Instrument Science Reports

We discuss the behavior of the default “wavecal” spectra obtained together with most STIS spectroscopic exposures, which are needed for proper wavelength calibration of the science data. Because the Pt/Cr-Ne lamps used for the wavecals have been fading (especially at the shortest wavelengths), some changes in the default lamp and/or exposure time have been implemented in recent years to maintain accurate calibrations. To assess whether additional changes might be appropriate, we examine the trends in the SHIFTA1 and SHIFTA2 values derived from the wavecals (the x and y offsets of the spectral image on the detector), we re-visit the wavelength-dependent fading of the lamps, and we perform simulations to estimate the exposure times that would be needed to obtain accurate SHIFTA values. While the current wavecals do appear to yield reasonable SHIFTA, increases in the default exposure times for some of the shortest-wavelength settings would help to ensure reliable wavelength zero points as the lamps continue to fade.

We evaluate uncertainty calculations in the calstis pipeline for data in the low-count regime. Due to the low dark rate and read-noise free nature of MAMA detectors, observations of UV-dim sources can result in exposures with 0 or 1 counts in some pixels. In this regime, the “root-N” approximation widely used to calculate uncertainties breaks down, and one must compute Poisson confidence intervals for more accurate uncertainty calculations. The CalCOS pipeline was updated in 2020 to account for these low-count uncertainties. Here, we assess how STIS observations are currently affected by this phenomenon, describe a new Jupyter notebook exploring the issue, and introduce a new utility, stistools.poissonerr, to manually calculate Poisson confidence intervals for 1D STIS spectra. Additionally, we describe a related software bug in the stistools.inttag utility, which splits TIME-TAG data into sub-exposures. This newly fixed bug serves as a useful case-study for the proper use of Poisson confidence intervals.

The sensitivities of STIS first-order, medium resolution modes were redetermined from on-orbit observations and CALSPEC models (version 11) of the primary white-dwarf spectrophotometric standard stars G191-B2B, GD 71, and GD 153. The sensitivity of an additional configuration was updated by comparing observations of the secondary standard BD +75°325 with the STIS low-resolution spectrum that has been calibrated consistently with the version 11 models. The procedures used to derive the sensitivities and verify the PHOTTAB reference files prior to their activation in CRDS (on May 1, 2025) are described. Results are presented in graphical form in an extensive appendix. Issues and uncertainties are discussed briefly, along with recommendations for future work.

Re-derivation of the sensitivities of all of the Space Telescope Imaging Spectrograph (STIS) observing modes were required after major updates were introduced to the model atmospheres of the three primary standard stars. The new predicted continuum fluxes were up to 2–3% different from the models used to originally calibrate STIS. This work focuses on the re-derivation of spectral sensitivities for the five STIS low-resolution (L-mode) gratings: G140L, G230L, G230LB, G430L, and G750L, which span wavelengths from the far-ultraviolet through the near infrared. Updated photometric throughput tables were delivered to the Calibration Reference Data System (CRDS) on April 7, 2022 and April 14, 2023, which triggered a recalibration of all historical STIS datasets taken with these modes. The sensitivities derived from each of the standard stars typically agree with one another to better than 1%, though discrepancies as large as 1.5% are found in spectral regions most impacted by hydrogen absorption.

We discuss the suite of STIS calibration programs executed during HST Cycle 30, covering the period 2022 Nov 07 through 2023 Nov 05. For each of the 19 current regular calibration programs, we provide brief descriptions of the objectives, observations, analysis procedures, and results – with comparisons to the results from previous cycles and to desired accuracies, as well as references to more detailed analyses of the calibration data. Many of these calibration programs produce routine reference file deliveries or demonstrate the continuing applicability of existing reference files for processing STIS observations. This ISR provides a brief snapshot of the current instrument performance, similar to those given in annual reports for Cycles 7–10 and 17–21. Two Appendices briefly discuss the state of the onboard calibration lamps and the ongoing major effort to revise the flux calibration for the many STIS spectroscopic and imaging modes.

The STIS team re-derived on-orbit sensitivity curves for the echelle modes for post-servicing mission 4 observations using the standard DA white dwarf G 191-B2B. These new updates relied on the recent CALSPECv11 models, which introduced improvements in the fluxes of the primary standard stars of the order of ~1-3% depending on the wavelength of interest. As part of this effort, the team also released new blaze shift coefficients and echelle ripple tables. We present a detailed description of the procedures followed in the derivation of these new throughputs and the accompanying updates.

The STIS echelle gratings can be used with a variety of different central wavelength settings. “Secondary” wavelength settings, designed to cover select absorption or emission lines, have not been calibrated as precisely as their primary mode counterparts. In particular, secondary echelle mode traces (and subsequent extraction regions) have been previously defined using straight line fits to each spectral order. In this work, we define a new general method for defining echelle traces that utilizes Gaussian process regression and accounts for the detailed curvature of each order across the detector. Across a variety of echelle grating and central wavelength settings, we find that this method can improve flux throughput by ~4% especially near wavelengths located close to the edge of the detector. We have used this method to provide new traces and update reference files for 9 different echelle modes for both pre- and post-Servicing Mission 4 (SM4; in 2009) observations.

Recent improvements to stellar atmospheric models have merited updated flux calibration for high priority STIS observing modes. Specifically, in the FUV and NUV, continuum differences of 1-3% are present between the newest models (CALSPECv11) and previous models (CALSPECv04-v07). As a result of these improvements the STIS team has derived updated sensitivity curves and blaze shift coefficients for a variety of echelle modes in order to meet targeted flux accuracies. The first series of echelle sensitivity updates primarily targeted post-Servicing Mission 4 (SM4; in 2009) observations. In this ISR, we investigate instead applying a simple scaling (derived from the ratio of new vs old CALSPEC model continua) to the previously determined throughputs of STIS echelle modes. This alternative approach has a straightforward implementation and provides reasonable accuracy, especially in cases where available calibration data are lacking (e.g., pre-SM4 era). Adopting this scaling approach, we delivered pre-SM4 throughput updates for 8 echelle modes, resulting in typical improvements of 0.5-2.4% across the FUV and NUV.

For observations of supernovae, kilonovae, tidal disruption events, and other non-repeatable observations, it is important the science data is taken successfully within a specific time window. Part of obtaining that data is often centering objects in the aperture to a higher accuracy than is available from Hubble Space Telescope’s (HST’s) blind pointing. On the HST Space Telescope Imaging Spectrograph (STIS) the sequence of exposures responsible for this centering is the target acquisition or STIS ACQ sequence, and it is most often placed only at the beginning of a visit. Unfortunately, STIS ACQ sequences will fail if the observatory experiences issues locating guide stars in time for the start of the required exposures. If the guide stars are located at a later point in the visit, the remaining science exposures can be taken but the pointing might not be as accurate as is required. This work discusses both the frequency of this issue and the feasibility of placing redundant or “safety” STIS ACQ sequences in a multi-orbit visit to regain the desired pointing accuracy in an affected visit. To do so we select a subset of all 113 STIS ACQ sequences from September 2018 to September 2023 which have experienced this issue. We find that this problem occurs in ∼5% of the total STIS ACQ sequences taken during that time period, with a recent increase in the rate to ∼9% from March to September 2023. Since the observatory goes through periods of better or worse pointing performance, this recent increased failure rate is not guaranteed to continue. For those failed visits which span multiple orbits, ∼39% never obtain a lock on the guide stars and thus take no data. Of the multi-orbit visits that do recover the guide stars, the majority (∼78%) do so before the beginning of science exposures in the second orbit. We also provide advice for users on how to make a risk assessment based on the analysis presented here.

The correction formula for Charge Transfer Efficiency (CTE) that is used in the HSTCAL CALSTIS pipeline CCD data reduction has not been significantly updated since 2006. Correcting for CTE losses is crucial to the goal of 1% precision in the STIS spectrophotometric fluxes that are the basis for all HST and JWST flux calibrations. Precision in the CTE correction is especially relevant for the faintest flux standards, where the amount of correction can exceed 20%. The comparison of new datasets of STIS spectra and ACS photometry of faint stars reveals the required updates to the parameters of the STIS CTE correction formula. After replacing the original with the new parameters, the change in the spectral energy distribution (SED), i.e. the flux decrease in physical units, for a very faint star NGC2506-G31 ranges from 4 to 6% over most of the G430L and G750L spectral wavelength ranges. No observations of the faint stars with G230LB or the medium dispersion modes were made; but the CTE correction depends only on signal level and should apply to all CCD spectroscopic modes. With the new formulation for the STIS CTE correction the STIS, ACS, and WFC3 flux measures are in accord at the 1% level, not only for the primary standards and other stars in the neighborhood of V=13 and brighter, but also now for fainter stars in the V=16 range.

The G230LB grating used with STIS's CCD detector scatters red light. In red objects, the scattered light mingles with the ultraviolet signal, causing spurious short-wavelength flux and weakening absorption features. Recent calibration observations characterize the scattered light using duplicate observations with the MAMA detector and similar grating G230L. The full two-dimensional spectrum contains little helpful information to mitigate the scattered light problem. For one-dimensional, extracted spectra, the scattered light can be approximately modeled as a ramped pedestal whose amplitude is proportional to the object's V-band flux. We present formulae for scattered light corrections. For stars warmer than G0 spectral type, correction is superfluous. Off-slit-center positioning appears not to affect the properties of the scattered light. Therefore, we are able to extrapolate correction formulae for extended objects from the point source formulae. Polynomials for flux corrections due to off-center slit positioning in the 0.2 arcsec slit are also tabulated.

The data reduction of STIS raw spectra was originally designed to apply three different flat fields: pixel-to-pixel level corrections (P-flats), low-frequency corrections in scales of tens of pixels (L-flats), and time variability corrections (D-flats). However, in the case of FUV-MAMA, the L-flat reference file currently only corrects for vignetting of the G140L mode and no other low-frequency corrections are applied. In this document, we analyze calibration data obtained in Cycle 28 across the detector to test whether any uncorrected low spatial frequency variations exceed the accuracy specifications of the instrument. We find that the FUV-MAMA fluxes are mostly repeatable at different cross-dispersion positions in the detector, with the exception of G140M centered at 1567 A ̊ . We also provide recommendations of when (or ever) users should request disabling monthly offsets, which is an available but unsupported mode starting in Cycle 30.

In 2012, the blaze function shapes (normalized sensitivity as a function of wavelength) of E140M’s spectral orders began exhibiting changes that could not be accounted for with simple blaze shift coefficients in the PHOTTAB reference files. In February 2018, a special calibration program observed the HST standard star G191B2B in order to recharacterize the E140M blaze function shape and obtain a snapshot of the current sensitivity order-by-order. To best characterize the evolving shape changes across all post Servicing Mission 4 (post-SM4) data, 3 new PHOTTAB files and 2 new RIPTAB files were delivered in 2020. One pair of PHOTTAB and RIPTAB files correspond to the new blaze shape and associated blaze shift coefficients and are applied to data taken after July 01, 2016. The second RIPTAB file is associated with 2 PHOTTAB files that contain a rederivation of the original post-SM4 blaze shapes (covering data taken May 11, 2009 through July 01, 2016), with two different sets of blaze shift coefficients covering data taken before and after July 01, 2012. Two sets of blaze coefficients were needed to better calibrate the data where the shape was most actively evolving. As a side consequence of this work, spectral order 86 is newly flux calibrated for all post-SM4 data. These new sensitivity derivations benefited from the availability of new line blanketed model atmospheres of G191B2B that allowed a more robust identification of the stellar continuum in the observed data. These line blanketed models also predict continuum fluxes in the E140M bandpass that differ by a few percent relative to pure hydrogen models, and they became the new flux standard for the Hubble Space Telescope soon after the 2020 reference file delivery. Thus, all five reference files were redelivered in 2022 based on the new underlying flux model. While this ISR primarily describes the work and methods implemented for the 2020 new sensitivity curve derivations, it also describes both the blaze shift updates prior to 2018 that motivated the new observations, as well as the more recent update to the new CALSPEC v11 standard.

The STIS CCD detector suffers from charge transfer inefficiency (CTI) which can be corrected for using a pixel-based or empirical flux correction (CTI = 1− CTE). Here we present a comparison of these two CTI correction methods and compare these to the magnitudes derived from non-CTI corrected CCD images. We use data spanning 2010 to 2022 and derive photometry for the same sources for each CTI method to compare the magnitudes. We explore the absolute differences between the CTI corrected magnitudes, and their spatial, time and magnitude dependence. The offsets are smallest for the brightest stars and deviate further from zero with increasing magnitude (< 18 mag: 0.020 mag, 0.12%; 18–19 mag: 0.037 mag, 0.20%; 19–22 mag: −0.084 mag, −0.35%). Stars brighter than 19 mag are marginally over-corrected with both CTI methods. Stars fainter than 19 mag are slightly under-corrected by the pixel-based CTI method and slightly over-corrected with the empirical flux CTI method. Generally, we find that the offsets between the codes are small (< 1%), consistent with past results, and well within the quoted ∼ 5% STIS photometric errors.

The three detectors on STIS, one CCD and two MAMAs, are subject to time-dependent sensitivity (TDS) changes on both short- and long-timescales. These variations are corrected for in the STIS calibration pipeline CALSTIS with TDS models derived from spectroscopic data. In this analysis, we measure residual TDS trends in the data after these corrections are applied. We update the analysis presented in STIS ISR 2013-02 (using data from 1997 to 2012) with the goal of tracking the efficacy of these TDS corrections for data taken up to 2022. We measure aperture photometry of sources in standard stellar fields (NGC 5139 for the CCD, NGC 6681 for the MAMAs) and derive magnitude trends for each star with time. We then determine overall residual TDS effects for each detector, and by filter for the NUV and FUV MAMAs (with data in three filters each). We find roughly consistent results to those from STIS ISR 2013-02 measured over the same time period, that show magnitude trends are within the ~1% STIS flux calibration accuracy. We observe stronger negative magnitude trends (i.e., sources appearing brighter with time) when including more recent data. This implies that the TDS models are over correcting the data which could mean that the loss of imaging sensitivity is slowing at a more rapid rate than the spectroscopic TDS models predict, as determined independently for all three STIS detectors. We also measure point spread functions for each image and find no significant trends in their full-width-half-max values with time for any detector.