Data Release 4

December 14, 2021

ULLYSES Data Release 4 (DR4) consists of high-level science products (HLSPs) for 25 new ULLYSES targets and updated HLSPs for 226 targets previously released in DRs 1-3, for a total of 251 ULLYSES targets to date. Products are also provided for 8 companion targets present in STIS long-slit observations. Updated HLSPs from previous DRs now include recent COS and STIS pipeline and/or calibration improvements. HLSPs from previous DRs should be replaced with newly downloaded products that incorporate these upgrades.

DR4 Target Breakdown

251 ULLYSES targets: 8 non-ULLYSES targets Finally, imaging HLSPs exist for 1 low-metallicity galaxy, NGC3109.

Changes to the _sed File Extension

Beginning in DR4 and all subsequent DRs, the filename extension used for level 4 HLSPs has been changed. Previously this extension was “_sed.fits”, it has now been renamed “_preview-spec.fits”. Level 4 products are created by abutting all coadded input spectra for a given target. These products are not spectral energy distributions (SEDs), as the units are in flux density (erg/cm2/s/Angstrom). Additionally, the selection of the transition wavelength from one abutted spectrum to the next is currently defined to be the middle of the overlap region. As such, it is possible that these products may use lower-quality data when higher-quality data exist. While these preview spectra may be suitable for many forms of scientific analysis, it is recommended that they be carefully evaluated (e.g. by comparing with the HLSPs for each individual spectrographic configuration) before being used for detailed measurements.

Time-series Spectra

Spectroscopic time-series products are available for both survey and monitoring T Tauri stars. For survey stars, only exposure level time-series with LCOGT data (with extension “_tss.fits”) are available. It should be noted that LCOGT time-series “spectra” consist of only two wavelength samples corresponding to the central wavelengths of each imaging filter used. HST COS and STIS UV data are time-tagged, which allows for sub-exposure time-series spectra. Sub-exposure products (with extension “_split-tss.fits”) are only generated from HST data of monitoring stars. HST exposure-level time-series spectra are also created for monitoring stars. The data formats of both exposure- and sub-exposure-level products are identical regardless of input data type (LCOGT or HST) or target type (survey or monitoring star), but the input data calibration and exact methods of creation differ. See the Data Description webpage for complete details.

In DR4, new LCOGT exposure-level products are available for: New HST exposure- and sub-exposure-level products are available for: Additionally, updated DR3 time-series products are available for download. These products benefit from updated calibration and processing.

Time and Wavelength Sampling

The time and wavelength sampling for sub-exposure time-series spectra are optimized to probe the smallest time interval possible while maintaining a S/N ≥ 5 per resolution element at the peak of the most important emission lines: C IV (1548 Å), and Mg II (2800 Å). The final sampling is also chosen to ensure that products are ready for use out-of-the-box and require no additional binning for most scientific goals. However, if a higher S/N is required, data can be binned further in time or wavelength by the user.

For all NUV observations, the optimal time sampling and wavelength sampling were determined to be, respectively, 10s and the native wavelength grid (i.e. no wavelength binning, or ~0.415 Å). For FUV spectra, the following time and wavelength bins have been adopted for the four monitoring T Tauri stars: For reference, the COS/FUV resolution element is 6 pixels, or ~0.073 Å, in the dispersion direction.

COS Calibration Updates

In DR3, routine data quality checks revealed a region of transient elevated counts, known as a “hotspot”, on the COS/FUVB segment. The hotspot is located in one of the background regions used by CalCOS, and therefore its flux is subtracted from the science spectrum resulting in a corresponding flux depression. Analysis performed by the COS team indicated the hotspot has been present since February 2018. Hotspots can be identified and flagged for removal using a COS reference file called the SPOTTAB. On November 9, 2021, the COS team delivered a new SPOTTAB that now flags and ignores all pixels affected by this hotspot. All affected COS HLSPs in DR4 have been created using this SPOTTAB.

Caveats and Known Issues

Data Description & Download

A full description of the ULLYSES data products and how they are created can be found here. Data may be downloaded from this website (HLSPs and their contributing data), the MAST Data Discovery Portal (HLSPs and their 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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