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).
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:
|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|
|Combined spectra, with common instrument, different gratings and cenwave settings, and grouped by resolution||cos||g130m-g160m-g185m||cspec|
|All instruments and settings: the SED product||cos-stis||uv||sed|
Note: Not all products are included in DR-1.
The High Level Science Products are in FITS format, and are organized as described in the following tables.
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|
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.
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.
Combining Spectra with Different Gratings and Instruments
For all other cases spectra are spliced, meaning that:
Known Issues and Caveats
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: