ASTRAL is a Hubble Space Telescope (HST) Large Treasury Project, whose aim was to collect high-quality ultraviolet spectra of representative bright stars utilizing the high-performance Space Telescope Imaging Spectrograph. In Cycle 18 (2010–2011), ASTRAL focused on eight iconic late-type stars, devoting 146 HST orbits to the purpose. In Cycle 21 (2013–2015), the program shifted gears to the warm side of the H-R diagram, to capture 21 diverse early-type objects with an allocation of 230 orbits. Main objective was to record the targets — including well-known bright stars like Procyon, Betelgeuse, Sirius, and Vega — with broad uninterrupted UV coverage (1150–3100 Å) at the highest signal-to-noise and highest echelle spectral resolution achievable within the alloted orbits and observing constraints. These UV "atlases" have enormous interpretive value in their own right, and complement efforts from ground-based observatories, which now routinely achieve comparably high resolution and S/N in optical and near-infrared spectra of bright stars.

Broad ultraviolet coverage was achieved by splicing together echellegrams taken in multiple FUV (1150–1700 Å) and NUV (1600–3100 Å) grating settings of STIS. The observing strategy was designed to maximize S/N by spreading the total desired exposure depth in each setting over 2–5 separate "visits" of up to four orbits each, in a series of independent relatively short integrations (1–3 kiloseconds duration). Because of a slight randomness in the STIS grating positioning mechanism, and because of the changing projected velocity of the target (owing to telluric and spacecraft motions), each independent exposure will shift slightly on the detector and thus experience a different "fixed pattern noise." Combining the independent spectra after the fact mitigates these systematics, improving S/N. In addition, the observing sequences were designed to have at least one exposure of each distinct setting immediately after a target-centering "peak-up" so that the velocity zero point of the echellegram, which can be affected by thermal drifts, would be as accurate as possible. Other exposures of that type can be registered to the reference observation by cross-correlation. Furthermore — if practical, depending on target brightness — a few exposures in each sequence were taken through the photometric aperture (0.2"×0.2"), and again close to a peak-up, to ensure that the radiometric scale of the final spliced spectrum would be as close to the true absolute level as practical.

Post-processing begins at the level of the calstis pipeline "x1d" file, a tabulation of extracted wavelengths, flux densities, photometric errors, and data quality flags for the up to several dozen orders of the particular grating setting (e.g., E140M-1425, where the first part is the mode and the second is central wavelength in Å). The x1d file contains at least one — sometimes several — sub-exposures, which were treated as separate observations. The initial processing includes a post facto correction for subtle wavelength distortions identified in a previous study of the STIS dispersion relations ("Deep Lamp Project"); a Bayesian reformulation of the photometric error; and more aggressive edge trimming of E230 settings to avoid unflagged "dropouts" that sometimes occur at the beginning of the low orders. Also, spurious tilts of the spectra, due to an unfortunate interaction between narrow slits and the telescope "breathing," are corrected, if necessary. The x1d orders then are merged, averaging the overlap regions weighted by the individual sensitivity functions s _λ, but accounting for bad pixels and wavelength gaps. During this process, an "active blaze correction" determines an optimum blaze shift to balance the fluxes in the overlaps between adjacent orders.

Next, a series of different layers of coaddition and splicing are applied to the sets of order-merged 1D spectra of each object:

STAGE ZERO — sub-exposures, if any, of an observation are combined. The individual spectra are aligned in velocity by cross-correlating against the observation with the highest apparent throughput, determined by comparing the total net count rates integrated over the central zone of the echelle format. With the narrow slits used extensively in ASTRAL Hot-Stars, the effective throughput can vary significantly (up to ~30%) during a single orbit owing to telescope breathing. Next, the sub-exposures are interpolated onto the wavelength scale of the reference spectrum; scaled to the reference exposure in flux density according to the total net count rates; then coadded, weighting by the total net counts, but taking into account bad pixels and gaps. Resulting files are called "o-type" and have the same rootname as the original observation, e.g., obkk52040.

STAGE ONE/TWO — Same-setting exposures of an object taken in different orbits of a visit, or in different visits, are combined. This is a hybrid of the Stages ONE and TWO described in StarCAT, to take advantage of the specially designed observing strategy of ASTRAL. As in Stage ZERO, the cross-correlation alignments are relative to the exposure exhibiting the highest apparent throughput. Also, the individual observations are scaled to the reference exposure according to flux ratios determined by a global average over high S/N intervals. Again, the weighting was by the total net counts, which allows for possibly different integration times in an exposure set (or use of different apertures in different visits). Resulting files are called "E-type," and have an appended aperture code and MJD date range to reflect the diversity of the constituent exposures, e.g., "E140M-1425_020X020_55543-55554."

STAGE THREE — Different wavelength segments of an object are spliced together to produce a seamless spectrum covering the full FUV + NUV range. As in the other steps, wavelengths of adjacent segments are aligned by cross-correlation. A "bootstrapping" calibration procedure takes advantage of the intentional broadly overlapping spectral coverage to refine the velocity zero point and the absolute flux scale (by pair-wise comparisons of the overlap regions in velocity and flux). Resulting file is called "U-type," e.g., "UVSUM_1X_55543-55554." The "UVSUM" part signals that a multi-wavelength splice was involved; the middle numeral indicates a particular grouping of spectra that were spliced; the adjacent letter tells whether the spectra all were medium resolution ("M"), all were high resolution ("H"), or mixed ("X"; latter is the case for all but one of the ASTRAL Cool Stars targets, but several of the Hot Stars are purely M and a few are purely H); and the trailing date range summarizes the minimum and maximum starting MJDs of the spliced group.

NOV 2018: datasets reprocessed (mid-2017) with new CALSTIS reference files (created by author). Improvements made to post-processing protocols (2018). New data product for sub-exposures.

NOV 2018: added two STIS Calibration White Dwarfs (G191-B2B and BD+28°4211).

JUN 2017: added four metal-poor turn-off stars from GO-14161 (Peterson, PI); and sunlike α Cen A & B from various historical and current STIS programs.

As of 2015 January, the STIS observations for the Hot Stars episode of ASTRAL had been successfully completed.

If highest resolution is desired, say for ISM studies, avoid top-level spliced (U-type) spectrum below 1350 Å, and instead consult E140H-1291 co-add one level down (if available).

There remain spurious defects in STIS spectra, especially noticeable in Hot Stars targets with long stretches of smooth continuum. Most appear as weak, narrow "emission lines," mainly in NUV. They apparently result from flat-field issues. Occasionally defects occur in FUV where local areas of sensitivity degradation on detector were not flagged: these appear as broad (few Å) shallow absorptions. Also, new flag (EPSILON=450) marks places between adjacent echelle settings where difference between co-added fluxes was larger than 3 times average photometric noise. Flag introduced to help identify possible processing glitches.

Because of necessity to de-tilt and rescale many sub-exposures, owing to "breathing" effects, ASTRAL Hot Stars material not well suited for temporal studies of continuum variations (0.2″×0.2″ Cool Stars o-type exposures should be better in time-domain regard). Also, fact that exposures of each setting were scaled to observation with maximum throughput could introduce bias, if source had been variable over span of STIS visits. Thus, absolute fluxes of ASTRAL spectra should be viewed cautiously; although relative flux distributions should be reliable.

Contents of the Catalog

There are several kinds of processed data. First, each page displays a PNG preview of the coadded (o-type or E-type) or spliced (U-type) spectrum. Second, at the top level, a graphical timeline of all the observations is provided, color-coded by mode. Third, FITS files of the fundamental data are linked for downloading. Fourth, for the top-level datasets, an "ETC-ready" ASCII file is available. It is a highly streamlined version of the final spectrum, specifically intended to be used with an HST Exposure Time Calculator (or any that adheres to the HST ETC format standard), but NOT SUITABLE FOR ANALYSIS (see: ETC Summary). A description of the streamlining procedure and numerous warnings concerning the use of these ETC files can be found in ETC Summary. Finally, links to flat ASCII versions of all the coadded data files can be found here.

Fits File Data Formats. The "o-type" files (individual observations) have basic header information in 0-th extension describing target, exposure properties, and splice points from order-merging; and one or more trailing data extensions. If observation had only single sub-exposure, there is one extension (EXTEN=1), containing WAVE – wavelength (Å); FLUX – flux density (erg/cm²/s/Å); ERROR – photomeric error (same units as flux density); and DQ – data quality (0 for no issues; higher values to flag various conditions: bad pixels, camera blemishes, gaps, and so forth). If two or more sub-exposures, EXTEN=1 holds Stage ZERO co-added spectrum; subsequent extensions have parameters of individual sub-exposures: sub#n in EXTEN=n+1. EXTEN=0 header now contains additional information, concerning cross-correlation template and derived velocity shifts for Stage ZERO co-addition.