Instrument Science Reports (ISRs)

We describe the re-creation of the FUV XTRACTAB reference file for BOXCAR extraction at lifetime positions (LPs) 1–6, as well as the PROFTAB, TRACETAB, and TWOZXTAB reference files for LPs 3–6, where TWOZONE extraction is enabled. These files were re-created due to the 2024 improvement of the geometric distortion, delta-geometric, X-walk, and Y-walk corrections. In most cases, we closely follow the methods used to create these reference files at the time each LP was commissioned. The major exception is that, for the first time, we create traces and profiles for the ``blue modes'' (G130M cenwaves 1055 and 1096) to enable TWOZONE extraction for these modes at LP2, so their extraction is less affected by gain sag. In general, we find that the spectral traces at each LP become much straighter with the application of the new walk and geometric corrections, demonstrating the improvement these updated corrections have on reporting the location of photon events in the cross-dispersion direction.annel for continued high productivity through the 2030s.

The far-ultraviolet (FUV) detector of the Cosmic Origins Spectrograph (COS) accumulates gain sag where photons land, and without continued mitigation this degradation would render the most-used modes unusable. To extend COS FUV operations through the 2030s, the COS team developed the COS2035 strategy, which builds on the existing COS2025 rules with four technical breakthroughs and two new usage policies. First, SPLIT-wavecals decouple wavelength calibration from science exposures and open detector real estate above the Pt-Ne lamp light leak. Second, a hybrid lifetime position (LP) architecture allows different gratings to operate at different LPs simultaneously. Third, the LP-infinity framework removes the dependence on the eight-LP limit in the COS flight software, supported by a new table-based APT and TRANS rules architecture. Fourth, a revised gain-sag flagging method evaluates integrated column count loss against the maximum achievable signal-to-noise (S/N) per mode. The two new usage policies cap per-target S/N at the maximum achievable value set by fixed-pattern noise, and limit any single program to 2% of the lifetime at any single LP. With LP7 and LP10 enabled in Cycle 33 and LP11 and LP12 in active commissioning for Cycles 34 and 35, the COS2035 strategy positions the FUV channel for continued high productivity through the 2030s.

Beginning in HST Cycle 25, all COS FUV target-acquisition algorithms work by stepping the target across the aperture, measuring the flux at each dwell point, calculating a flux-weighted centroid based on those measurements, and slewing the telescope to that centroid position. This entire procedure is performed by the flight software, and no images are recorded. To confirm that the target acquisition procedures are working as expected, each year we run a series of tests that simulate this process by acquiring exposures at specified offsets in the cross-dispersion direction. Here, we present the results of these tests from Cycle 33.

The enabling of new Lifetime Positions 7 and 10 for the Cosmic Origins Spectrograph (COS) Far-Ultraviolet (FUV) detector began in the fourth quarter of 2024. At the beginning of Cycle 33, COS G130M/1055 and 1096 modes move from Lifetime Position 2 (LP2) to Lifetime Position 7 (LP7), G130M/1222, and 1291 modes move from Lifetime Position 5 (LP5) to Lifetime Position 7 (LP7), and G160M/1533, 1577, 1589, 1600, 1611, and 1623 modes move from Lifetime Position 6 (LP6) to Lifetime Position 10 (LP10). The work involved in the enabling phase includes four observing programs to determine the optimal target placement and focus positions, verify target acquisitions at each new LP, and check whether the onboard deuterium lamps can illuminate the LP7 region for gain-map monitoring, reducing reliance on external targets.

The calibration of Cosmic Origins Spectrograph (COS) data is performed by the data processing pipeline known as CalCOS. CalCOS utilizes several reference files that store information required for data processing (e.g., dispersion solutions), and these reference files can be updated for the purpose of improving calibrated data products. In 2024 a major effort to improve the far-ultraviolet (FUV) geometric distortion, delta-geometric, and walk corrections was completed. Those three corrections occur early in the CalCOS workflow and shift photon events into the corrected reference frame ((XCORR, YCORR) coordinate system) in which subsequent calibration steps—and their reference files—are defined. This change necessitated the re-derivation of all “downstream” reference files in use at the time. Here we summarize the effort to create new versions of 56 reference files applicable to lifetime positions 1 through 6 and intended for use with the new geometric distortion, delta geometric, and walk corrections. The primary outcome of this work is improved wavelength and flux calibrations for COS FUV spectra.

Observations of HST spectrophotometric standard stars show that the COS NUV detector has a time-dependent sensitivity (TDS) that must be monitored and accounted for in flux calibration. Regular observations monitor the changes in sensitivity for three NUV gratings: G230L, G185M, and G225M. The fourth NUV grating, G285M, was removed from routine monitoring because its sensitivity has become too low. G285M continues to be available but unsupported for General Observer programs. In this ISR, I present the results of the Cycle 30, Cycle 31, and Cycle 32 NUV TDS programs.

We present updated time-dependent sensitivity (TDS) corrections for the far-ultraviolet (FUV) channel on board the Cosmic Origins Spectrograph (COS). Updated TDS corrections were required due to the recently derived geometric distortion, delta-geometric, X-walk, and Y-walk corrections introduced in the COS calibration pipeline. The new high-voltage-dependent sensitivity (HVDS) correction was also applied to all COS FUV data via the HVDSCORR module in CalCOS and incorporated into the TDS corrections. Monitoring data were binned in wavelength and fit using a piecewise linear function of time. The analysis included TDS monitoring data of calibrator white dwarfs from the start of monitoring in September 2009 until February 2024. Comparison of the recalibrated data with CALSPEC models WD0308-565 and GD71 showed that the time-dependent flux calibration is within the 5% absolute and 2% relative requirements for all FUV gratings. The residuals between data and model over time show 2-10% improvements across all wavelength bins for all Lifetime Positions (LPs).

The new Hubble Spectroscopic Legacy Archive (HSLA) provides coadded spectra of individual targets that have been observed with the Cosmic Origins Spectrograph (COS) and the Space Telescope Imaging Spectrograph (STIS) over their operating lifetime. HSLA uses data available in the Mikulski Archive for Space Telescopes (MAST). It automatically produces coadds whenever new data become publicly available or when there is newly recalibrated data. HSLA defines individual targets by their associated coordinates, accounting for proper motions, and uses SIMBAD, NED and the Phase II observing proposals to obtain astronomical classifications for each object. Coadded spectra are produced for each observing mode. In the case of COS far-ultraviolet observations there is one coadded spectrum for each lifetime position (LP). Additionally, a spectrum spanning the entire wavelength range covered by the observations is produced by abutting the spectra from a selection of individual modes. For each individual target, HSLA also provides a human-readable metadata file with key information that can be used in searches or for further exploration of the data. The HSLA project also makes the code used for coadding spectra publicly available along with several other tools (using Jupyter notebooks) for custom coaddition required in special cases. In this report we will describe the main components of HSLA and provide a brief description of how the data and metadata can be accessed.

In preparation for the activation of Lifetime Positions 7 (LP7) and 10 (LP10) on the Cosmic Origins Spectrograph (COS) Far-Ultraviolet (FUV) channel aboard the Hubble Space Telescope (HST) in Cycle 33, we conducted a series of focus sweep observations in December 2024 and January 2025 to optimize the focus settings for the COS G130M/1222, 1291, 1055, and 1096 modes at LP7 and the G160M modes at LP10. The focus varies with lifetime position, requiring a re-determination of the Optics Select Mechanism 1 (OSM1) absolute focus position for each grating and central wavelength configuration. Using spectra from the subdwarf stars Feige 48 and AG+81-266, we performed a spectral line-width analysis via autocorrelation functions of focus offsets relative to an estimated absolute focus value to identify the OSM1 positions that produced the sharpest absorption features. Our results yielded final absolute focus values of -993, +37, and -2556 focus steps for G130M/1222, 1291, and 1096 at LP7, and +124 focus steps for G160M/1600 at LP10. We then extrapolated from these results to find the focus values for G130M/1055 and the remaining G160M central wavelengths. The LP7 values were incorporated into the COS flight software table pcmech_OSMTbl and the LP10 values were included within the commanding instructions to ensure continued optimal spectral resolution and instrument performance.

We present an update to the time dependent sensitivity (TDS) corrections for the near-ultraviolet (NUV) channel aboard the Cosmic Origins Spectrograph (COS). These new TDS corrections account for wavelength dependence, whereas the previous the TDS corrections were assumed to be grating and stripe dependent only. The TDS corrections are derived by monitoring flux calibration standard white dwarfs twice a year and fitting a linear function to the monitoring data binned in wavelength as a function of time. The analysis included TDS data from the start of monitoring in August 2009 until August 2023. Additionally, to increase the wavelength coverage of the G225M settings, we included in our analysis calibration observations from Cycle 17 in conjunction with Cycle 30 supplemental monitoring data. Comparison of the data with the CALSPEC models G191-B2B and WD1057+797 and comparison between the old and new calibrations applied to HIP 66578 showed that the new TDS corrections significantly improved the flux calibration. The new NUV TDS corrections maintain the flux accuracies to within the 5% absolute calibration requirements for most central wavelengths (cenwaves). The wavelength-dependent TDSTAB (8a818589l tds.fits) was delivered to operations on 8 October 2024.

The wavelength calibration procedure employed by CalCOS relies on measuring the separation in the dispersion direction between Pt-Ne lamp spectra that are observed concurrently with science exposures (or immediately before and after a science exposure in some cases), and a template Pt-Ne spectrum for which the dispersion solution of the Primary Science Aperture is defined. Template spectra for each Cenwave/Segment/FPPOS combination are stored in the LAMPTAB reference files, which are specific for each lifetime position (LP). Improvements to the geometric distortion, delta-geometric, X-walk, and Y-walk corrections applied to the Cosmic Origins Spectrograph far ultraviolet detector necessitated updates to all “downstream” reference files used by CalCOS, including the LAMPTAB files at all LPs. This document describes the re-creation of the LAMPTAB files.

While generally stable with time, the count rate over the duration of a COS exposure can occasionally vary, either due to the intrinsic nature of the object being observed or due to issues with the telescope assembly or COS detector. Depending on the nature of this variability, reduction of these observations may result in poor flux calibration. We use a method called Local Outlier Factor to identify COS exposures that exhibit significantly variable count rates, and classify them based on their variability. Out of all COS spectroscopic TIME-TAG data (FUV and NUV) taken through Spring 2025, we identify a total of 260 variable exposures, 11 of which exhibit variability due to telescope/detector issues, and the rest exhibiting variability due to the intrinsic variable nature of the object. We demonstrate that the problematic exposures can be corrected using the BADTCORR step in CalCOS. We provide lists of all variable exposures in addition to the explicit “bad time” intervals that can be entered into the BADTTAB reference file to correct the 11 problematic exposures.

Due to a light leak through the flat-field calibration aperture of the Cosmic Origins Spectrograph (COS), some observations with the FUV detector require the wavelength calibration aperture to move to a position where the lamp can safely flash on the detector and track wavelength solution drifts. This is applicable for any science observations taken above 5.4” in relation to Lifetime Position (LP) 1 (such as at LP6, and LP7). This procedure, also known as SPLIT-wavecal, incur significant additional overheads due to this movement. To minimize these overheads, the standard pipeline adopts a simulated wavelength calibration lamp flash at the 600-s exposure time based on historical COS data. In this report, we analyze historical COS data between 2009 and 2024 to re-derive SPLIT-wavecal parameters that define the simulated lamp flash. We find no time dependence in the values of these parameters within the range 2009-2024 and refine the wavelength solution drift of G130M and G160M Observations.

We report on the analysis and implementation of updates to the Bad Pixel Table (BPIXTAB) reference file for the Hubble Space Telescope (HST) Cosmic Origin Spectrograph (COS) far-ultraviolet (FUV) detector before the moves to Lifetime Positions 5 and 6 (LP5 and LP6). The BPIXTAB is a critical component of the COS calibration pipeline that identifies problematic or less-than-optimal detector regions through data quality (DQ) flagging. We employed a semi-automated detection procedure to identify new very low response regions (DQ = 16) and low response regions (DQ = 1024) at and near the areas of the new lifetime positions. Additionally, we adjusted the boundaries of several existing DQ regions to account for distortions in pixel coordinates. Technical and scientific testing confirmed that the updated reference files correctly flagged problematic areas and function properly within the CalCOS pipeline. The updated BPIXTAB reference files for the LP5 and LP6 eras were delivered to the HST Calibration Reference Data System in November 2021 and September 2022.

The COS FUV detector comprises two microchannel plate segments (FUVA, FUVB) with cross delay line anodes to report the dispersion (x) and cross-dispersion (y) location of the charge cloud generated from a photon event. The electronic settings used to measure the locations were optimized during ground testing, but as the detector ages with usage, these positions shift due to the lowered pulse height amplitude (PHA) of the charge cloud, a phenomenon known as “walk”. In this work, we use a wide range of archival COS data across Lifetime Positions (LPs) 1–5 to determine the corrections needed to remove walk in the y direction. Prior to this work, a Y-walk correction that was independent of detector location was applied to all COS FUV data. However, we find that the Y-walk shifts, dy, become increasingly steeper with PHA at locations above LP1 and shallower below. We use these dy measurements to create a YWLKFILE reference file that applies a correction based on YCORR location. This new correction, when combined with the updated GEOFILE, results in straighter spectral traces at LPs 1–6, and ensures that low gain events registered at the LP furthest below LP1 (i.e. LP4) remain in the spectral extraction region as expected.