June 2025 STAN

In this STAN, the STIS team presents updates on the STIS flux recalibration of the M-modes (release 4), as well as information on the relevant STIS presentation that will be given at the 246th American Astronomical Society (AAS) meeting in Anchorage, Alaska.

Updated STIS Flux Calibration (Release 4)

The STIS team recently delivered a new set of reference files, including new photometric throughputs (PHOTTABs) for the first-order MAMA and CCD spectroscopic M-modes. The new delivery is part of the larger STIS flux recalibration effort to incorporate the recent improvements to the atmospheric models for the standard stars (Bohlin et al. 2020, CALSPECv11), along with re-examination of the Vega flux.

M-modes

With a few exceptions, the spectrophotometric calibration of the first-order medium resolution (M-mode) gratings rely on sensitivities determined by comparison with CALSPEC models of the primary white-dwarf standards G191-B2B, GD 71, and/or GD 153. These sensitivities were last updated in 2005 – 2006 by referencing spectra of the standards obtained in 2000 – 2002 to version 4 (for G191-B2B and GD 153) or 5 (for GD 71) of their respective CALSPEC models (Proffitt 2006). Figure 1 shows that the updated (CALSPECv11) models can differ by up to ~3% from their previous versions over the wavelength range covered by the STIS M-mode gratings. 

Generally, throughputs for each cenwave were derived by first computing the sensitivity curve from each individual standard star, merging the sensitivity curves when multiple observations of one or more standard stars were present, and finally, fitting a smooth cubic spline to this merged sensitivity curve. While this process is similar in principle to recalibration efforts of other first-order STIS spectroscopic modes (see April 2022 STAN, and May 2023 STAN), the sensitivity calibration of the M-modes is complicated by having relatively narrow wavelength ranges and interference fringes at the longest wavelengths. Our detailed consideration of these effects will be discussed in an ISR currently under internal review (a future STAN will announce its final publication). The sensitivity curves delivered in the new photometric throughput files (PHOTTABs) result in updated absolute flux calibration for the STIS medium resolution observations, including pre- and post-Servicing Mission 4 (SM4; in 2009).

Through a series of three reference file releases, the STIS team has updated the calibration of the first-order low-resolution (L-mode) gratings, high-priority echelle modes, and NUV imaging modes. This fourth release includes updates to all MAMA and CCD M-modes with the necessary data to perform an updated calibration. In Table 1, we present the complete list of affected cenwaves. Cenwaves lacking the necessary calibration data were left unchanged and are indicated in bold. In Table 2, we show the affected PHOTTAB files and new file names of the updated calibration that were delivered to CRDS. 

All of the archival data impacted by this new release have been reprocessed (completed on May 9th, 2025). Users who require the most accurate flux calibrations are advised to re-retrieve their science observations and confirm that the calibrated products were made using the latest reference files (see Table 2). For any questions regarding the updated reference files delivered as part of this recalibration release, please contact the HST help desk (link: https://stsci.service-now.com/hst). Please consult the STIS Flux Recalibration webpage (https://www.stsci.edu/hst/instrumentation/stis/flux-recalibration) for any additional information on the status of ongoing and future flux calibration updates. 

Additional details on these recent updates and the overall STIS flux recalibration effort will be presented in an iPoster at AAS 246 (Session 306), as described in the article below.

STIS Presentation at the 246th Meeting of the American Astronomical Society

The STIS team will present an iPoster at the 246th Meeting of the American Astronomical Society (AAS) in Anchorage, AK, during the week of June 9 - 12, 2025.

Recent Improvements to the Calibration of HST/STIS Spectroscopic Modes
Matthew Siebert, TalaWanda Monroe, Joleen Carlberg, Alex Fullerton, Matilde Mingozzi, Amy Jones, Svea Hernandez, Serge Dieterich, Leonardo Dos Santos, Sean Lockwood, Daniel Welty, Robert Jedrzejewski

The HST Space Telescope Imaging Spectrograph (STIS) team has been working to update the absolute flux calibration of STIS spectroscopic and imaging modes to benefit from improved stellar atmospheric models of the primary HST standard stars published in CALSPECv11. So far, the STIS team has released 3 sets of reference file updates that prioritized the most used spectroscopic modes. We will summarize these improvements and describe new methods developed for the calibration of observations taken pre-Servicing Mission 4 (SM4; in 2009), and for the derivation of new secondary echelle mode spectral traces. New updates will be presented that are planned to be incorporated in our team’s 4th release of reference files. These updates will include recalibrated throughputs for the STIS first-order medium-resolution modes and recalibrated throughputs, blaze shift coefficients, and secondary mode spectral traces for all echelle E230H cenwaves. Furthermore, we will discuss plans for the creation of a new type of pipeline reference file for echelle blaze shift coefficients.

The iPoster also discusses ongoing work for improving the STIS CCD wavelength calibration accuracy for the CCD spectroscopic modes at the nominal and E1 pseudo-aperture locations. There is a slow, long-term rotation of the CCD detector, recently confirmed in both STIS spectroscopic and imaging modes, causing an increase in the zero point offsets at E1. The team has now evaluated the impact of this rotation on wavelength calibration accuracy and will provide updates on revisions to the CALSTIS pipeline aimed at improving the wavelength calibration.