Instrument Science Reports (ISRs)

We describe the effort to enable and verify COS FUV target acquisitions at Lifetime Position 5. Four HST visits were executed as part of program 16432. The first three visits verify target acquisition modes ACQ/SEARCH, ACQ/PEAKD, and ACQ/PEAKXD. The last visit verifies that these modes are still working after they are installed as part of the on-board flight software at the beginning of HST Cycle 29. We find that all modes are working within requirements. This Instrument Science Report is also meant as a case study of FUV target acquisition analysis, and several procedures are explained in detail. Appendix A describes the virtual target mechanism, and explains how to set up virtual targets in APT.

The majority of COS science observations have used concurrent TAGFLASH Pt-Ne lamp exposures for wavelength calibration, whereby the wavelength calibration lamp is flashed at certain intervals during an exposure. However, the COS wavelength calibration lamp cannot be flashed when the aperture block is at a position greater than 113 steps, which corresponds to +5.4” arcseconds on the sky relative to Lifetime Position (LP) 1, due to a light leak through the Flat-field Calibration Aperture (FCA). Therefore at LP6, located at +6.5” above LP1, the aperture block must be moved to a different position such that the lamp can be flashed and the wavelength zero point can be correctly accounted for, a process called ‘SPLIT-wavecals’. To reduce overheads due to SPLIT-wavecals, we undertook a study to determine whether a lampflash could be effectively removed without significantly increasing the uncertainty on the wavelength calibration or smearing the line profile resulting from uncorrected Optics Select Mechanism (OSM) drift. This ISR first describes the tests of four methods used to simulate observations without the 600 s lampflash while correcting for OSM drifts, and then presents a description of the adopted method. The chosen method minimizes the uncertainty on the FUV wavelength calibration. We found that the along-dispersion shift (in pixels) at 600 s was best replicated by a fractional value of the shift at the end of the exposure, with an additional dependence on exposure time. This correction was implemented in CalCOS starting with version 3.4.0 for LP6 observations.

We report results of the Cycle 29 COS NUV dispersion solution zero point monitoring program 16538. Monitored modes include G130L cenwaves 2635, 2950, and 3000, G225M cenwave 2217, and G185M cenwave 2010. Spectra of target star HD 6655 where obtained approximately one year from the Cycle 28 iteration of this program. Results from cross-correlations with reference COS and STIS data show that all monitored modes are found to be within specifications.

The COS data quality (DQ) flags are used to identify events in FUV and NUV datasets that are abnormal in some way. Here we describe each flag and how they are assigned in the COS calibration pipeline. Additionally, we report on updates to the FUV Bad Pixel Table (BPIXTAB), which is one of several reference files used to assign DQ values to data during calibration. The BPIXTAB contains a list of rectangular regions on the FUV and NUV detectors where the data quality is sub-optimal. In preparation for the move to lifetime position 4, we examined all regions listed in the existing reference file using recent dark and deuterium datasets and resized or removed them as necessary. We also searched for new areas to include. This updated FUV BPIXTAB was delivered in June 2019 for use in the calibration pipeline.

In preparation for the enabling of Lifetime Position 6 (LP6) and future lifetime positions, we measured the COS FUV spectral resolution at +7”, +9” and +11” above LP1 on the detector. We performed observations of the star AzV80 using cenwaves G130M/1222 and G160M/1577. Our analysis shows a decrease in the resolution from R ∼ 9000 − 13000at+7”,R ∼ 8000−12000at+9”,toR ∼ 7000−10000at+11’,withthe spectral resolution being on average 30% lower at +7” than that at LP2 (+3.5” above LP1). The decrease in resolution motivated the placement of LP6 as close as possible to LP5, +6.5” above LP1 both to maintain high resolution at LP6, and allow a future LP above LP6 with sufficient resolution for most science cases.

The Cycle 30 COS/FUV spectroscopic sensitivity monitor ran from December 2022 to October 2023. Observations of the G160M modes were obtained at Lifetime Position 6 (LP6), 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, G130M/1222 was obtained at LP4, and the G140L modes were obtained at LP3. Regular connection visits were obtained for the G160M modes to monitor the flux between LP4 and LP6. The Time-Dependent Sensitivity (TDS) slopes of all modes ranged from 0% to −6% 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 LP6.

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.

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.

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.

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.

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.

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.

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.

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.

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.