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 (see This Data Release, below).

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

Description Instrument Opt-Elem Product-Type
STIS echelle single grating, where the orders have been extracted and merged. stis e140h mspec
Combined spectra, with common instrument and grating, and in some cases with different cenwave settings. cos g130m cspec
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 cos g130m-g160m-g185m cspec
stis e140h-e230h
stis e140m-e230m
stis g230l-g430l-g750l
All instruments and settings: the SED product cos-stis uv sed
cos-stis uv-opt
fuse-cos-stis uv-opt

Note: Not all products are included in DR-1.

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:

For DR-1, only single-epoch spectra are stored per file.

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

This Data Release

What is Being Released

This release consists of both coadded and abutted COS and STIS spectra for 115 of the approved ULLYSES targets.

What is Not Being Released

Certain planned products will be included in a future release:

The full set of targets that will be included, once all observations are complete, may be found on the ULLYSES target pages.

Spectrum Preparation

Spectra of program targets were obtained with multiple instruments, multiple gratings, and in some cases multiple settings of the grating central wavelength (i.e., a grating rotation). In future releases spectra will be combined from multiple telescopes (e.g. HST and FUSE). Some of the spectra will be obtained with an echelle grating, some with single-order small 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. Each input spectrum was obtained by running its instrument's calibration pipeline (CALSTIS for STIS data, CALCOS for COS data). The pipeline products used as input to the ULLYSES processing are the _x1d products, not the _x1dsum products for COS, as these are created using a linear interpolation method that introduces noise correlation between neighboring output pixels.

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 total gross counts from each input pixel are used as the weighting factor, 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:

Known Issues and Caveats

  1. The selection of the transition wavelength from one abutted spectrum to the next is defined in this release to be the middle of the overlap region. In future releases the transition wavelengths will be selected such that the signal-to-noise ratio in the input spectra is very similar.
  2. The computation of the error arrays includes contributions from the flat-field uncertainty and background contribution for COS data, but only from the celestial target+background counts for STIS data. This calculation is a good approximation in the high signal limit which applies to nearly all data in DR-1.
  3. For STIS data, all Data Quality (DQ) flags in the standard CALSTIS Serious Data Quality Flags (SDQFlags) definition are used with the exception of DQ=16 (pixels with dark rate >5-sigma times the median dark level) as this DQ flag was not deemed to have significant impact on the computed flux.
  4. The output flux is weighted by:
    • GCOUNTS for COS
    • GROSS counts times the expusure duration for STIS. For STIS the GROSS counts arrays include some negative values due to the subtraction of the scattered light contribution in the CALSTIS pipeline. Therefore, for STIS data only, we currently weight input flux by absolute gross counts multiplied by the exposure duration.
  5. For COS G130M/1096 data of FP-POS setting 1 only: a flux calibration issue at the reddest ~1.5 Angstroms of both FUVA and FUVB segment spectra causes incorrect flux measurements. As a temporary solution until new sensitivity curves are implemented, the discrepant flux is currently being excluded from coaddition.

Download DR-1 Data

Included in DR-1 are 53 massive stars located in the LMC and 62 massive stars located in the SMC. Data products for these targets 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:

Charting young stars’ ultraviolet light with Hubble.

Space Telescope Science Institute
3700 San Martin Dr.
Baltimore, MD

Questions? Contact the
STScI Help Desk