INSTRUMENT SCIENCE REPORTS (ISRs)

We summarize the calibration activities for the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope during Cycle 24, which ran from November 2016 through October 2017. We give an overview of the COS calibration plan, 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 24 are also given here. Results of the analysis attached to each program are published in separate ISRs.

We present revised wavelength solutions for the COS blue modes, i.e. central wavelength (cenwave) settings of G130M/1055 and G130M/1096 at lifetime position 2 (LP2). We utilized COS observations of two stars, AV75 and HD 93205, taken at the two cenwaves of interest, along with FUSE and STIS data of these stars. The dispersion solutions (dispersion coefficient and zero-point) of detector FUVA for both cenwaves were obtained by using AV75 observations. However, for FUVB, we used HD 93205 data to derive the dispersion coefficients while we used AV75 to obtain the zero-points. We updated the newly derived dispersion solutions to the DISPTAB reference file for LP2, which was delivered in September, 2020, for use by the COS calibration pipeline, CALCOS. With these new wavelength solutions, the internal accuracies of the blue modes solutions are within the target of ±3 pixels (~ ±9 km s1) like those of the other G130M modes. Offsets of ~5 pixels can occur for FUVB due to the lack of wavelength calibration lamp spectra.

Here we summarize the Cycle 26 FUV Dark Monitoring Program for the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST) covering dates November 2018 to October 2019. We give an overview of the calibration plan and summary for this calibration program.

Program 15534 used the onboard deuterium lamp to illuminate the regions of the COS FUV detector used during normal operations in Cycle 26. Data were taken two times during the year at detector positions corresponding to Lifetime Position 2 (LP2), LP3, and LP4. The pulse height information obtained was used to create gain maps in order to monitor the detector gain sag. The results obtained are consistent with those from previous cycles.

The Cycle 26 COS/FUV spectroscopic sensitivity monitor ran from December 2018 to November 2019. The standard mode observations were obtained at Lifetime Position 4 (LP4), the nominal position for COS starting October 2, 2017, and the blue modes (G130M/1055 and G130M/1096) were obtained at Lifetime Position 2 (LP2). The new modes introduced in Cycle 26 (G130M/1533 and G140L/800) were included in the monitoring observations starting in February 2019. The Time-Dependent Sensitivity (TDS) slopes of the standard and blue modes were found to remain steady, and range from 0% to −4% per year. The TDS slopes of the new cenwaves 1533 and 800 were calculated from the data obtained in this program. In this ISR we describe the program and its execution, and provide a summary of the analysis and results.

In preparation for the move to COS FUV Lifetime Position 5 (LP5) and future LPs located above +5.4′′, we verify that there are no systematic effects introduced by calibrating science exposures taken at LP5 (+5.4′′) with wavecal exposures taken at LP2 (+3.5′′). We demonstrate this using a cross-correlation technique that quantifies the accuracy with which Pt-Ne lamp spectra return to the same position in the dispersion (x) and cross-dispersion (y) directions after the aperture block moves between LP2 and LP5. We find that successive LP5 exposures separated by aperture moves are aligned at the 0.1–0.4 pixel level in x, and that these small offsets can be explained by OSM drift, and at the 0.026 pixel level (1σ) in y, indicating the movement is reproducible. Together these results indicate that calibrating COS FUV science data taken at LP5 with lamp flash data taken at LP2 would not adversely affect the wavelength calibration or spectral resolution of the data.

We report on the monitoring of the zero points of the COS NUV dispersion solutions during Cycle 26 in program 15541. Select cenwaves were monitored for all NUV gratings except for G285M, which is no longer monitored due to its rapidly declining sensitivity and lack of general observer use since Cycle 21. Comparisons to COS monitoring data obtained in previous cycles indicate internal stability within the allowed ranges of zero points. Comparisons to STIS data indicate small but persistent COS offsets of 1 – 3 pixels toward shorter wavelengths. The cause of this is not yet well understood.

We report on the monitoring of the zero points of the COS FUV dispersion solutions during Cycle 26 in program 15536. Select cenwaves were monitored for all FUV gratings. Comparisons to COS monitoring data obtained in previous cycles indicate internal stability within the allowed ranges. Comparisons to FUSE and STIS data indicate satisfactory absolute calibration. All measured offsets are within the established thresholds of 6 pixels for G130M/1096, 3 pixels for the other G130M and G160M cenwaves, and 9 pixels for the G140L cenwaves.

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, is now very low, it was removed from the routine monitoring program, and its use is discouraged for General Observer (GO) programs. Results from the Cycle 26 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 significant sensitivity decline of −2.87% ± 0.18% yr−1. It was discovered during Cycle 26 that the current NUV TDSTAB overestimates the rate of sensitivity loss for each grating. NUV fluxes have therefore been slightly overcorrected by an amount that grows with time and have, in recent years, become inconsistent with the 5% accuracy specification. Work is underway to correct this.

We summarize the calibration activities for the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope during Cycle 26 which ran from November 2018 through October 2019. We give an overview of the COS calibration plan, 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 26 are also given here. Results of the analysis attached to each program are published in separate ISRs.

Program 15370 used the onboard deuterium lamp to illuminate the LP2, LP3, and LP4 regions of the COS FUV detector at the time of the move to LP4 on October 2, 2017. The data show that the modal gain for the deepest hole at LP3 was very close to three, and thus it was the appropriate time to move. The gain maps at LP4 show that the gain is consistent with what is expected from similar data taken in the enabling phase of the LP4 process, and there is no sign of a hole due to airglow.

Program 14941 used the onboard deuterium lamp to illuminate the regions of the COS FUV detector used during normal operations in Cycle 25. Data were taken two times during the year at detector positions corresponding to LP2, LP3, and LP4. The pulse height information obtained was used to create gain maps in order to monitor the detector gain sag.

We summarize the calibration activities for COS during Cycle 25, which ran from 2017 October through 2018 September. 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 25 are also given. Results of the analysis attached to each program are published in separate ISRs.

This report summarizes the calibration plan, analysis, and results for the Cycle 25 NUV Dark Monitoring Program (14942) for the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST). Observations for this program were taken for the entirety of Cycle 25, spanning from the end of 2017 October to the end of 2018 October.

The Cycle 25 COS/FUV spectroscopic sensitivity monitor ran from 2017 December to 2019 April. The time-dependent sensitivity (TDS) slopes were found to remain steady and range from 0% to −3%/yr. The program included two additional visits, during which the standard modes were observed at Lifetime Position 3 (LP3) to provide a check on the TDS at the previous lifetime position. It was found that the TDS is largely independent of lifetime position.