ULLYSES Data Products

The High Level Science Products (HLSPs) in this release are combined from individual 1-D calibrated spectra obtained with the COS and STIS instruments onboard HST. Products are made using both archival HST data and new HST observations obtained through the ULLYSES program. Data products are available from this website (HLSPs and contributing data), the MAST Data Discovery Portal (HLSPs and contributing data), or directly as a High-Level Science Product collection using the DOI (HLSPs only).

Data Product Description

This section describes the structure and format of most products that will be provided in the ULLYSES project. However, not all products will appear in the early releases.

File Name Templates

The file names for ULLYSES science data products follow a naming scheme which encodes the target designation and the instruments and observing configuration(s) that contribute to the product. The names have the form:


The <telescope>, <instrument>, <opt_elem>, and <product-type> templates take names from the following table:

Description Telescope Instrument Opt-Elem Product-Type HLSP Level
Custom calibrated STIS 1D spectra hst stis g230l spec.fits 0
STIS custom calibration parameter files hst stis g230l spec.yaml 0
STIS echelle single grating, where the orders have been extracted and merged. hst stis e140h mspec.fits 1
Combined spectra, with common instrument and grating, and in some cases with different cenwave settings. hst cos g130m cspec.fits 2
cos g160m
cos g185m
cos g230l
stis e140h
stis e140m
stis e230h
stis e230m
stis g230l
stis g430l
stis g750l
Combined spectra, with common instrument, different gratings and cenwave settings, and grouped by resolution fuse fuv lwrs or mdrs cspec.fits 3
hst cos g130m-g160m-g185m
stis e140h-e230h
stis e140m-e230m
stis g230l-g430l-g750l
All instruments and settings: the SED product hst cos-stis uv sed.fits 4
cos-stis uv-opt
hst-fuse fuse-cos-stis uv-opt

High-Level Science Product Format

The High Level Science Products are in FITS format, and are organized as described in the following tables.

File Structure

Spectral data and information will be stored in two BINTABLE extensions:

Primary Header Metadata common to all spectra
Extension 1 Header Metadata specific to single-epoch spectrum
Table 1 Data Science data specific to single-epoch spectrum
Extension 2 Header Metadata specific to provenance
Table 2 Data Metadata specific to contributing spectra

Science Table Extension

Various elements of a single spectrum of M wavelength bins are stored in a single table row; each element is stored in a separate field (i.e., column). The table extension headers also contain informative metadata.

Field Name Dimensions Units Data Type
WAVELENGTH M Angstrom Array, single-precision float
FLUX M erg/cm2/s/Angstrom Array, single-precision float
ERROR M erg/cm2/s/Angstrom Array, single-precision float
SNR M Array, single-precision float
EFF_EXPPTIME M s Array, single-precision float

Provenance Table Extension

Select metadata for each spectrum that contributes to the combined spectrum in the SCIENCE extension will populate a row in the provenance table. The fields in the following table are metadata harvested from the headers of the contributing spectra.

Field Name Units Data Type
CENWAVE string
MINWAVE Angstrom double-precision float
MAXWAVE Angstrom double-precision float
SPECRES double-precision float
CAL_VER string
MJD_BEG d double-precision float
MJD_MID d double-precision float
MJD_END d double-precision float
XPOSURE s double-precision float

Spectrum Preparation

Spectra of ULLYSES targets were obtained with multiple instruments, multiple gratings, multiple settings of a grating central wavelength, and multiple telescopes. Some of the spectra are obtained with an echelle grating, some with single-order small or large aperture, and some with single-order long-slit configurations. The approach for combining data depends upon whether the input spectra share a common instrument and grating. Input COS spectra are obtained by running the instrument calibration pipelines, CalCOS. For COS, _x1d products are used, not _x1dsum products, as these are created using a linear interpolation method that introduces noise correlation between neighboring output pixels. Virtual Observatory (VO) files are used as the input data for FUSE. All input STIS CCD data, and a subset of STIS NUV-MAMA data, are obtained using custom calibration routines written by the ULLYSES team. All other STIS data are obtained by running CalSTIS.

Combining Spectra with a Common Grating

This approach applies to combining spectral orders within a single echelleogram, and to different exposures obtained with a common grating with the same or different central wavelength settings. Each input pixel measurement is treated as an estimate of the monochromatic flux at its assigned wavelength. The output flux is obtained by calculating a weighted average of all the flux measurements that fall within the output pixel's bounds. The throughput (net count rate divided by flux) times the exposure duration is used as the weighting factor for each input pixel, so that measurements derived from more counts have higher weights. Only input pixels with corresponding Data Quality (DQ) flags that are not considered serious contribute to the output flux. Figure 1 shows an example of how fluxes from two overlapping spectra are mapped to the imposed wavelength grid of the output spectrum.

The error array is calculated as the square root of the total counts that contribute to the output pixel, and the signal to noise ratio (SNR) is the ratio of the flux to the error. This method of combination avoids correlating errors in neighboring pixels, at the cost of a very small loss in spectral sampling. The SNR values are calculated per pixel.

Figure 1 — Flux from two or more spectra (blue and red circles) that fall within a wavelength bin (vertical dashed grey lines) are averaged to form the output flux (grey diamonds), with weight proportional to the relative counts within each bin. (The X's denote pixels rejected for poor quality.) The wavelength bin size is constant, with sampling equal to the coarsest wavelength sampling of all input datasets.

Combining Spectra with Different Gratings and Instruments

For all other cases spectra are spliced, meaning that:

Latest Data Release

The latest data release notes can be found here.

Download Data

Data products for are available from this website (HLSPs and contributing data), the MAST Data Discovery Portal (HLSPs and contributing data), or directly as a High-Level Science Product collection using the DOI.


A description of the ULLYSES observations and data products is given in:

For more information on how to cite ULLYSES data, see ULLYSES References.

Charting young stars’ ultraviolet light with Hubble.

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