Instrument Science Reports (ISRs)

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.

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.

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.

We describe the creation of wavelength calibration lamp template reference files (LAMPTABs) for the COS/FUV central wavelength settings G130M/1291, 1300, 1309, 1318, and 1327 at Lifetime Position 5 (LP5), as well as G140L/800 at Lifetime Position 3 (LP3). We obtained Pt-Ne lamp exposures with Program 16469 in 2021 March. Analysis of the data involved determining the effects of drift from the Optics Select Mechanism 1 (OSM1) as a function of time, then removing this drift from each photon event to create the lamp template spectra. Accurate shifts for each FP-POS were determined by cross-correlating the lamp template data at each FP-POS with that taken at FP-POS = 3. These FP PIXEL SHIFT values were entered into the LAMPTABs, along with the drift corrected lamp template data at each cenwave, segment, and FP-POS. The files were tested for scientific accuracy and then delivered to the reference file database system in September 2021 for use with the COS calibration pipeline for Cycle 29 and beyond.

The exploratory study of lifetime position (LP) 5 resulted in the decision to commission LP5 at +5.4 arcsec above LP1 for the G130M setting only. G160M, however, could not be positioned at the same location because its spectral profile projects lower on the detector and would have been impacted by gain sag from LP2. Following this assessment, the LP6 Exploratory Study was conducted to determine the optimal placement of LP6 for the G160M setting above +5.5 arcsec from LP1. Operating COS in this region is complex due to a light leak that occurs above +5.5 arcsec on the detector which prevents concurrent wavelength calibration (wavecal) exposures, combined with the inability to use the deuterium lamp for gain measurements in this region. Here we provide a summary of the investigations conducted to optimize LP6 at this previously unused region with regards to its lifetime and resolution. We additionally describe the process involved with implementing a new mode of wavelength calibrations, ‘SPLIT wavecals.’ An assessment of both the spectral resolution and gain modeling above LP5 on the detector determined the optimal placement of LP6 to be at +6.5 arcsec on the detector. This position provides the highest resolution possible for LP6, while also enabling a future lifetime position above LP6.

We report on the zero point update of the COS NUV G230L grating at cenwave 2950, stripe B. An investigation of ULLYSES T Tauri spectra obtained with the COS NUV G230L grating in summer 2021 revealed offsets between cenwaves 2635 and 2950. This document presents the subsequent analysis of the alignment of cenwaves 2635, 2950, and 3000 compared to overlapping STIS E230M data. The zero point of G230L/2950 was found to be consistently shifted with respect to cenwaves 2635 and 3000. This shift was corrected and an updated DISPTAB was delivered on March 22, 2022. Possible offsets of 1–2 pixels between all G230L cenwaves and STIS E230M data remain, but were unable to be verified or corrected due to insufficient data. However, all G230L cenwaves now meet the required accuracy goals of 175 km s−1 (1.8 ˚ A, or 1.4–2.6 pixels).

Exposures were taken with the onboard deuterium lamp in order to illuminate the regions of the COS FUV detector used during normal operations in two programs during Cycle 28. Visits 3E and 4E of program 15772 provided an initial measure of the gain for new High Voltage (HV) levels for LP3 and LP4, and Program 16323 was executed to collect data two times during the year at 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.

Program 15772 used the onboard deuterium lamp to illuminate the regions of the COS FUV detector used during normal operations in Cycle 27. Data were taken two times during the year at detector positions corresponding to the three active Lifetime Positions (LPs): LP2, LP3, and LP4. 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.

High-frequency variations in the flat-field of the COS FUV detector result in fixed- pattern noise that limits the signal-to-noise achievable by the instrument. The effects of fixed-pattern noise can be reduced via spectral dithering by utilizing multiple FP-POS positions, which correspond to one motor step rotation of the optical select mechanism. There are four FP-POS offsets available for each central wavelength. Here, we mea- sure the maximum achievable S/N, (S/N)max, as a function of the number of FP-POS included in COS spectra for each segment and cenwave of the G130M, G160M, and G140L gratings. We use deep calibration observations aimed at measuring the two- dimensional spectral profiles of the different COS observing modes at LP3 to estimate the fixed pattern noise and resulting limiting S/N. As expected, we find that (S/N)max increases as the number of FP-POS used increases. (S/N)max also varies with wave- length (i.e., across a segment), grating, and cenwave. This variation can be attributed to changes in the width of the cross-dispersion profile. For settings or detector regions with wider cross-dispersion profiles, the fixed pattern noise is averaged over a larger number of two-dimensional pixels, resulting in a lower level of fixed pattern noise in extracted, one-dimensional spectra. With G130M, users can reach S/N of about 25, 30, and 35 with 2, 3, and 4 FP-POS, respectively. With G160M, users can expect to reach S/N of 22, 28, and 32 with 2, 3, and 4 FP-POS, respectively. And with G140L, a maxi- mum S/N of 23, 29, and 36 can be reached with 2, 3, and 4 FP-POS, respectively. Users are encouraged to use as many FP-POS as allowed by science requirements in order to maximize the legacy value of their data.

The LP5 Exploratory Study was conducted to determine if and where LP5 could be placed at the top of the COS FUV detector. Due to a light leak known to occur above +5.5 arcsec on the detector, the complexity of operating COS in this previously unused region required a detailed investigation, which we present here. Firstly, we describe how the detector characteristics above +5.4 arcsec were explored, including accurately measuring the position of the light leak at +5.5 arcsec from the Pt-Ne2 medium current lamp, mapping the modal gain above +5.4 arcsec, and determining the 2D spectral profiles at these locations. With these details in hand, it was decided that while G130M can be operated at 5.4 arcsec, right below the light leak, for 4–5 years, the effects of LP2 gain sag on the G160M profile (which projects slightly lower on the detector) were too restrictive on its lifetime and G160M should be placed above +5.5 arcsec (LP6) where non-concurrent wavecals are required. We describe our preliminary investigation into the feasibility of operating G160M at this location - an essential step in finalizing the design of LP5 itself. We found that while the stability of the aperture placement was sufficient for non-concurrent wavecals, it resulted in significant increased overheads which we successfully mitigated as part of the LP6 Exploratory study. The culmination of this study was a plan for operating COS at several LPs simultaneously and thereby enabling operations until 2030 and beyond.

The wavelength calibration procedure employed by CalCOS at LP6 relies on measuring the separation in the dispersion direction between Pt-Ne lamp spectra that are observed immediately before and after a science exposure, and a template Pt-Ne spectrum for which the dispersion solution of the Primary Science Aperture is defined. Template spectra for each Cenwave/Segment/FP-POS combination are stored in the LAMPTAB reference file, and must be updated with each new lifetime position due to changes in the absolute focus. This document describes the creation of the LAMPTAB file for use at LP6 at the beginning of HST Cycle 30 (2022 Oct 1).

We report measurements of the COS FUV spectral resolution at Lifetime Position 5 (LP5). We validated theoretical Line Spread Functions (LSFs) by convolving a high-resolution STIS spectrum of AV 75 with model LSFs and comparing narrow-line spectral features to newly observed COS spectra of the same star taken at LP5 with cenwaves G130M/1291 and G130M/1327. The resolving power at LP5 ranges from approximately 12,000 to 17,000, and is generally higher than at LP4. We found that when using the model-supplied LSFs the corresponding resolving power R was accurate to within 2,000 units, or approximately 15 percent. The model LSFs are available for download from the COS website1.

A sixth lifetime position (LP6) was enabled on the Cosmic Origins Spectrograph (COS) in October 2022 in an effort to maximize the lifetime and scientific output of the far- ultraviolet (FUV) detector. We present the results of Visit 5C for Program 16829, which used the on-board Deuterium lamps to illuminate the LP6 region of the COS FUV detector with the goal of measuring the modal gain for the start of LP6 operations. The resulting gain maps indicate that the starting high voltage values are appropriate for both the FUVA (167) and FUVB (169) segments. No gain sag holes are found on the FUVA segment, but the FUVB segment has several sagged columns in regions that overlap with LP5 G130M/1291 Lyman α. These holes were expected due to the close proximity of LP5 and LP6 on the detector, and the majority of the pixels in the LP6 rows have modal gain larger than 6.

We determine the position of the aperture center of the Cosmic Origins Spectrograph (COS) at Lifetime Position 6 (LP6). This work determined the location of maximum instrument throughput for LP6 in the HST focal plane, enabling targets to be centered in the aperture. Program 16849 was observed on 20 December 2021 and was comprised of one two-orbit visit of the target star WD1337+705, during which position target (POS TARG) values were varied over several step sizes (typically ∼0.2 arcsec) across both the dispersion and cross-dispersion axes. By comparing throughputs at each position relative to the measured maximum, mean offsets of −0.07208 arcsec and −0.00143 arcsec were calculated for the V2 and V3 coordinates, respectively. These adjustments to the Science Instrument Aperture File (SIAF) were tested for scientific accuracy and then uploaded to the flight software (FSW) in 2022 February for use in HST Cycle 30.

New stellar atmosphere models are available for several flux standard White Dwarf stars, including WD0308-565 and GD71, which are both used for flux calibration of the Cosmic Origins Spectrograph (COS) instrument on the Hubble Space Telescope (HST). These models were normalized to match revised Vega and Sirius magnitudes. Compared to previously used models, the predicted flux increases by an average of 2% across the COS wavelength range, with changes of up to 10% at short wavelengths found for WD0308-565. We, therefore, updated the COS Photometric Throughput Tables, or FLUXTAB reference files, for both the FUV and NUV observation modes. We applied a wavelength-dependent scaling factor to the sensitivity curves found in the FLUXTABs for the FUV at Lifetime Positions (LP) 1 through 4 and for the NUV. We have validated our updated FLUXTAB files by re-calibrating our data and confirming that it matches the CALSPEC standard models to within 2% or less. Seven new FLUXTABs were delivered in February and June 2022 and are used for the COS data that is available on MAST.