the alpha release of the Pandeia-based ETC for HST instruments and the complete delivery of the updated G750L reference file.  


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

J. 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 (not currently active). 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.

Updated STIS Flux Calibration for G750L

The STIS Team released a new reference file on April 14, 2023 (74e15479o_pht.fits) containing updated throughput information for the low-resolution G750L grating. Similar to previous flux calibration releases (April 2022 STAN and January 2023 STAN), this updated calibration file includes the recent improvements to the stellar atmospheric models by Bohlin et al. 2020 (CALSPECv11) along with the re-examination of the spectral flux for Vega.

The sensitivity curve for the G750L mode (with a single supported central wavelength of 7751) was derived from observations of the standard DA white dwarfs G 191-B2B, GD 71, and GD 153. At the wavelengths covered by the G750L configuration, 5240 - 10,270 Å, the improvements to the models of the primary standard stars changed their fluxes by ≲1% when compared to those from CALSPECv5, which were used in deriving the previous version of the STIS sensitivities (see Figure 1).

We note that the new sensitivity included in the recently delivered photometric throughput file (PHOTTAB) impacts the absolute flux calibration of the STIS G750L observations for both pre- and post-Servicing Mission 4 (SM4).

As usual, we advise users who require the best flux calibration to re-retrieve their observations and verify that their observations have been calibrated with the new reference file. Lastly, we remind the HST community of the STIS Flux Recalibration webpage. Users can refer to this website to obtain the latest information on the upcoming releases and the status of current and future work. We note that the timeline for the implementation of the flux recalibration is tentative and subject to change. For any questions concerning the flux recalibration updates, please contact the HST Help Desk

Updated Flux Recalibration of G750L
Figure 1. Ratio of the CALSPECv11 over CALSPEC v5 models. We note that the CALSPECv5 models were previously used to derive the sensitivities of the low-resolution modes. We show a different standard star in each panel, and label the ratio accordingly.



HST Help Desk 

Please contact the HST Help Desk with any questions.