About This Article
In this STAN, the STIS team announces the release of a new Hubble Spectroscopic Legacy Archive that encompasses all publicly available COS and STIS spectra including more than 60,000 datasets and over 6,000 individual targets. The STIS team also presents four new Instrument Science Reports that were published between July and September.
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Announcement of the New Hubble Spectroscopic Legacy Archive (HSLA)
The new Hubble Spectroscopic Legacy Archive (HSLA) provides scientifically validated coadded spectra of individual targets that have been observed with the Cosmic Origins Spectrograph (COS) and the Space Telescope Imaging Spectrograph (STIS) over their operating lifetime. HSLA uses data available in the Mikulski Archive for Space Telescopes (MAST) and automatically produces coadds whenever new data become publicly available, or when there is newly recalibrated data.
A key feature of the new HSLA is that it automatically defines individual targets, groups multiple observations of a single target into associations, and produces a classification for each target. Target associations make use of the dataset coordinates accounting for proper motions, and uses SIMBAD, NED and the Phase II observing proposals to determine which datasets should be associated with each unique target. Then, using the SIMBAD, NED, or Phase II keywords, a detailed classification is determined for an object to aid in the spectroscopic study of classes of astrophysical objects. The classifications consist of three tiers of detail, mapped to the Unified Astronomy Thesaurus (UAT). For example, the HSLA target Markarian 817 is classified at Tier 1 as a Galaxy, Tier 2 as an Active Galaxy, and at Tier 3 as a Seyfert. It corresponds to the UAT object Seyfert Galaxies (1447). The new HSLA facilitates scientific analysis and creation of diagrams similar to that in Figure 1, which shows the galactic coordinates of all COS and STIS spectra of white dwarfs that cover two silicon (Si) lines that can be used to study the local ISM (Si II 1261 Å) or the accretion of rocky material onto white dwarf atmospheres (Si II 1265 Å). In total, the HSLA contains over 800 white dwarfs (WDs). In addition, the HSLA has spectra for over 3000 individual stars and over 2000 individual galaxies.
Coadded spectra with rigorous input criteria are produced for each observing mode, and in the case of COS far-ultraviolet observations for each lifetime position (LP). Additionally a quicklook spectrum spanning the entire wavelength range covered by the observations is produced by abutting the spectra from a selection of individual modes. An example HSLA composite STIS and COS spectrum of Beta Pictoris is shown in Figure 2, covering a wavelength range from 1135-3095 Å and over four orders of magnitude in flux.
For each individual target HSLA also provides a human-readable metadata file with key information that can be used in searches or for further exploration of the data. The metadata file includes the target name and unique identifiers, the target coordinates in J2000, information on the target's name and classification information, and a summary of the programs and instrument modes that are included in the target association.
HSLA data products, including quicklook coadds (_aspec files), coadded single grating products (_cspec files), metadata files (_metadata files), and code output logs (.trl files) are available at the MAST Portal, the HST Mission Search, or via astroquery. Since the HSLA is fully automated, it will be updated routinely as new HST spectroscopic data are taken or if data are reprocessed with improved calibrations. Data access may change with time and users are encouraged to visit the HSLA Webpage for the latest details on how to access data.
The coadd code and its HASP/HSLA wrapper are available for public use in Space Telescope’s GitHub Organization (coadd: ULLYSES repository, wrapper: HASP repository). Both codes are scripted in Python (compatible with versions 3.9 and above) and are installable using pip. Two notebooks specifically address the HSLA data products and the challenges inherent in combining data from observing programs that are separated in time by many years.
The first notebook, "Introduction to the HSLA Data Products and Tools," explores the standard HSLA data files returned by MAST. It provides a simple example of a custom coadd, demonstrating how to adjust the logic by which the abutting routine decides which coadded spectra (_cspec files) to use for each wavelength region.
The second notebook, "Combining COS Data from Multiple Lifetime Positions and Central Wavelengths," explores how the COS line-spread function (LSF) varies with LP and CENWAVE. Because the HASP/HSLA script sums all of the spectra from a particular grating without regard to LP or CENWAVE, its final products may not have the highest-possible spectral resolution. The notebook presents several techniques for dealing with these effects.
For comprehensive details, please visit the HSLA webpage or the HSLA Summary ISR.
Recent STIS Documentation
We are pleased to announce the publication of four new STIS Instrument Science Reports, the full list of STIS ISRs can be accessed here: https://www.stsci.edu/hst/instrumentation/stis/documentation/instrument-science-reports
ISR 2025-02:Updated Sensitivities of the Five STIS L-mode Gratings
Amy M. Jones, Svea Hernandez, Joleen K. Carlberg, Daniel Welty
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.
ISR 2025-03: Recalibrating the Sensitivities of the STIS First-Order, Medium-Resolution Modes
Alex Fullerton
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.
ISR 2025-04: Uncertainties in Low-Count STIS Spectra
Joshua D. Lothringer, Leonardo dos Santos, Joleen Carlberg, Sean Lockwood, Jacqueline Brown
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.
ISR 2025-05: Status of the STIS Auto-wavecal Exposures
D. Welty and S. Lockwood
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.