About this Article

Background

A new cross-mission flux calibration working group was established at the Space Telescope Science Institute (STScI) in November 2021. The main charter for the group is to coordinate the flux calibration of all instruments of the three major missions at STScI: Hubble Space Telescope (HST), James Webb Space Telescope (JWST), and the Nancy Grace Roman Space Telescope.

The group, led by A. Calamida, includes flux calibration representatives from all the instruments across the three missions, together with their team managers and deputies, for a total of about 30 members. Meetings are held approximately every two months and focus on a specific flux calibration issue, with time for general discussions.

In more detail, the cross-mission flux calibration working group is currently focusing its effort on the following important objectives.

• Improve cross-instrument coordination, the quality of the calibration itself, and the efficiency of the work. Until now, each HST instrument performed its own flux calibration with its own software tools. However, several aspects are common to the different HST instruments, as is the software needed to perform a precise flux calibration. Leveraging these commonalities will provide an effective means of better addressing all calibration aspects, resulting in an improved end product. The cross-mission flux calibration working group will serve as a bridge between the HST instrument teams to ensure effective cross-communication between their respective calibration experts. For instance, after the update of the primary HST standard white dwarf models and the reference flux of Vega in 2020 (Bohlin et al. 2020, AJ, 160, 21), all HST instruments required an update to their calibration. The cross-mission calibration group is currently facilitating this effort, now complete for the Wide Field Camera 3 (WFC3) and the Advanced Camera for Surveys (ACS), with the Cosmic Origins Spectrograph (COS) and the Space Telescope Imaging Spectrograph (STIS) ongoing, to ensure that all HST instruments are properly calibrated on the same photometric system.
• Coordinate JWST flux calibration with HST and Roman. JWST flux calibration for its different instruments is managed by the telescope's absolute flux calibration working group, led by Karl Gordon. However, the participation of the JWST calibration group members in the cross-mission calibration group is fundamental to ensure the consistency of HST and JWST calibration and to coordinate the development of tools useful to both missions. For instance, STIS/HST spectroscopy of all the JWST calibration stars have been or will be obtained for all the JWST calibration stars, and additional STIS/HST and WFC3/HST observations will be obtained to support the calibration of the JWST Time Series Observations data. Furthermore, the participation of the Roman representatives to the cross-mission calibration working group is vital to establish the foundation of this future telescope flux calibration.
• Support the improvement of the CALSPEC database of standard stars. In particular, a systematic validation of the stability of all the CALSPEC stars is underway with Transiting Exoplanet Survey Satellite (TESS) observations. Many of these stars have TESS light curves and a few were found to be variables with amplitudes above 0.25% at one sigma (Mullally et al. 2022, AJ, 163, 136). A second TESS proposal has been submitted to monitor stars with existing light curves and expand the number of CALSPEC stars having TESS light curves. The information on the star stability or variability will be added to the CALSPEC database.
• Enhance transfer of knowledge. The cross-mission calibration working group will ensure that the methodology and any updates to the HST, JWST, and later Roman, flux calibration are made available to the Institute staff and the external users. This goal will be achieved by publishing flux calibration papers, by improving the current documentation and web pages, and presenting contributions to workshop and conferences, such as AAS and IAU. The publication of articles describing flux calibration methods has been a long tradition at STScI, and the most recent contributions include a manuscript on the new time-dependent WFC3-UVIS photometric calibration and the calibration of the WFC3-IR detector (Calamida et al. 2022, AJ, submitted), another on the design and standard stars for the JWST absolute flux calibration program (Gordon et al. 2022, AJ, in press), and another one detailing a new CALSPEC based comparison of the HST and Spitzer IRAC calibration (Bohlin et al. 2022, AJ, submitted).

In summary, the cross-mission flux calibration working group will contribute to improving and homogenizing the absolute flux calibration of all instruments on the STScI telescopes, HST, JWST, and the future Roman.

The links below provide more details about the flux calibration of the different telescopes/instruments:

• WFC3 photometric calibration
• STIS calibration
• STIS flux recalibration
• ACS zeropoints
• COS sensitivity
• JWST data absolute flux calibration

The new cross-mission flux calibration working group welcomes feedback from observers. Please send any comments or suggestions to the helpdesk, who will forward the input to the group members.