August 2023 STAN

In this STAN we discuss items of interest to those who are preparing Phase II proposals for Cycle 31 and the incorporation of updated dark rates into the COS exposure time calculators.

Best Practices for COS Phase II Submissions

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.
• If a target consists of multiple sources, a target field is crowded, or a target is faint, consider executing an offset target acquisition.
• 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, more restrictive scheduling constraints, 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.

New Dark Rates Adopted for ETC v31.2 are Significantly Higher than in v31.1

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 31.2 have changed significantly since the previous version. The new dark rates are 3.83E-6 counts/sec/pixel for FUVA (up by 74%) and 4.93E-6 counts/sec/pixel for FUVB (up by 79%). The dark rates for Spectroscopic Target Acquisition are now 7.10E-6 counts/sec/pixel for FUVA (up by 100%) and 7.88E-6 counts/sec/pixel for FUVB (up by 66%). Note that the increased dark rates are expected due to their correlation with increased solar activity.

The NUV dark rate adopted by ETC version 31.2 is 1.25E-3 counts/sec/pixel (up by 23%).

PIs with very faint or extended targets should consider investigating whether their Phase I estimates for target signal-to-noise are altered significantly. If the change is enough to put the science goals of the program at risk, they are encouraged to reach out to their Contact Scientist for further guidance.

Additionally, the ETC has been updated with the latest throughputs for all COS modes, incorporating changes in sensitivity with time. PIs are strongly encouraged to use the latest version of the ETC for determining exposure times for use in their proposals.

Reminder of the COS2025 Policies

Users preparing Cycle 31 Phase II submissions are reminded that the COS2025 policies are still in effect and are expected to help extend COS' lifetime until the 2030's. 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 lifetime position where G130M observations are taken (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.

Signal-to-Noise Considerations when Choosing the Number of FP-POS

Historically, COS users are strongly encouraged to use four FP-POS positions for each grating and cenwave combination in order to increase the legacy value of observations and minimize the impact of detector fixed-pattern noise on science data. Due to COS2025 usage rules and the considerations for G160M discussed below, some grating and cenwave combinations no longer require four separate FP-POS. COS ISR 2023-11 provides details on the maximum achievable signal-to-noise ratio (SNR) as a function of number of FP-POS for each grating, which can change with mode and wavelength, primarily due to how the width of the cross-dispersion profile of a given mode varies across the FUVA and FUVB detectors. The maximum SNR for each grating is:  1) G130M: 25, 30, and 35 for 2, 3, and 4 FP-POS, respectively 2) G160M: 22, 28, and 32 for 2, 3, and 4 FP-POS, respectively 3)G140L: 23, 29, and 36 for 2, 3, and 4 FP-POS, respectively. Users are still encouraged to use as many FP-POS as allowed by their science requirements in order to maximize the legacy value of their data.

Spectroscopy with the G160M Grating at LP6

In Cycle 31, 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 1 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.
   

  

  G160M Cenwave	Gap Range (Angstroms)
  1533	1439.0 – 1440.0
  1577	1483.0 – 1484.0
  1589	1494.9 – 1495.9
  1600	1506.6 – 1507.6
  1611	1518.3 – 1519.3
  1623	1531.0 – 1532.0

   

Regardless of the number of FP-POS required, FP-POS 1 and 4 should be included among them for the best wavelength coverage. The savings in overhead from using fewer than four FP-POS must be balanced against the potential reduction in wavelength coverage. Continuous coverage of the broadest possible wavelength range may be obtained by using either all four FP-POS or by using three FP-POS: 1, 4, and either 2 or 3. If there is a well-justified reason to use only two FP-POS, 1 and 4 are recommended. At LP6, this maximizes the wavelength coverage when two FP-POS are in use, leaving a gap of only 0.3 Å on Segment B. The 0.3 Å gap is expected to lie within the 1 Å ranges tabulated above, but its precise location cannot be predicted in advance due to mechanical uncertainty.