For each source, the ULLYSES team combines 1) adjacent spectral orders within a single echelle exposure and 2) different exposures obtained with a common grating, using the same or different central wavelength settings. This approach is used to create level 2 cspec.fits files.

Each input pixel measurement is treated as an estimate of the monochromatic flux at its assigned wavelength. The wavelength bin sizes are constant across the entire wavelength range, where the bin sampling is equal to the coarsest wavelength sampling of all input datasets. 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, converting to flux units by multiplying by the flux/net counts at that wavelength. If the net counts and flux are zero, the conversion ratio is interpolated using neighboring values. The signal to noise ratio (SNR) is calculated for each wavelength bin as 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.

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For each source that has spectra from different gratings, instruments, and/or telescopes, the contributing level 2 cspec.fits spectra are spliced together. This approach is used to create level 3 aspec.fits and level 4 preview-spec.fits products.

For non-overlapping spectra, the arrays are abutted as-is and the spectral sampling will be discontinuous at the transition point(s).

For overlapping spectra, a transition wavelength is selected. The input spectra are truncated and abutted at that wavelength, so the spectral sampling will be discontinuous at the transition. The transition point(s) are determined automatically by the HASP (Hubble Advanced Spectral Products) software that ULLYSES utilizes. For a given source, all contributing gratings have a pre-defined wavelength range within which the data are considered "good". Each grating also has a preassigned priority number. Iterating over all contributing gratings for the full wavelength range, data from the highest priority grating is selected over lower priority gratings. In practice, this is implemented by assigning 2 transition wavelengths to each grating spectrum, one for the shortest wavelength that fits in the fiducial wavelength range, and one for the longest wavelength. The full list of transition wavelengths from each input grating product is then traversed, and at each transition a decision is made as to which grating has the highest priority and will be used until the next transition wavelength. The transitions are abrupt, and the resolution and wavelength spacing will change from grating to grating. While this method works well for most targets, there are a select number of targets where the abutted products might not be ideal. In these rare cases, we encourage use of the level 2 cspec.fits files instead. We emphasize that the level 4 preview-spec.fits products are intended primarily for visual inspection, and level 2 or 3 products should be used for scientific analysis.

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Level 5 time-series spectra are created for all stars whose flux is variable over time. Both HST and LCO data are used to create spectral time-series products. Time-series spectra have two different suffixes: tss.fits and split-tss.fits. Exposure-level time-series spectra, tss.fits files, are created using default x1d files from MAST. Sub-exposure-level time-series spectra, split-tss.fits files, are created by splitting a default spectrum into smaller exposure chunks. For example, a 100s exposure could be split into ten 100s subexposures, to probe variability on a very fine timescale.

Exposure-level time-series products are created for survey and monitoring T-Tauri stars. LCO, HST/COS, and HST/STIS data are all used. Sub-exposure-level time-series products are created for monitoring T-Tauri stars only and use only HST/COS data, as that is the only mode where time-tag data can be further split.

Input Data

For all survey and monitoring stars observed by the ULLYSES team, LCO images are taken over five epochs. The first four epochs occur approximately 90 and 10 days before, and 10 and 90 days after, HST observations. Each of these epochs consists of daily observations over a 10-day interval. The fifth epoch occurs during the HST observations, and where LCO images are taken every 15 minutes. Each LCO observation includes back-to-back images in the filters applicable for the star type. Monitoring stars use SDSS u’ and i’ filters as well as the Bessel V filter. Survey stars only use the SDSS i’ and V filters. Unsuccessful observations will cause some gaps in either time or wavelength coverage. LCO “wavelength arrays” are limited to the central wavelengths of each filter. LCO data is used to create exposure-level tss.fits files, and is not obtained for survey stars with purely archival data.

For survey stars, HST exposure-level tss.fits files are created for any sources that 1) have multiple epochs, either through ULLYSES observations or archival observations, and 2) exhibit variability across epochs.

Monitoring T Tauri stars in the ULLYSES program are observed with HST/COS. Each visit is executed in the following order: a COS/G230L/2950 observation, a COS/G230L/2635 observation, COS/G160M/1589 exposures at FP-POS 3 and 4, and COS/G160M/1623 exposures at FP-POS 1 and 2. These observations are taken in two epochs, separated by approximately a year. There are four observations per rotation period, and three rotation periods in each epoch, yielding 24 exposures per target per central wavelength setting. These data are used to both exposure-level tss.fits and sub-exposure-level split-tss.fits files. In addition to this data obtained by the ULLYSES team, any additional archival data of monitoring stars is used to create exposure-level tss.fits files.

Input Data Calibration

For LCO time-series products, aperture photometry and flux calibration are performed on calibrated images. Full details on LCO data calibration are included below.

For exposure-level time-series products using HST data, default x1d files from MAST are generally used as input to the HLSP creation code. When it is required, custom-calibrated spectra may be used, e.g. when custom extraction is needed for STIS data or wavelength offsets are required for COS data.

For sub-exposure time-series products using HST/COS data, default corrtag files files from MAST are split into smaller time bins using the costools.splittag routine. This creates multiple “split corrtags”, each of which is then calibrated using CalCOS to produce individual “split x1d” products.

Time and Wavelength Sampling

The time sampling for exposure-level time-series products is determined by the time of observation of each exposure. Native wavelength sampling is always used for exposure-level time-series spectra (i.e. data are not binned in wavelength).

The time and wavelength sampling for sub-exposure time-series spectra is 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). For all COS/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 the following time and wavelength bins have been adopted for the four monitoring T Tauri stars:

• TW Hya: time = 30s, wavelength = 3 pixels
• RU Lup: time = 30s, wavelength = 6 pixels
• BP Tau: time = 30s, wavelength = 6 pixels
• GM Aur: time = 90s, wavelength = 6 pixels

For reference, the COS/FUV resolution element is 6 pixels in the dispersion direction.

The final sampling is chosen to ensure that products are ready for use out-of-the-box and require no additional binning for most science goals. However, if a higher S/N is required, data can be further further in time or wavelength by the user, or use the exposure-level files may be used.

Creation and Format of Time-series Spectra

While the exact method of creating the time-series products differs when using LCO versus HST data, the data formats of the output products are identical.

For LCO data, an ASCII photometric file is used as input— this file includes the LCO image filename, MJD start, MJD stop, central wavelength, flux, and error of each observation. The LCO central wavelength is defined as 3560 Å for the u' filter, 5450 Å for the V filter, and 7670 for the i' filter.

For HST data, default MAST x1d files, or custom-calibrated x1d files, are used as input.

For each time-series spectral product, all input data are rebinned onto the same wavelength grid. The exact wavelength sampling is detailed in the Time and Wavelength Sampling section. Data are then assembled into a 2-D array by inserting each rebinned spectrum into a row of a 2-D image. The X-axis of the image represents wavelength, and Y-axis represents time, with time increasing with increasing Y. The same process is used to create a 2-D image for the flux error. Separate 1-D arrays in the data extension give the wavelength as a function of the column number, and the MJD start and stop times as a function of the row number. An illustration of the 2-D flux array is shown below in Table 1.

For an example of how to read in and explore these products, see the "ULLYSES Walkthrough" Jupyter Notebook section "Time-series spectra".

Table 1: An illustration of the 2-D flux arrays computed for the spectral time-series products. The values represent the flux at each time and wavelength sample. Values of zero correspond to times or wavelengths where no data were obtained. In this exaggerated example, red/italic text could correspond to G230L observations while blue/normal text could correspond to G160M observations. Both gratings’ wavelength coverage overlap at wavelength index 1 (“Wavelength1”), which then has flux values at all time samples. An identically formatted table is made for the error measurements as well. The corresponding wavelength and time arrays are provided as separate 1-D arrays in the same data extension.

Time3	0	1.5	1.0	1.0
Time2	0	1.4	1.0	1.0
Time1	1.0	1.3	0	0
Time0	1.0	1.2	0	0
	Wavelength0	Wavelength1	Wavelength2	Wavelength3

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The input for ULLYSES processing of LCO data are the LCO BANZAI products. After ULLYSES calibration of BANZAI products, level 5 exposure-level time-series spectra are produced, with suffix tss.fits.

LCO images reduced with the BANZAI pipeline (McCully et al. 2018) are available in the LCO archive (see DDT2020B, DDT2021A). BANZAI performs bad-pixel masking, bias and dark removal, and flat-field correction. It also determines the astrometric solution and extracts a catalog of sources. Using the BANZAI-reduced images, an absolute flux calibration is determined based on magnitudes cataloged by the AAVSO Photometric All-Sky Survey (APASS, funded by the Robert Martin Ayers Sciences Fund and NSF AST-1412587).

The ULLYSES LCO calibration pipeline is written in IDL. It first performs aperture photometry on sources in the BANZAI-generated catalog using aper.pro from the IDL Astronomy User's Library (Landsman 1993). It sums counts in a five-pixel radius and subtracts the modal signal in an annulus extending from 10 to 20 pixels. These sources are matched to sources in the APASS tables, using a matching radius of 2 arcsec. The relationship between APASS magnitudes and –2.5 times the logarithm of the LCO counts is fit with a line, ignoring three-sigma outliers. The slope of this line can vary to account for non-linearity in the instrument response; this term is typically very close to 1.

Next, the pipeline attempts to find a point source at the expected coordinates of the ULLYSES target. If a source is found, aperture photometry is performed using the same parameters described above. The measured counts are converted to a magnitude with the relationship determined above. Magnitude is then converted to a flux density using the zero-magnitude flux for the observed bandpass. Central wavelengths and zero-magnitude fluxes come from Bessell et al. 1998 for the Bessell V filter and from Fukugita et. al 1996 for the Sloan Digital Sky Survey (SDSS) i' filter. The uncertainty is assumed to be dominated by the uncertainty in the measured counts of the ULLYSES target. Measurements with uncertainties greater than 20% of the fluxes (S/N < 5) are discarded. Output photometry files are then used as input to the spectral time-series creation code.

For the u' photometry, there are some differences in the procedure due to the sparseness of the science fields at this wavelength. Separate calibration fields are observed close in time and airmass to the science fields. If BANZAI is unable to determine an astrometric solution for the calibration field, a 500 pixel box is searched for the brightest star around its expected location, and 200 pixel boxes are searched for additional calibration stars at their expected offsets from the brightest calibration star. Magnitudes for the calibration stars at u' are obtained from Version 2.4.2 of STScI's Guide Star Catalog. For the science field, each u' image is almost always followed by a V image with no intervening telescope motion, so the u' astrometry is copied from the subsequent V image. The flux vs. counts relationship found for the calibration field is applied to the science target after correction for the different exposure times and airmasses. The airmass correction, central wavelength, and zero-magnitude flux for the SDSS u' filter come from Fukugita et. al 1996.

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All archival FUSE data used in the ULLYSES sample are examined and vetted by the ULLYSES team. Generally, FUSE Virtual Observatory (VO) files are used with no modification. However, some targets exhibit various issues in their spectra, such as spectral channel drifting. Beginning in DR6, the ULLYSES team began to deliver improved spectra for such targets. These custom-calibrated FUSE spectra are delivered as level 3 aspec.fits products, since FUSE VO files are several individual channel spectra abutted together.

Using the strategy outlined below, FUSE data for 26 targets previously excluded from the sample were able to be rectified and included in data products. Even with extra processing, FUSE data for 4 targets were still unable to be rectified and are not included in the ULLYSES sample:

1. PGMW-3120: multiple stars in aperture
2. AV-22: multiple stars in aperture
3. AV-287: target not in aperture
4. SK-69D220: multiple stars in aperture

Flux Differences Due to Guiding

Some ULLYSES FUSE data suffer from drifts among the spectral channels; FUSE was essentially four independent spectrographs and thermal instabilities on orbit could cause each one to drift out of alignment. One of the four channels was used for guiding, and the flux in this channel was generally the most accurate. Usually, thermal drifts in the other channels led the target to drift out of the aperture, resulting in lower count rates and thus spectral fluxes. In crowded fields, these drifts could allow a neighboring star to drift into the aperture, resulting in higher fluxes in the final spectra. The following strategy was used to repair these data:

1. Begin by examining the VO file, which was initially created by splicing together pieces of the extracted spectra from the eight FUSE detector segments.
• If the VO file does not meet data quality needs (e.g., depressed flux or mismatching flux at channel transition points), a new VO file is created by using the eight individual extracted spectra in the “ALL” files. These eight spectra are shifted to a common wavelength zero point and rescaled to create a new VO file.
2. The guide channel is identified (LiF1A for the first half of the mission, and LiF2A for the second) and its spectrum adopted as a reference.
3. If the spectra from other channels are less than 50% brighter than the reference, then they are rescaled to match the reference in the region of overlap.
4. If they are more than 50% brighter than the reference, they are assumed to be contaminated by nearby stars and not included in the final spectrum.

Even with these corrections, some flux mismatches still remain at the transition point between FUSE and HST data— these are not corrected by the ULLYSES team. If smooth transitions are required, one of the contributing spectra may be manually scaled to the other.

Background Subtraction Corrections

It was also necessary to run parts of the FUSE calibration pipeline (CalFUSE) in cases where the original background subtraction failed. FUSE did not have a shutter, so the detector received light from all three apertures (LWRS, MDRS, and HIRS) at all times. CalFUSE assumes that only the target aperture contains a star and fits a background model to the rest of the detector. In crowded fields, nearby stars occasionally fell in a non-target aperture, leading to an over-subtraction of the background. In these cases, the region of the detector used to model the background (stored as header keywords in the intermediate data file) was modified, and spectra were re-calibrated.

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Occassionally, a star is not exactly centered in the COS 2.5" aperture. When this occurs, there can be systematic offsets in the final wavelength scale. This effect is easily corrected by measuring the offset and supplying it to CalCOS (see Section 5.3.2 of the COS Data Handbook). After correcting offsets, level 0 spec.fits are created.

Four stars in the ULLYSES sample exhibit such COS wavelength offsets:

1. LMC079-1: visit le2701
2. SZ-10: visits leilac, leil2d, and leilad
3. V-GM-AUR: visit lek71f
4. V-TW-HYA: visit le9d1c

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The COS/NUV channel suffers from vignetting in the shortest wavelengths of each NUV stripe (see Section 5.1 of the COS Data Handbook). For ULLYSES observations of the four monitoring T Tauri stars (V-BP-TAU, V-GM-AUR, V-RU-LUP, V-TW-HYA), vignetting is corrected by scaling the flux in vignetted regions to the flux in the same region with a different COS configuration (that does not suffer from similar vignetting). For all other targets, however, COS/NUV observations lack such overlapping spectra. For these targets, the vignetted regions— defined as the first 200 pixels of each NUV stripe— are flagged and discarded.

For the monitoring stars, the vignetting scaling is performed as part of automated pre-processing of time-series spectra, therefore no corrected spectra for individual exposures are delivered. Instead, the spectra present in the level 5 time-series products includes the vignetting correction.

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All STIS CCD, and some STIS single-order MAMA data, require manual calibration. For these targets, the resulting spectra are delivered as level 0 spec.fits files. All corresponding non-standard calibration parameters are recorded in YAML configuration files, with suffix spec.yaml. Both spec.fits and spec.yaml files are considered level 0 HLSPs.

The following detector and data effects are corrected in STIS level 0 products, and each is described in detail below:

• CCD only: DQ=16 overflagging (pixels having dark rate >5σ times the median dark level)
• CCD only: NIR fringe pattern
• CCD and MAMA: custom extraction(s)

DQ=16 Flagging
When creating ULLYSES HLSPs, all spectral elements with a DQ value considered serious are excluded when coadding and abutting data. The DQ=16 flag, which notes pixels having a dark rate >5σ times the median dark level, can affect more than 5% of STIS/CCD pixels. When a spectrum is extracted, however, very large portions (>30%) of the spectrum can be marked as DQ=16, due to the boxcar extraction method. The majority of these flagged spectral elements are indeed good and should be included in data products. To address this this problem, the ULLYSES team applies a less aggressive dark flagging threshold. This is done by creating new superdarks with modified DQ arrays. The DQ arrays from the dark are then inherited by the science images themselves. Using this method, it is possible to reduce the incidence of DQ=16 flags by an order of magnitude. See Figure 2 for an example of this improvement.

Defringing

STIS CCD data suffer from an interference pattern called fringing at wavelengths > 7000A, a result of multiple reflections between CCD surfaces (Goudfrooij et al. 1998). Fringing is corrected by obtaining a contemporaneous flat— a fringe flat— using the tungsten lamp and the same grating as the science exposure. For most ULLYSES STIS/G750L exposures, a contemporaneous 0.3x0.09 fringe flat is used. If multiple sources are present in the slit, a contemporaneous 52x0.1 fringe flat may be used instead. If a fringe flat in the appropriate slit was not taken contemporaneously, an archival fringe flat closest to the science observation date is used.

The stistools package defringe tool is used to correct fringing, following the method outlined in the STIS Data Handbook. First, the fringe flat is normalized, then the shifts and scale factors needed to match the fringes in the flat and science spectra are computed. This is often an iterative process; if the best shift and scale values are found at the edge of the supplied range, the range is expanded and the values recomputed. Finally, the science spectra are divided by the scaled and shifted fringe flat. In rare cases, defringing actually degrades the science spectrum quality. For such scenarios, defringing is skipped altogether. See Figure 2 for the comparison between a G750L spectrum with and without fringe correction.

Multiple Sources in Slit

The extraction location and size, cross-dispersion search range, background region locations and sizes, and wavelength offset can all be manually specified when performing a custom spectral extraction.

When multiple sources are present in the slit, multiple, individual, extractions are required. For both ULLSYES source and the "companion" source(s), the following extraction parameters may be modified: extraction location and size, cross-dispersion search range, background region locations and sizes, and wavelength offset. The naming comvention of such companions depends on how close they are to the intended ULLYSES source.

If the ULLYSES source and the companion are close enough to be unresolved in the COS aperture (i.e., they are within 1.25" of each other), three products will be delivered:

1. The ULLYSES source only, with the target name in the HLSP filename being <TARGNAME>A, e.g. CVSO-109A
2. The companion only, with the target name in the HLSP filename being <TARGNAME>B, e.g. CVSO-109B
3. The "blended" source (both ULLYSES and companion flux), with the target name in the HLSP filename being <TARGNAME>, e.g. CVSO-109

If the companion source is close enough to the ULLYSES source that their PSFs are indistinguishable, the blended spectrum is simply a single extraction box encompassing both sources. If there is separation between the companion and ULLYSES source extraction boxes, then the two separate spectra are coadded to create the blended source.

If there is sufficient separation between the ULLYSES and companion source such that they are resolved in the COS separation (i.e., they are separated by more than 1.25"), two products will be delivered.

1. The ULLYSES source only, with the target name in the HLSP filename being <TARGNAME>, e.g. CVSO-104
2. The companion only, with the target name in the HLSP filename being the source's catalog name, e.g. GAIA-DR3-3217634157789741952

Miscentering in Slit

If the source is poorly centered in the slit, in either the along- or cross-dispersion direction, custom extraction can correct the issue. In most cases, CalSTIS's automatic source location algorithm will successfully extract sources offset in the cross-dispersion direction. However, it may fail for faint sources, and in such cases the extraction location may be manually supplied. When the source is miscentered in the along-dispersion direction, the systematic offset can be calculated and supplied when re-calibrated, rectifying the issue.

Extraction Box Size

Sometimes adjusting the extraction box size can result in a higher SNR for the extracted 1D spectrum. In such cases, the height of the extraction box is manually supplied during recalibration.

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CalSTIS flags the lowest orders of commonly used echelle configurations as having a poorly determined residual background. However, the fluxes of ULLYSES sources in these orders are accurately determined. To maximize wavelength coverage, the ULLYSES team manually removes the affected Data Quality (DQ) flag, DQ=2048, for these orders only. After removal, the data are included in all subsequent HLSPs. Level 0 spec.fits are also created for these cases.

The affected echelle grating, central wavelength, and orders are:

• E140M/1425 order 86
• E230M/2707 order 66
• E230M/2415 order 73

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Misalignments in the STIS echelle blaze function can cause flux mismatches in the overlapping regions of adjacent orders. For a subset of STIS echelle datasets (461 medium-resolution and 11 high-resolution spectra), the ULLYSES team used the stisblazefix tool to empirically correct for such misalignments.

This correction empirically adjusts the alignment of the blaze function by minimizing discrepancies in the overlapping flux of neighboring orders. The linear fit to the applied shift sometimes converges on a false solution, so further inspection is required in the signal-to-noise (SNR) regime where stisblazefix gives reliable results. To do so, the STIS echelle datasets are parameterized by their SNR, and subsampled high-SNR calibration datasets from the STIS MAMA Spectroscopic Sensitivity and Focus Monitor (from Cycle 7-31), to determine the behavior of the algorithm at lower SNR. Only datasets with nonzero exposure time and a "NORMAL" exposure interruption indicator flag (sci_expflag in MAST) are used. Additionaly, some datasets were excluded; OE5CE4010, OE5CE4050, O3ZX050D0, and O3ZX050E0 exhibited issues in their target acquisitions, and O4LU01060 was obtained during a detector power reset. This resulted in a test sample of 225 high-resolution and 236 medium-resolution echelle observations of the white dwarf flux standards BD+28D4211, BD+75D325, and G191-B2B/G191B2B. Datasets were obtained from 1997-09-15 to 2023-09-28. This final suite of data was used to determine the SNR limits to apply to the ULLYSES sample.

The average continuum SNR of each echelle x1d dataset was then calculated as follows: dividing the flux by the error, excluding the and first and last 100 pixels from each order, calculating the 75th percentile value for each order, and finally taking the mean across all orders. This proved to be robust to emission and absorption lines, and to the blaze function edges. See Figure 3 for an example SNR estimate.

The the monitor datasets were subsampled at levels of [0.5, 1, 2, 5, 10, 25, 50]%, giving a range of continuum flux levels and hence SNR. The subsampled datasets were processed with CalSTIS and stisblazefix to derive the blaze shift offset at the end orders and the mean across all orders. These values are subtracted from those derived from the corresponding full dataset. A more detailed analysis may wish to derive additional data points from more subsamples of the input datasets, especially at lower SNR levels.

When plotted against SNR (see Figures 4 and 5), the scatter in the stisblazefix mean blaze correction increases to ~3 pixel at SNR=10 for H-modes and >2 pixels at SNR=5 for the M-modes. These thresholds also conservatively correspond to a sudden increase in scatter of the calstis-calculated blaze shift value (Δ in the SCI ext "BLZSHIFT" keyword). These SNR thresholds are them used when determining which ULLYSES datasets should be corrected with stisblazefix. Additionally, the stisblazefix diagnostic plots were manually reviewed for obvious bad fits, but nothing was rejected. For M-mode data we used an SNR=5 threshold to apply stisblazefix to 461/590 ULLYSES datasets. For H-mode data we used an SNR=10 threshold to apply it to 11/66 ULLYSES datasets. For each datasets that had stisblazefix applied, all corresponding ULLYSES HLSPs have COMMENT keywords (in the SCI extension) noting so.

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The ULLYSES program obtained spectra of six massive stars in the low-metallicity galaxies Sextans A and NGC 3109. To estimate the exposure time of these stars, pre-imaging, using the HST Wide Field Camera 3 (WFC3) was obtained, using filters F225W, F275W, F336W, F475W, and F814W. Drizzled combined images for each galaxy and filter are delivered as level 6 drc.fits files.

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To fully capitalize on the legacy of the ULLYSES data sample, the ULLYSES team delivered community-created data products from two large external research groups: PENELLOPE and X-Shooting ULLYSES (XShootU). Both collaborations obtained complimentary data of ULLYSES stars using VLT. Community-submitted products are delivered as level 7 prodducts with suffix vltspec.fits.

The PENELLOPE collaboration obtained VLT data of T Tauri stars in the ULLYSES sample, including non-flux-calibrated high-resolution optical spectra (ESPRESSO and UVES instruments) and flux-calibrated medium-resolution UV-to-NIR spectra (X-Shooter instrument). X-Shooter data of 74 T Tauri stars was delivered as ULLYSES level 7 products in early 2024. While the contents of the data arrays were not modified, the original spectra from the three X-Shooter arms (UVB, VIS, NIR) are stored as three separate FITS files by the PENELLOPE team. While the ULLYSES team did not modify the contents of the data arrays in these files, the three files are combined into a single FITS file, with the spectrum (and associated header) of each arm stored in a separate BinTableHDU data extension. The 0th header follows the same format used by all other ULLYSES HLSPs, as is required by MAST HLSP guidelines. Finally, a PROVENANCE BinTableHDU extension, following the same format as all other ULLYSES HLSPs, is added after all data extensions (as a 4th extension).

The XShootU collaboration obtained VLT X-Shooter data of massive stars in the ULLYSES sample. This includes flux-calibrated medium-resolution optical and NIR spectra of 129 LMC stars, 103 SMC stars, and 3 low-metallicity stars. This data will be delivered as ULLYSES HLSPs in the near future.

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FITS File Structure

Spectral data and information is stored in two BINTABLE extensions:

• a Science extension for the combined product, and
• a matching Provenance extension to record attributes of each spectrum that contributed to the combined product

Primary Header	Metadata common to all contributing spectra
Extension 1 Header	Metadata specific to science EXTNAME = 'SCIENCE'
Table 1 Data	Science data specific to single-epoch spectrum
Extension 2 Header	Metadata specific to provenance EXTNAME = 'PROVENANCE'
Table 2 Data	Metadata specific to contributing spectra

Single-epoch Science Table

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.

Column Name	Dimensions	Units	Data Type
WAVELENGTH	M	Angstrom	single-precision float
FLUX	M	erg/cm2/s/Angstrom	single-precision float
ERROR	M	erg/cm2/s/Angstrom	single-precision float
SNR	M	—	single-precision float
EFF_EXPTIME	M	s	single-precision float

Single-epoch Provenance Table

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.

Column Name	Units	Data Type
FILENAME	—	string
PROPOSID	—	string
TELESCOPE	—	string
INSTRUMENT	—	string
DETECTOR	—	string
DISPERSER	—	string
FILTER	—	string
CENWAVE	—	string
MINWAVE	Angstrom	double-precision float
MAXWAVE	Angstrom	double-precision float
APERTURE	—	string
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

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Time-series Spectra File Structure

Spectral data and information is stored in two BINTABLE extensions:

• a Science extension for the combined product, and
• a matching Provenance extension to record attributes of each spectrum that contributed to the combined product

Primary Header	Metadata common to all contributing spectra
Extension 1 Header	Metadata specific to science EXTNAME = 'SCIENCE'
Table 1 Data	Science data specific to multi-epoch spectra
Extension 2 Header	Metadata specific to provenance EXTNAME = 'PROVENANCE'
Table 2 Data	Metadata specific to contributing spectra

Time-series Spectra Science Table

The time-series spectral products have slightly different table columns compared to single-epoch spectra. The FLUX and ERROR arrays are 2D arrays with wavelength increasing along X, and time increasing along Y. The wavelength values for each column of the 2D data are stored in the WAVELENGTH array, while the MJDSTART and MJDEND columns store the start and end times for each row of the FLUX and ERROR arrays.

Column Name	Dimensions	Units	Data Type
MJDSTART	M	d	double-precision float
MJDEND	M	d	double-precision float
WAVELENGTH	N	Angstrom	single-precision float
FLUX	M x N	erg/cm2/s/Angstrom	single-precision float
ERROR	M x N	erg/cm2/s/Angstrom	single-precision float

Time-series Spectra Provenance Table

Select metadata for each spectrum that contributes to the time-series product will populate a row in the provenance table. The fields in the following table are metadata harvested from the headers of the contributing spectra. Some columns are only present for HST data or LCO data.

Column Name	Units	Data Type
FILENAME	—	string
PROPOSID	—	string
TELESCOPE	—	string
INSTRUMENT	—	string
DETECTOR	—	string
DISPERSER (HST data only)	—	string
FILTER (LCO data only)	—	string
CENWAVE (HST data only)	—	string
MINWAVE (HST data only)	Angstrom	double-precision float
MAXWAVE (HST data only)	Angstrom	double-precision float
APERTURE	—	string
SPECRES (HST data only)	—	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

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FITS File Structure

For a complete description of the original PENELLOPE data products, see the ESO release information. Spectral data and information is stored in four BINTABLE extensions:

• a UVB extension for the UVB arm spectra
• a VIS_TELL extension for the VIS arm spectra
• a NIR_TELL extension for the NIR arm spectra, and
• a matching Provenance extension to record attributes of each spectrum that contributed to the final product

Primary Header	Metadata common to all contributing spectra
Extension 1 Header	Metadata specific to UVB spectra EXTNAME = 'UVB'
Table 1 Data	Science data specific to UVB spectra
Extension 2 Header	Metadata specific to VIS spectra EXTNAME = 'VIS_TELL'
Table 2 Data	Science data specific to VIS spectra
Extension 3 Header	Metadata specific to NIR spectra EXTNAME = 'NIR_TELL'
Table 3 Data	Science data specific to NIR spectra
Extension 4 Header	Metadata specific to provenance EXTNAME = 'PROVENANCE'
Table 4 Data	Metadata specific to contributing spectra

PENELLOPE Science Tables

The following science table format applies to the UVB, VIS, and NIR data extensions. 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.

Column Name	Dimensions	Units	Data Type
WAVELENGTH_AIR	M	Angstrom	single-precision float
FLUX	M	erg/cm2/s/Angstrom	single-precision float
ERR	M	erg/cm2/s/Angstrom	single-precision float
QUAL	M	—	single-precision float

PENELLOPE Provenance Table

Select metadata for each spectrum that contributes to the the science extensions will populate a row in the provenance table. The fields in the following table are metadata harvested from the headers of the original PENELLOPE products.

Column Name	Units	Data Type
FILENAME	—	string
PROPOSID	—	string
TELESCOPE	—	string
INSTRUMENT	—	string
DETECTOR	—	string
DISPERSER	—	string
CENWAVE	—	string
MINWAVE	Angstrom	double-precision float
MAXWAVE	Angstrom	double-precision float
APERTURE	—	string
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

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FITS File Structure

For a complete description of the original XShootU data products, see the ESO release information. Spectral data and information is stored in three or more BINTABLE extensions:

• a UVB extension for each UVB arm spectra
• a VIS extension for each VIS arm spectra, and
• a matching Provenance extension to record attributes of each spectrum that contributed to the final product

There is at least one UVB and one VIS extension for each source, but some sources contain multiple epochs of data. In such cases, there will be a separate extension for each epoch.

Primary Header	Metadata common to all contributing spectra
Extension 1 Header	Metadata specific to UVB spectra EXTNAME = 'XSH-U.UVB.<DATE>' where <DATE> is the MJD start date
Table 1 Data	Science data specific to UVB spectra
Extension 2 Header	Metadata specific to VIS spectra EXTNAME = 'XSH-U.VIS.<DATE>' where <DATE> is the MJD start date
Table 2 Data	Science data specific to VIS spectra
Extension 3 Header	Metadata specific to provenance EXTNAME = 'PROVENANCE'
Table 3 Data	Metadata specific to contributing spectra

XShootU Science Tables

The following science table format applies to the UVB, and VIS data extensions. 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.

Column Name	Dimensions	Units	Data Type
WAVELENGTH	M	Angstrom	double-precision float
WAVELENGTH_AIR	M	Angstrom	double-precision float
SCI_FLUX	M	erg/cm2/s/Angstrom	single-precision float
SCI_FLUX_ERR	M	erg/cm2/s/Angstrom	single-precision float
SCI_FLUX_QUAL	M	—	single-precision integer
SCI_FADU	M	ADU	single-precision float
SCI_FADU_ERR	M	ADU	single-precision float
SKY_FADU	M	ADU	single-precision float
SKY_FADU_ERR	M	ADU	single-precision float
SCI_FLUX_SLC	M	erg/cm2/s/Angstrom	single-precision float
SCI_FLUX_SLC_ERR	M	erg/cm2/s/Angstrom	single-precision float
SCI_FLUX_TAC	M	erg/cm2/s/Angstrom	single-precision float
SCI_FLUX_TAC_ERR	M	erg/cm2/s/Angstrom	single-precision float
TELLURIC_SPECTRUM	M	—	single-precision float
SCI_FLUX_ABS	M	erg/cm2/s/Angstrom	single-precision float
SCI_FLUX_ABS_ERR	M	erg/cm2/s/Angstrom	single-precision float
SCI_FLUX_NORM	M	—	single-precision float
SCI_FLUX_NORM_ERR	M	—	single-precision float

XShootU Provenance Table

Select metadata for each spectrum that contributes to the the science extensions will populate a row in the provenance table. The fields in the following table are metadata harvested from the headers of the original XShootU products.

Column Name	Units	Data Type
FILENAME	—	string
PROPOSID	—	string
TELESCOPE	—	string
INSTRUMENT	—	string
DETECTOR	—	string
DISPERSER	—	string
CENWAVE	—	string
MINWAVE	Angstrom	double-precision float
MAXWAVE	Angstrom	double-precision float
APERTURE	—	string
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

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FITS File Structure

Drizzled and weight images are stored in two IMAGEHDU extensions, and PROVENANCE information stored in a BINTABLE extension:

• a Science extension for the drizzled science image
• a Weight extension for the weight map image
• a matching Provenance extension to record attributes of each image that contributed to the combined product

Primary Header	Metadata common to all contributing images
Extension 1 Header	Metadata specific to drizzled science images EXTNAME = 'SCI'
Image 1 Data	Drizzled science image
Extension 2 Header	Metadata specific to drizzled images EXTNAME = 'WHT'
Image 2 Data	Weight map image
Extension 3 Header	Metadata specific to provenance EXTNAME = 'PROVENANCE'
Table 3 Data	Metadata specific to contributing images

Drizzled Image Provenance Table

Select metadata for each image that contributes to the drizzled image in will populate a row in the provenance table. The fields in the following table are metadata harvested from the headers of the contributing images.

Column Name	Units	Data Type
FILENAME	—	string
PROPOSID	—	string
TELESCOPE	—	string
INSTRUMENT	—	string
DETECTOR	—	string
FILTER	—	string
APERTURE	—	string
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

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All STIS CCD, and some STIS single-order MAMA data, of require tailored calibration. For these targets, all non-standard calibration parameters are recorded in YAML configuration files that are included with ULLYSES data relases. YAML files are considered level 0 products.

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