June 2026 STAN

In this STAN, the STIS Team presents the fifth release of the updated STIS flux calibration, in addition to an instrument science report on a new tool for barycentric corrections with HST/STIS data.

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Updated STIS Flux Calibration (Release 5)

The STIS team recently delivered a new set of reference files, including new photometric throughputs and blaze shift coefficients (PHOTTAB), ripple tables (RIPTAB), spectral traces (SPTRCTAB), and extraction parameters (XTRACTAB). The new delivery is part of the larger STIS Flux Recalibration effort to incorporate the most recent improvements to the atmospheric models for the standard stars (Bohlin et al. 2020, CALSPECv11), along with the re-examination of the Vega flux.

This fifth release includes three different updates to the echelle modes affecting both pre- and post-Servicing Mission 4 observations (SM4; in 2009): (1) new photometric throughputs and blaze shift coefficients for post-SM4 E230H data (24 total modes) (2) new characterization of the spectral traces for the secondary E230H modes (16 total modes), (3) minor adjustments to the background extraction regions of E230H and E230M edge orders to improve data quality flagging. The affected central wavelength settings and reference file updates are shown in Tables 1-3 below.

Affected datasets were reprocessed on May 18, 2026, and additional info on the newly delivered reference files can be found on CRDS. We encourage users to visit the STIS Flux Recalibration webpage to find additional information on current progress and future planned releases.

 

Post-SM4 updates:

Similar to previous updates to the E230H throughputs (July 2023 STAN), the sensitivity curves and blaze shift coefficients for E230H modes were derived through the analysis of the standard DA white dwarf G191 B2B observations. The STIS E230H settings provide high-resolution echelle spectra with coverage between ~1600 and 3100 Å. The newly derived throughputs for these modes were obtained following a similar approach as that described in STIS ISR 2022-04 . The post-SM4 throughputs for the E230H modes were last updated using the CALSPECv7 spectral models (STIS ISR 2012-01). The updated throughputs relying on the CALSPECv11 models increase the NUV fluxes by ~3% around 1700 Å, and ~2% at 3000 Å (Figure 1/left), and the new blaze shift coefficients significantly improve agreement in the order overlap regions, particularly for more recently acquired observations (Figure 1/right). Additionally, the previous version of the NUV PHOTTAB did not flux calibrate several detector edge orders (21 total across the affected central wavelength settings), and the recently delivered PHOTTABs now include these orders in the calibration. In this release, we have updated both NUV PHOTTAB reference files which are nearly identical but differ in their USEAFTER dates to accommodate a breakpoint in their E230M/1978 blaze shift coefficients.

Detector	Updated Setting	Old PHOTTAB	New PHOTTAB	Old RIPTAB	New RIPTAB	USEAFTER Date
NUV-MAMA	E230H (Primary: 1763, 2013, 2513, 2762, 3012. Secondary: 1813, 1863, 1913, 1963, 2063, 2113, 2163, 2213, 2313, 2363, 2413, 2463, 2563, 2613, 2663, 2812, 2862, 2912, 2962)	95118573o_pht.fits	a5e19538o_pht.fits	77o1827bo_rip.fits	a5e1953bo_rip.fits	May 11, 2009
NUV-MAMA	E230H (Primary: 1763, 2013, 2513, 2762, 3012. Secondary: 1813, 1863, 1913, 1963, 2063, 2113, 2163, 2213, 2313, 2363, 2413, 2463, 2563, 2613, 2663, 2812, 2862, 2912, 2962)	9511857eo_pht.fits	a5e1953do_pht.fits	77o1827bo_rip.fits	a5e1953bo_rip.fits	March 31, 2018

 

Table 1. Replaced and recently released photometric throughput files and ripple tables for the spectroscopic modes included in the fifth flux recalibration release.

 

Updated Spectral Traces:

The shape of the spectral traces for the different echelle orders vary with grating and wavelength, and prior to the start of of the STIS flux recalibration effort, were last delivered in 2006.  At that time, only the primary echelle modes had traces that followed the curvature seen in the data. The STIS echelle secondary mode traces, on the other hand, had been defined as straight lines, which in some cases were not ideal for accurately extracting the flux of a given order (particularly on the edges, where many orders tend to exhibit more complex shapes). Information on the spectral traces, or 1-D spectrum trace table, is stored in the SPTRCTAB reference file and used during the X1DCORR calibration step to locate and extract 1-D spectra for each echelle order. In Figure 2/left we show an example of a previous straight-line trace in blue, labeled “Reference SPTRCTAB”, for order 339 of the secondary mode E230H/2313. Using the Gaussian process trace derivation method described in STIS ISR 2024-03 and used in our third release, we have re-derived traces for 16 total E230H secondary modes, leading to major improvements particularly at the order edges (see Figure 2/left, orange curve). We highlight that while pre-SM4 throughputs have not been changed, these updates significantly improve their order shapes (see Figure 2/right). We note that the improvement is in shape only and these observations are not fully on the CALSPEC v11 system yet. New pre-SM4 throughputs will be delivered in a future release. For post-SM4 observations, the updated trace shapes have been incorporated in the sensitivity derivations above.

Detector	Updated Setting	Old SPTRCTAB	New SPTRCTAB	USEAFTER date
NUV-MAMA	E230H (Secondary: 1813, 1863, 1913, 1963, 2063, 2113, 2163, 2213, 2313, 2363, 2413, 2563, 2613, 2663, 2862, 2962)	77o18279o_1dt.fits	a5e1953fo_1dt.fits	October 1, 1996
NUV-MAMA	E230H (Secondary: 1813, 1863, 1913, 1963, 2063, 2113, 2163, 2213, 2313, 2363, 2413, 2563, 2613, 2663, 2862, 2962)	77o1827jo_1dt.fits	a5e19539o_1dt.fits	June 5, 2000

 

Table 2. Replaced and recently released 1-D spectrum trace tables (SPTRCTABs) for the secondary modes included in the fifth flux recalibration release.

 

Edge order background regions:

In previous versions of the extraction slit parameters reference file (XTRACTAB), background regions for a given spectral order did not originally have a central wavelength setting dependence. This format precluded the ability to define order-specific background regions since the parameters of one order could affect same-numbered orders present in other central wavelength settings. In this release we have updated this reference file to have central wavelength dependency and have used this functionality to improve the background parameters of echelle orders at the edge of the detector for post-SM4 observations.

It was noticed in previous reference file deliveries that for echelle orders that are located on the bottom and top edges of the detector, one of the two pre-defined background regions falls off the detector resulting in the assigning of data quality (DQ) flag 2048 (>30% of background pixels rejected by sigma-clip). However, comparisons of the extracted fluxes of CALSPEC stars to the models confirmed that the high quality of the echelle scattered-light removal algorithm (which precedes the local background subtraction) resulted in accurate flux calibration even in regions that were flagged with DQ=2048. By setting the off-detector background extraction width to 0 pixels (effectively ignoring this region), the extracted flux is identical to the two-background region case but the DQ = 2048 flags are no longer set. The affected NUV central wavelength settings, reference file, and specific orders are shown in Tables 3 and 4. These updates only apply to post-SM4 era observations. Pre-SM4 era echelle spectra underwent monthly offsetting in the spatial and spectral directions, therefore, edge orders present on the detector could change from observation to observation.

Detector	Updated Setting	Old XTRACTAB	New XTRACTAB	USEAFTER date
NUV-MAMA	E230H (Primary: 1763, 2013, 2513, 3012. Secondary: 1813, 1863, 2063, 2363, 2463, 2563, 2613, 2663, 2713, 2812, 2862)	l3m1437to_1dx.fits	a5e1953co_1dx.fits	May 11, 2009
NUV-MAMA	E230M (Primary: 2707. Secondary: 2415)	l3m1437to_1dx.fits	a5e1953co_1dx.fits	May 11, 2009

 

Table 3. Replaced and recently released slit extraction parameter trace table (XTRACTAB) for modes included in the fifth flux recalibration release.

Grating	Central Wavelength	Orders Updated
E230H	1763	407
E230H	1813	459
E230H	1863	386
E230H	2013	359
E230H	2063	350, 399
E230H	2163	336
E230H	2113	343
E230H	2263	322
E230H	2313	316
E230H	2363	347
E230H	2463	297
E230H	2513	291, 324
E230H	2563	286, 318
E230H	2613	281
E230H	2663	306
E230H	2713	272, 300
E230H	2812	263
E230H	2862	258
E230H	2912	278
E230H	3012	245
E230M	2415	73
E230M	2707	66

 

Table 4: NUV echelle orders affected by updates to XTRACTAB background regions to improve DQ flagging.

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New STIS Instrument Science Report

We are pleased to announce the publication of a new STIS Instrument Science Report (ISR). The full list of STIS ISRs can be accessed here: https://www.stsci.edu/hst/instrumentation/stis/documentation/instrument-science-reports

ISR 2026-02: Barycentric Corrections for HST/STIS Data

Joshua Lothringer, Joleen Carlberg, Sean Lockwood

We describe stistools.barycentric correction, a new Python utility for calculating barycentric timing corrections for HST/STIS observations. This tool replaces the deprecated stsdas.hst_calib.stis.odelaytime IRAF function that was previously used for HST barycentric corrections. Our new utility uses astropy for conversion between time formats and standards and introduces a new way to calculate HST’s position through JPL Horizons, replacing the need to download separate HST orbital ephemeris files. Here, we describe the methods used in the new utility, the tests that were carried out to verify its accuracy, and explain some of the complexities involved in determining light travel times to accuracies down to a millisecond for HST. We also summarize the current understanding of the absolute accuracy of STIS time stamps.