About this Article
HSLA Team - 2025 Nov 10
The new Hubble Spectroscopic Legacy Archive (HSLA) provides scientifically validated coadded spectra (coadds) 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 are newly recalibrated datasets.
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 closest Unified Astronomy Thesaurus (UAT) concept. 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 HSLA has spectra for over 3000 individual stars and over 2000 individual galaxies and a full listing of classifications is available at the HSLA website.
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, redshift or radial velocity 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, HST Mission Search Form or via astroquery. The HSLA website gives users details on which option is preferred for different use cases. 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.
An example aspec file for b Pictoris is shown in Figure 1. β Pictoris is a well-known nearby A-star that possesses a debris disk and two giant planets. Additionally, the stellar spectrum shows evidence for both stable and time variable absorption features due to exo-comets that break up and evaporate. The aspec file covers multiple COS and STIS gratings from many individual observing programs and over four orders of magnitude in flux.
The HSLA products work best for astrophysical sources that have compact angular sizes and are not variable, which ensures flux continuity across different gratings and apertures over the lifetime of COS and STIS. In those cases, the coadditions should match each instrument’s requirements for flux and wavelength repeatability and accuracy.
The automatically-generated HSLA products do not fit all science cases or requirements, so an integral part of the project is to provide user-friendly access to custom coaddition for situations when more individualized approaches might need to be taken. The HSLA project has a notebook that introduces users to custom coaddition of multiple HST observing programs and another that introduces users to the multiple COS lifetime positions and how that may impact the line spread function of their spectra. An example from the HSLA Introduction Notebook is shown in Figure 2.
(top) Signal-to-noise (S/N) achieved by each of the two gratings (orange and blue curves) that participated in a quicklook aspec spectrum (green curve). The default grating priority results in lower S/N data being used between 1650 and 1720 Å.
(bottom) A user adjusted grating priority table changes the transition wavelength for a custom aspec spectrum that ensures the higher S/N data are used in the overlap region. This is one of several ways in which users can explore the HSLA Introduction Notebook that provides step-by-step guides on creating custom coadded datasets, enabling users to tailor spectral data combination to fit their unique research needs.
HSLA is seamlessly integrated into the Hubble MAST search form and the MAST Portal (See Figure 3) as a standard component of data downloads from the archive. Researchers looking for HSLA data can access them by using the Hubble Search form to select individual objects or discover objects of different classifications. The MAST Portal is also useful for searching for different classes of objects by using the Advanced Search page, setting Project=”HSLA” and using the Target Classification column to filter desired object types, with detailed instructions available at the HSLA website. The most efficient approach to downloading a large number of spectra (such as getting all the spectra of objects classified as “Stars”) would be to use astroquery.mast, a Python package that is part of astropy. Detailed instructions on how to use astroquery for large numbers of targets are present in the HSLA introductory Notebook in the HST Jupyter Notebook repository.
For comprehensive details on HSLA, please refer to the HSLA webpage or HSLA Instrument Science Report.
(top) A snapshot from the Hubble MAST Portal showcasing the integration of the Hubble Spectroscopic Legacy Archive (HSLA) products using the Advanced Search page. In this example the filters for “Project” and “Target Classification” are used to select all stars in the HSLA, a total of 3891 unique targets. For more instructions on how to perform searches, visit the HSLA webpage.
(bottom) A snapshot of the results from the above search, focusing on the “Target Name”, “Target Classification”, and “Observation ID” result columns. The Target Classification column provides the four methods of identifying classes of targets.