Instrument Science Reports (ISRs)

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.

COS lifetime position 6 (LP6) is located towards the upper edge of the COS FUV detector such that lamp flashes used for wavelength calibration cannot be taken without first moving the aperture block to another location on the detector, resulting in longer overheads per exposure. Depending on the S/N requirements of an observation, the increased overhead time can be reduced if LP6 observations consist of fewer than four exposures at each fixed pattern position (FP-POS), a requirement intended to mitigate the effect of fixed-pattern noise on limiting S/N. In this work, we use LP6 data taken during 2024 to determine which FP POS settings are required to create a combined spectrum that is continuous in wavelength coverage across each segment. We find that FP-POS = 1 and FP-POS = 4 is the only combination of two FP-POSs that results in continuous wavelength coverage for both FUVA and FUVB. However, for cenwave 1533, which is the G160M cenwave with the widest profile in the cross- dispersion direction, this combination results in a wavelength discontinuity of size ∼0.3 ˚A at ∼1439.5 ˚A on FUVB due to one of the gain-sag holes from LP5 overlapping with a very low response region. Cenwave 1623 observations could also be affected by this very low response region depending on the exact cross-dispersion location of an observation. Therefore, users of cenwaves 1533 and 1623 at LP6 should consider observing with three FP-POSs if they want continuous wavelength coverage on FUVB.

We discuss the gain map measurements made during Cycle 30. Exposures were taken with the onboard deuterium lamp in order to illuminate the regions of the COS FUV detector used during Cycle 30, which ran from October 2022 to October 2023. Cycle 30 program 17248 was the standard yearly gain map program that collected data at two times during the year for all Lifetime Positions. The pulse height information obtained was used to create gain maps in order to monitor the detector gain sag, and thus to determine when to adjust the commanded high voltage on the detector. Higher than expected usage on LP5 resulted in a more rapid decrease in gain than had been anticipated, and will result in a shorter life at that location.

We discuss the gain map measurements made during Cycle 29. Exposures were taken with the onboard deuterium lamp in order to illuminate the regions of the COS FUV detector in use during Cycle 29, which ran from October 2021 to October 2022. Data from four calibration programs (PIDs 16333, 16471, 16829, and 16833) were used. The pulse height information obtained was used to create gain maps in order to monitor the detector gain sag, and thus to determine when to adjust the commanded high voltage on the detector.

This report details the generation of reference files employed in the BOXCAR and TWOZONE extraction algorithms for COS/FUV G130M cenwaves 1291, 1300, 1309, 1318 and 1327 at the new lifetime position (LP) 5 and G140L cenave 800 at LP 3. Observations of the bright white dwarf standard star WD0308–565 were conducted under Program 16466 in March 2021. Utilizing this dataset, we analyzed the 2-D profile and spectral trace across each detector segment to derive parameters for the 1-D spectral extraction, trace correction, profile, and TWOZONE spectral extraction reference files. Following validation for scientific accuracy, these files were submitted to the reference file database system in November 2021 for integration into the COS calibration pipeline.

We report on the creation and validation of the photometric throughput table (FLUXTAB) and flat-field file (FLATFILE) for the sixth lifetime position (LP6) of the Cosmic Origins Spectrograph (COS) Far Ultraviolet (FUV) detector, which became active in October 2022 at the beginning of Cycle 30. Using observations of the flux-standard star WD0308-565, we derive new sensitivity curves and low-order flat fields (L-FLATS) at LP6 for both segments of the G160M central wavelengths. The L-FLATS and sensitivity curves for LP6 agree within 2% of those at LP4, the previous LP of the G160M cenwaves. In addition, the LP6 FLUXTAB and FLATFILE reference files meet our absolute and relative flux calibration requirements of less than 5% and 2%, respectively.

The dark rate for the far-ultraviolet (FUV) cross delay line detector on the Cosmic Origins Spectrograph is known to be sensitive to the Solar activity cycle. As a result, the dark rate can change significantly with time and requires tracking in order to properly update the exposure time calculator. This ISR provides an overview of the calibration plan and summary of the FUV dark monitoring program (Program ID: 17247), and compares the Cycle 30 dark rates against those of previous cycles. We find that the Cycle 30 FUV Segment A dark rates increased by 58-92% compared to Cycle 29 while the FUV Segment B dark rates increased by 81-100%. These values are consistent with expectations given that the Sun is near the maximum activity level for its current cycle. However, the Cycle 30 dark rate values do not exceed the peak rates observed during the previous solar maximum, which spanned 2011 through 2015.

Dark rates for the COS NUV MAMA detector are routinely monitored by taking a series of dark exposures every two weeks throughout each cycle. This Instrument Science Report describes the data analysis and results for the Cycle 30 dark monitoring calibration program (Program ID: 16935) that ran between November 2022 and October 2023. We found a 95% mean dark rate value of 1.25 × 10−3 counts pixel−1 second−1 over the full time range of Cycle 30, which is an increase of 0.8% from Cycle 29. We also confirm that a previously discovered periodic signal in the dark rate is driven largely by the MAMA temperature, as measured from telemetry. When the dark rate’s temperature dependence is removed, the scatter in the dark rate is reduced by a factor of ∼1.5 and the periodic signal is diminished.

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 cycles 25, 26, 27, 28, 29, 30, and 31.

We summarize the calibration activities for the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope during Cycle 30, which ran from November 7, 2022 through November 6, 2023. We present an overview of the COS calibration plan and COS usage statistics, and we briefly describe major changes with respect to the previous cycle. High-level executive summaries for each calibration program comprising Cycle 30 are also given here. Results of the analysis attached to each program are published in separate ISRs.

We summarize the calibration activities for the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope during Cycle 29, which ran from November 1, 2021 through November 6, 2022. We give an overview of the COS calibration plan and COS usage statistics, and we briefly describe major changes with respect to the previous cycle. High-level executive summaries for each calibration program comprising Cycle 29 are also given here. Results of the analysis attached to each program are published in separate ISRs.

We summarize the calibration activities for the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope during Cycle 28, which ran from November 2020 through October 2021. We give an overview of the COS calibration plan and COS usage statistics, and we briefly describe major changes with respect to the previous cycle. High-level executive summaries for each calibration program comprising Cycle 28 are also given here. Results of the analysis attached to each program are published in separate 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.