Instrument Science Reports (ISRs)

September 29, 2025L.P. Miller et al.

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.

September 29, 2025J. Hernandez et al.

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.

September 10, 2025N. Indriolo et al.

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.

August 28, 2025A. Damoah & S. Hasselquist

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.

August 08, 2025L. Dos Santos et al.

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.

August 04, 2025L.P. Miller et al.

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.

July 14, 2025S. Hasselquist et al.

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.

July 11, 2025S. Hasselquist et al.

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 electronics 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 determine the corrections needed to remove walk in the x direction. We find that, while the shifts are ∼1-2 pixels for photon events at and near the gain nominal values of PHA = 10-12, events with PHA < 7 can have X-walk of up to ∼7 pixels, particularly low PHA events occurring on the left side of FUVA. We have used these measurements to create a new XWLKFILE reference file that contains the corrections to be applied by the CalCOS XWLKCORR module to correct these shifts.

July 02, 2025D. Kakkad et al.

We describe the derivation of a new geometric distortion correction reference file for the far-ultraviolet (FUV) detector of the Cosmic Origins Spectrograph (COS) aboard the Hubble Space Telescope (HST). The geometric distortion correction aims to achieve a uniform plate scale across both segments of the FUV detector by applying a transformation from thermally-corrected coordinates to a coordinate system where pixels measure 6 μm °ø 24 μm. The previous version of the geometric distortion correction reference file (GEOFILE) was known to have problems at some locations that negatively impacted the accuracy of fluxes and wavelengths in calibrated data products, hence the motivation to derive a new geometric distortion correction. To derive the distortion corrections in both the dispersion and cross-dispersion directions (dx and dy, respectively), we utilized Pt-Ne lamp spectra obtained during the Thermal Vacuum testing campaign in 2003. This process involved comparing the observed locations of individual emission lines from the Pt-Ne spectra with ray-trace models. Through interpolation and extrapolation we generated the reference file image arrays corresponding to the full active area of the detector. The new GEOFILE addresses previous artifacts and sub-optimal corrections, significantly enhancing performance across multiple detector regions in conjunction with newly implemented walk and delta-geometric corrections.

July 02, 2025N. Indriolo et al.

The geometric distortion correction—one of the earliest steps in the calibration of COS FUV data—enforces a uniform plate scale across the detector such that all digital pixels correspond to the same physical size. The walk correction, which follows the geometric distortion correction, ensures that photons observed at different gain levels are properly aligned on the detector. During the effort to measure X-walk (walk in the dispersion direction) and re-measure geometric distortion, it was determined that coherent, small-scale (≤ 5 pixels) errors in the dispersion direction remained, even after application of new geometric distortion and X-walk corrections. The decision was made to remove these residuals using a delta-geometric correction, to be applied after the initial geometric distortion correction, and before the X-walk correction within the flow of the CalCOS pipeline. Here we describe the motivations for using a delta correction, the data used to measure the correction, and the analysis used to create a new DGEOFILE reference file.

July 02, 2025N. Indriolo et al.

The process of assigning incident photons to digital pixels on the far ultraviolet (FUV) detector of the Cosmic Origins Spectrograph (COS) suffers from distortions. Nominally these are removed via the geometric distortion correction and walk correction steps within the CalCOS pipeline, but over time it has become apparent that at some locations on the detector the existing corrections are inadequate. Here we describe the overall effort to revise the COS FUV geometric distortion and walk corrections, first explaining the need for new corrections, and then presenting results that demonstrate improved calibration.

April 28, 2025N. Indriolo

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 32.

April 01, 2025E.M. Frazer et al.

We report on the observations and analysis completed to find the optimal placement that centered the target in the fixed lifetime position (LP) 5 aperture of the Cosmic Origins Spectrograph (COS). In November 2020, we executed Program 16430, in which observations of the target star WD0947+857 were taken over a single two-orbit visit. During this visit, the target position was scanned across the fixed aperture in both the dispersion and cross-dispersion directions. By analyzing the throughputs at each position in comparison to the measured maximum, we calculated median offsets of -0.06690 arcseconds for the V2 coordinate and -0.00266 arcseconds for the V3 coordinate. These values were used to fine-tune the entries in the Science Instrument Aperture File (SIAF) that holds the locations of all COS apertures in spacecraft (V2, V3) coordinates at every LP. The SIAF was successfully updated in February 2021 for use with all LP5 calibration and science observations starting in Cycle 29.

April 01, 2025E. M. Frazer et al.

In Fall 2020, the enabling phase began for the Cosmic Origins Spectrograph’s (COS) next lifetime position (LP) moves: G130M cenwaves 1291, 1300, 1309, 1318, and 1327 to LP5 and G140L cenwave 800 to LP3. Data were taken as a part of three different observing programs in order to determine the target placement for LP5, find the best focus positions for LP5 and LP3/cenwave 800, and verify target acquisitions at LP5. Here we present a summary of the enabling activities, including each program and their deliverables.

February 25, 2025E.M. Frazer et al.

This report describes the creation of the XTRACTAB, PROFTAB, TRCETAB, and TWOZXTAB reference files used in the BOXCAR and TWOZONE extraction algorithms for COS/FUV G160M cenwaves 1533, 1577, 1589, 1600, 1611, and 1623, which moved from lifetime position (LP) 4 to the new LP 6 in October 2022. Observations of the white dwarf standard stars WD0308–565 and GD71 were obtained in Program 16906 between March and May 2022. Using this dataset, we analyzed the 2-D profile and spectral trace for each cenwave and segment to derive parameters for the 1-D spectral extraction, trace correction, profile, and TWOZONE spectral extraction reference files. After performing scientific and technical testing to ensure the files produced well-calibrated spectra, they were submitted to the reference file database system in September 2022 for use in the COS calibration pipeline.