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  1. ISR 2024-03 : COS Lifetime Position 6 Enabling Summary

    February 19, 2024K. Rowlands et al.
    We summarize the activities performed to enable lifetime position 6 (LP6) for the Cosmic Origins Spectrograph (COS) instrument on the Hubble Space Telescope (HST). LP6 is located +6.5” above LP1. All G160M cenwaves were enabled at LP6, which became the default LP for G160M exposures on 2022 October 3, at the beginning of Cycle 30. This ISR presents an overview of the LP6 enabling programs, which includes target placement, focus sweep, and target acquisition enabling and verification.
  2. ISR 2024-02: Cycle 29 COS FUV Wavelength Scale Monitor

    February 01, 2024D. French
    We report results of the Cycle 29 COS FUV dispersion solution zero point monitoring program 16534. Monitored modes include G130M cenwaves 1096, 1222, 1291, and 1327, G160M cenwaves 1577 and 1623, and G140L cenwaves 1105 and 1280. Spectra of target star AV 75 were obtained approximately one year from the Cycle 28 iteration of this program. Results from cross-correlations with reference COS, STIS and FUSE data show that the wavelength accuracy of all monitored modes are within the established specifications of 3 pixels for G130M and G160M settings, and 9 pixels for G140L settings.
  3. ISR 2024-01: The Hubble Advanced Spectral Product (HASP) Program

    January 16, 2024J.H. Debes et al.
    The Hubble Advanced Spectral Products (HASP) program is designed to robustly coadd Cosmic Origins Spectrograph (COS) and Space Telescope Imaging Spectrograph (STIS) spectra within the Mikulski Archive for Space Telescopes (MAST) in an automated fashion such that coadds are available for new data or archival data with updated calibrations. For each target within a visit or program, HASP employs a meticulous multi-stage filtering process to ensure data quality and creates coadded products for all central wavelengths (CENWAVEs) within specific gratings, as well as combined products using different gratings and instruments. The project also emphasizes making the code accessible to the user community for custom coaddition. As calibrations improve and new data are added to the archive, HASP products are re-created automatically so that they represent the best reduction of a given visit or program. Automated coadditions like those achieved by HASP can significantly enhance the combination of different CENWAVES, increase signal-to-noise ratios, and increase wavelength coverage. These properties make HASP a vital resource for astronomers using archival spectroscopic data from HST.
  4. ISR 2023-28: Derivation of the COS FUV Dispersion Solutions at Lifetime Position 6

    December 13, 2023W.J. Fischer et al.
    At strategically chosen times, the locations where spectra are obtained with particular central wavelength settings (cenwaves) on the far-ultraviolet detector of the Hubble Space Telescope’s Cosmic Origins Spectrograph are changed. These locations are known as lifetime positions (LPs). At the beginning of Hubble’s Cycle 30 (October 2022), LP6 became the default position for obtaining spectra with the G160M grating. At 6.5′′ above the original LP in the cross-dispersion direction, the best focus value differs from that at other LPs. Therefore, new dispersion solutions are required for each affected cenwave. Here we discuss the derivation and testing of dispersion solutions for the G160M cenwaves at LP6. These were incorporated in an updated DISPTAB reference file, delivered to Hubble’s Calibration Reference Data System on 2022 September 14.
  5. ISR 2023-27: Cycle 29 COS NUV Spectroscopic Sensitivity Monitor

    December 08, 2023W.J. Fischer
    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. Because the sensitivity of the fourth grating, G285M, has become very low, it was removed from the routine monitoring program, and it is now available-but-unsupported for General Observer programs. Results from the Cycle 29 NUV TDS program show that the G230L and G185M gratings, which are coated in MgF2, exhibit trends consistent with little or no change. On the other hand, the G225M grating, which is bare aluminum, shows a sensitivity decline of −2.86% ± 0.18% yr−1.
  6. ISR 2023-26: Monitoring COS Spectral Placement after an OSM1 Home Position Move

    November 30, 2023J.H. Debes et al.
    In ISR 2023-24 Hasselquist et al. (2023) described systematic trends and offsets with spectrum placement on the detector due to the Optics Select Mechanism 1 (OSM1) during Lifetime Position 4 (LP4) operations and after the start of operations at LP5. While these trends are within the requirements for COS and are usually corrected by internal WAVECALs, shifting the position on which Lyman-a falls has a potential impact on detector lifetime. These trends correlate with historical moves to the STScI Home Position for OSM1. To mitigate the observed behavior, the COS team, in consultation with NASA/Goddard Space Flight Center, Ball Aerospace, and the Hubble Space Telescope Mission Office, reverted the STScI home position for OSM1 back to G130M/1291 on 25 July 2022 from G130M/1222. Concurrent with this move, the special calibration program 17229, “COS Spectral Shift Monitor” was executed. The goal of the program was to monitor the location of the spectrum positioning after the home position move in order to better understand the physical origins of the behavior seen. This ISR describes the structure of the special calibration program and the resulting evolution in OSM1 positioning. We find that the choice of home position can impact the repeatability and average placement of spectra on the detector by the OSM1, potentially due to redistribution of lubricant from changes in mechanism motion patterns. In Program 17229, we find that a trend of increasing placement along the dispersion direction for G130M/1222 reversed and eventually flattened. For G130M/1291, the scatter in placements and offsets between small OSM1 moves and larger OSM1 moves is reduced by tens of pixels. The changes in behavior with home position generally supports the theory that lubrication may be a cause for what has been observed.
  7. ISR 2023-25: Overview of COS Lifetime Position 6 Calibration

    November 16, 2023R. Sankrit et al.
    The Cosmic Origins Spectrograph (COS) Far-Ultraviolet (FUV) modes G160M/1533, 1577, 1589, 1600, 1611 and 1623 moved from Lifetime Position 4 (LP4) to the new Lifetime Position 6 (LP6) on October 3, 2022 at the beginning of Cycle 30 operations. The procedures for calibrating these modes at LP6 followed those used for previous LPs. A number of observing programs were implemented, and the data analyzed to obtain calibration parameters, which were then incorporated into the reference files used by the COS pipeline. We present an overview of the LP6 calibration activities, brief descriptions of the individual observing programs, and a list of the output products. A series of Instrument Science Reports (ISRs) present the details of the calibration activities described here.
  8. ISR 2023-24: Investigating the Long-term Repeatability of the COS OSM1 Mechanism

    November 07, 2023S. Hasselquist et al.
    The Optics Select Mechanism 1 (OSM1) in the COS instrument is responsible for placing the spectrum on the FUV XDL detector along the dispersion direction in a repeatable way, to a required accuracy of better than half an FP-POS, or ~125 detector pixels. In May 2022, the COS team detected two separate changes in OSM1 motions. First, within the G130M grating positions at the 1291 central wavelength, small OSM1 motions (1 motor step) to the FP-POS = 3 and FP-POS = 4 locations resulted in spectral placement that was offset (~120 pixels) and less precise (𝜎 = 25 pixels to 𝜎 = 40 pixels) than large OSM1 motions (>200 motor steps) to these locations. This behavior began at the start of LP4 operations, and impacts the gain sag holes on the detector due to geocoronal Lyman alpha emission falling on different locations of the FUVB segment. Second, at the 1222 central wavelength, the FP-POS=2 and 4 locations showed a significant positive trend with time in spectrum positioning at a rate of nearly 0.5 pixels/day for close to six months before flattening out. Since this 1222 behavior was directly correlated with a change in OSM1 home position, the COS team hypothesized this change may be the cause of a significant fraction of the behavior. Aside from slightly larger gain sag holes, these changes in OSM1 motions have minimal science impacts on COS data, although users who are concerned with precise spectral placements on COS FUV might want to consider mitigation strategies.
  9. ISR 2023-23: Cycle 29 COS NUV Dark Monitor Summary

    October 30, 2023C.I. Johnson
    This Instrument Science Report describes the data analysis and results from the Cycle 29 NUV Dark Monitoring Program for the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope covering dates between November 2021 to October 2022. We present an overview of the calibration plan and summary of the derived NUV MAMA dark rates. We compare the dark rates reported for the Exposure Time Calculator (ETC), which was 1.20×10−3 counts pixel−1 second−1, to the final dark rate value measured over the full calibration cycle, which was slightly higher at 1.24×10−3 counts pixel−1 second−1. The full Cycle 29 NUV dark rate increased by about 12% compared to the Cycle 28 NUV dark rate. We measure the magnitude of the increase in the mean NUV dark rate with time, finding a slope of about 2.55×10−5 counts pixel−1 second−1 year−1. Additionally, we report on the discovery of a repeating sinusoidal pattern in the NUV dark rate with an amplitude of about 1.38×10−4 counts pixel−1 second−1 within each 366 day period.
  10. ISR 2023-22: Cycle 29 COS FUV Dark Monitor Summary

    October 16, 2023C.I. Johnson
    This ISR summarizes the results of the Cycle 29 FUV Dark Monitoring Program for the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST) covering dates between November 2021 and October 2022. We provide an overview of the calibration plan and summary, and compare the Cycle 29 dark rates against those reported for the exposure time calculator as well as Cycle 28. We find that the spectroscopic and target acquisition dark rates for Cycle 29 increased by about 7% for FUVA and 30% for FUVB compared to Cycle 28, which was expected given the recent increase in solar activity.
  11. ISR 2023-21: The LAMPTAB Reference File at Lifetime Position 4

    September 26, 2023E. Frazer & R. Plesha
    A new COS/FUV lamp template reference file (LAMPTAB) for Lifetime Position 4 (LP4) was delivered for use in the COS calibration pipeline in April 2018. The LAMPTAB contains reference spectra used during the wavelength calibration of COS/FUV spectroscopic data. The data used to derive the lamp spectral templates were acquired in August 2017 in Program 15369 as a part of the LP4 calibration activities. In this ISR, we document the creation and testing procedures for this reference file.
  12. ISR 2023-20: The Spatial Resolution of the COS FUV Channel at Lifetime Position 5

    September 25, 2023S. Dieterich et al.
    We report the spatial resolution for COS FUV cenwaves 1291, 1300, 1307, 1318, and 1327, all on the G130M grating, which are moved to Lifetime Position 5 in October 2021. We use observations of the calibrator white dwarf WD0308-565 to validate model cross-dispersion spread functions (CDSFs). When measured as the FWHM of the CDSF, the spatial resolution ranges from 1.6” for the central region of cenwave 1291 to 0.3” at the red end of cenwave 1327. We found overall good agreement in the shapes of model and observed CDSFs even though the model CDSFs are approximately 0.17” too narrow at the red half of the spectra. The model CDSFs discussed here are available for download at the COS website.
  13. ISR 2023-19: Focusing on New COS FUV Lifetime Positions: G130M/1222 and G160M at LP6

    August 07, 2023T.C. Fischer
    In the final two weeks of 2021, focus sweeps for the Lifetime Position 6 (LP6) were performed at a +6.5” sky offset with the G130M and G160M gratings through HST Program 16850. The focus sweeps were performed on the subdwarf B star Feige 48 at a range of focus settings. An auto-correlation technique was used to find the minimal line widths of absorption lines and therefore find the optimal focus for each setting. The final, absolute focus values for G130M/1222 and G160M/1600 are -881 steps and +108 steps, respectively.
  14. ISR 2023-18: Cross-validating Models of Flux-Standard White Dwarfs

    July 17, 2023R. Sankrit et al.
    The Cosmic Origins Spectrograph (COS) instrument is flux calibrated by comparing observations of standard white dwarf (WD) stars with models of those stars. The model parameters are derived from fits to the CALSPEC STIS and WFC3 spectrophotometry from 1350 ˚A to 1.7 μm, where these model fits agree with the CALSPEC spectral energy distributions to ∼1%. The lack of constraints on the model fit below 1350 A increases the model uncertainties above 1% in the COS 912–1150 A wavelength range. New contemporaneous observations of WD0308-565 and WD1057+719 better quantify these uncertainties. WD0308-565 is the standard used to calibrate all the COS modes covering the shortest wavelengths, and WD1057+719 is another standard accessible to COS. Based on our study, the model uncertainty contributes an additional ±5% uncertainty to the COS absolute flux calibration in the 912–1070 A wavelength range.
  15. ISR 2023-17: Cycle 29 COS/FUV Spectroscopic Sensitivity Monitor

    June 28, 2023K. Rowlands & R. Sankrit
    The Cycle 29 COS/FUV spectroscopic sensitivity monitor ran from December 2021 to October 2022. Observations of the G160M modes and G130M/1222 were obtained at Lifetime Position 4 (LP4), the blue modes (G130M/1055 and G130M/1096) were obtained at Lifetime Position 2 (LP2), the G130M standard modes (longwards of cenwave 1222) were obtained at LP5, and the G140L modes were obtained at LP3, starting Oct 4 2021. Connection visits were obtained in preparation for the start of G160M at LP6, and the second pair of LP3 and LP5 connection visits were obtained for G140L and G130M. The Time-Dependent Sensitivity (TDS) slopes of all modes ranged from 0% to −3% per year. In this ISR we describe the program and its execution, and provide a summary of the analysis and results. Based on the change in the net count rates over a one year timescale, and accounting for how a change in HV affects count rates, we find that the FUV TDS does not depend on LP between LP4 and LP3.
Last Updated: 06/02/2023

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