About This Article
In this STAN we discuss items of interest to those who are preparing Phase II proposals for Cycle 30. We also discuss the reappearance of a hot spot on the FUV detector and describe COS posters presented at the 240th AAS meeting.
The COS FUV detector experiences gain sag, where the ability of a given location on the detector to record photons diminishes with repeated exposure to light. To mitigate this, every few years a new location is introduced for the spectra to fall on the COS FUV detector. In Cycle 30, we introduce LP6, and nearly every LP will host a subset of the central wavelength settings (cenwaves). Figure 1 schematically illustrates the locations of each LP on the COS FUV detector segments and also serves as a guide to the policies discussed below.
- G130M cenwaves 1055 and 1096, the blue modes, will remain at LP2.
- G130M cenwave 1222 will remain at LP4.
- G130M cenwaves 1291, 1300, 1309, 1318, and 1327 will remain at LP5.
- G160M cenwaves will move from LP4 to a new LP6; see the next article for exceptions.
- G140L cenwaves will remain at LP3.
Although the spectral resolution and sensitivity vary slightly with LP, this arrangement is mostly transparent to users. An exception is the use of LP6, which carries increased overheads that are discussed in the next article. The characteristics of lifetime positions through LP4 are discussed in existing documentation, while the characteristics of LP5 and LP6 will be covered in current and forthcoming instrument science reports.
Figure 1. A portrayal of where available modes will fall on each segment of the COS FUV detector in Cycle 30. For simplicity, the widths of the regions do not represent the actual cross-dispersion widths of each mode. Annotations also serve as a guide to the COS2025 policies discussed below.
In Cycle 30, G160M spectroscopy will occur at LP6 by default. At 6.5" above the original LP1 on the detector, use of the wavelength calibration lamps at LP6 causes detector-damaging light to leak through the flat-field calibration aperture. To avoid this, wavelength calibration data must be obtained lower on the detector, before and after a science exposure instead of concurrently. These asynchronous wavelength calibration exposures, known as split wavecals, increase overheads. The overheads are significant only when more than two G160M science exposures are obtained per orbit. If more than two G160M science exposures are required per orbit, LP4 may be used to avoid the increased overheads. This strategy must be justified in the Phase I proposal and approved by the COS instrument team.
Figure 2 is a flowchart showing the cases for which LP4 may be requested. Recall that FP-POS settings are small offsets along the dispersion direction, and the use of multiple FP-POS settings with the COS FUV channel is required to reduce fixed-pattern noise.
If more than one G160M orbit is required per target, there will be at most two FP-POS exposures per orbit, and LP6 must be used. If only one orbit is required per target, the number of required FP-POS depends on the desired signal-to-noise (S/N) ratio. (For gratings other than G160M, all four FP-POS are normally required.)
- If the desired S/N is ≤ 20, only two FP-POS are required.
- If the desired S/N is > 20 and ≤ 25, only three FP-POS are required and LP4 may be used.
- If the desired S/N is > 25, all four FP-POS are required and LP4 may be used.
Regardless of the number of FP-POS required, FP-POS 1 and 4 should be included among them for the best wavelength coverage. Note that the G160M warning in APT is advisory, and there is no need to try to clear it. G160M spectroscopy remaining from previous cycles will continue to take place at LP4.
Figure 2. A flowchart showing the conditions under which LP4 may be requested for G160M spectroscopy. This also appears in Section 9.5.1 of the COS Instrument Handbook, which may be consulted for more information.
Users preparing Cycle 30 Phase II submissions are reminded that the COS2025 policies are still in effect. These policies consist of restrictions on the choice of detector segment and FP-POS positions for the G130M observing modes. The policies are designed to maximize the FUV detector lifetime by minimizing the exposure of the FUVB detector to geocoronal Ly α emission. They were introduced when COS FUV spectroscopy moved to Lifetime Position 4 (LP4) in 2017 and continue at the current LP5. Under COS2025, there are four G130M central wavelengths (cenwaves) that can be used with both detector segments on 1055, 1096, 1222, and 1291. For the other G130M cenwaves (1300, 1309, 1318, 1327) only segment FUVA can be on. Observations of the zero-redshift Ly α wavelength range can be performed at LP3. This strategy must be justified in the Phase I proposal and approved by the COS instrument team. Detailed information about the rules is available at the COS2025 policies page.
Submitting a safe, technically sound Phase II file that adheres to all our policies will reduce the risk of scheduling delays. Users are reminded to:
- Provide ETC ID numbers for all exposures, including acquisition exposures.
- Verify that each exposure is safe by running the Bright Object Tool (BOT) in APT, using the GALEX catalog whenever possible. Programs submitted with unexplained BOT warnings may lead to a delay in scheduling the observations.
- Specify the buffer time for all TIME-TAG exposures. Correctly calculating the buffer time is important to ensure that no data are lost during readout. In most cases, the buffer time should be 2/3 of the value calculated by the ETC, but there are exceptions for bright targets. Read full details.
- Follow the target list and instrument configuration specified in the approved Phase I proposal. Changes of grating, central wavelength, or lifetime position can be requested by a minor change request to your Contact Scientist, provided there is no change to the science goals. More substantial changes (such as target changes, instrument changes, and anything that alters the science goals) need to be requested by a major change request to the Telescope Time Review Board (TTRB), using the link from the Program Status webpage.
The dark rates for the COS FUV detector (FUVA and FUVB) and the COS NUV detector are monitored regularly. The dark rate of the FUV detector, primarily segment A, experiences occasional changes from its nominal baseline. See COS ISR 2019-11 for discussion of the variable, spatially structured component of the FUVA dark rate and recent efforts to more accurately account for it. Further details about the dark rate monitor, including links to the latest plots of the dark rates against time, may be found at the COS Monitoring page.
The FUV dark rates adopted by ETC version 30.2 have changed since the previous version. The new dark rates are 2.13E-6 counts/sec/pixel (up by 6%) and 2.14E-6 counts/sec/pixel (up by 16%) for FUVA and FUVB, respectively. The dark rates for spectroscopic target acquisition are now 3.60E-6 counts/sec/pixel (up by 14%) and 3.66E-6 counts/sec/pixel (up by 18%) for FUVA and FUVB, respectively.
The NUV dark rate adopted by ETC version 30.2 has been changed to 1.26E-3 counts/sec/pixel (up by 6%).
Additionally, the ETC has been updated with the latest throughputs for all COS modes, incorporating changes in sensitivity with time and changes due to the improved FUV flux calibration based on new white dwarf models discussed in the March 2022 COS STAN. PIs are strongly encouraged to use the latest version of the ETC for determining exposure times for use in their Phase II proposals.
The COS team recently discovered that a hot spot has reappeared on segment A of the FUV detector. It first appeared on September 7, 2014 but was dormant for many years until March 3, 2022. This hot spot falls within the two-zone extraction region of spectra taken at Lifetime Position 4. It therefore appears as an unexpected "emission line" in affected exposures. Since the hot spot’s first recent appearance, only a small number of exposures have been affected. The COS team is working on an update to the SPOTTAB reference file that will help remove this feature from affected exposures. Users with data obtained after March 3, 2022 at a cenwave listed below (and therefore at LP4) may check their spectra for this spurious emission line near the tabulated wavelength. Those who have questions about this matter are encouraged to contact the HST help desk.
Hotspot Wavelength (Å)
Two COS posters were presented at the AAS meeting. We invite all COS users to review these to find out about the latest developments and future plans for COS. Below we include links to the posters. Additionally, all COS posters are archived on the COS poster page.
In the thirteen years since a Space Shuttle Atlantis crew installed the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST), the ultraviolet instrument has continually expanded humanity’s understanding of the universe. The COS Team presents highlights and user-relevant status updates from recent instrument calibration. These include an improvement to the flux calibration of the FUV modes. New models for standard white dwarf stars prompted revisions to the fluxes of up to 10% at the shortest wavelengths. To overcome sensitivity losses from gain sag and keep the instrument scientifically productive into the 2030s, COS implemented a system of multiple lifetime positions (LPs). Most G130M modes moved to LP5 with HST Cycle 29 in October 2021. Beginning in Fall 2022, long exposures with G160M will move to LP6. Alongside these major changes, the COS team has continued to improve the instrument’s stability, calibration, and user support. By monitoring dark rates, sensitivities, and hotspots, the COS team better characterized the instrument’s detectors. Updates to the CalCOS pipeline enabled the new SPLIT wavecal mode which is essential to operations at LP6 and beyond. The COS team released an evolving suite of Jupyter Notebooks to aid new and returning COS users with common data analysis tasks. COS continues to provide observers with a sensitive view into the ultraviolet universe.
The Cosmic Origins Spectrograph's far-ultraviolet detector (COS/FUV) is affected by gain sag, where repeated exposure to photons results in a position-dependent loss of sensitivity as a function of time. To reduce the impact of gain sag and maximize the useful lifetime of COS, the location where spectra fall on the COS FUV detector is regularly changed to a new lifetime position (LP). Each move requires a new calibration effort, since resolution, sensitivity, the wavelength solutions, and the optimal focus change with detector position. For past LP changes, nearly all FUV settings have moved from the previous LP to the new one. In contrast, from Cycle 29 (2021--22) onward, nearly every LP is hosting a subset of the settings. For Cycle 29, LP5 was introduced for cenwaves 1291 through 1327 of the G130M grating, where the cenwave number is the approximate center of the wavelength range in angstroms. As was the case at LP4, there are restrictions on the use of detector segment B at LP5. Additionally, exposures with the G140L grating have moved from LP4 back to LP3. For Cycle 30, LP6 will be introduced for exposures with the G160M grating that are longer than approximately half an orbit. We discuss the calibration of the new LPs and key changes from the user perspective, and we encourage current and prospective COS users to monitor the COS website and other documentation for updates. These and future LP changes have the potential to extend UV science with COS into the 2030s.