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COS2025 Policies


The COS FUV microchannel plate detector’s efficiency at converting incoming photons into detectable events decreases with usage. This depletion of the detector’s gain (i.e. gain sag) results in unusable regions of the COS/FUV detector. The main culprit of gain sag in the COS/FUV detector is Lyα geocoronal emission at 1216 that fills the COS 2.5” aperture whenever the G130M grating is used with the 1291/1300/1309/1318/1327 cenwaves on Segment B.

In order to mitigate this gain sag, a number of strategies have been employed over the past 8 years of operations. These range from moving to different lifetime positions, to managing the high voltage to extract a smaller amount of charge, to re-distributing the cenwave usage so that Lyα does not produce a gain-sag hole in a given location.

We are now at a point where none of the strategies above will, without any other changes, allow us to continue operating the COS/FUV detector to 2025. To address this a new COS2025 policy has been developed. This new policy, with the goal of retaining full science capability of COS/FUV to 2025, places restrictions on the G130M cenwaves allowed at Lifetime Position 4.

New G130M/1223 Cenwave – No Longer Offered

During summer 2017, the COS team investigated the possibility of a new G130M central wavelength (cenwave) to be offered at LP4, known as the 1223 setting, intended to optimize FUVA resolution over the same wavelength range as 1222. This setting was advertised to the community for use in Cycle 25. However, following detailed programs to focus and characterize the spectral resolution of the 1222 and 1223 settings, it was found that 1222 has resolution on segment FUVB comparable to that of the G130M/1291 cenwave, and resolution on segment FUVA is within ~20% of that of G130M/1291. This was the resolution goal the new 1223 cenwave was trying to achieve, and so this finding removes the need for the 1223 cenwave. There is no need to offer two equivalent cenwaves, since there is significant internal overhead in maintaining the COS tools (ETC, pipeline, APT) for multiple cenwaves.

Therefore, the 1223 cenwave is no longer being offered, and all Cycle 25 COS observations using 1223 are being transferred to 1222. PIs of all relevant programs have been contacted. Information on the spectral resolution of each supported G130M setting is provided here:

Summary of Supported and Available Modes

Tables 1 and 2 summarize the supported and available science and target acquisition modes as a function of lifetime position, for each COS/FUV cenwave, following the new COS2025 policy.

Impact of COS2025 policy on Cycle 25 Programs

The new COS2025 policy might have some impact on the FUV portion of Cycle 25 COS programs, as the policy was not in place when the phase Is were prepared. As a result, some programs might need to change or add cenwaves, or request additional orbits to meet the approved science goal. In addition, 1222 and G160M exposures will also be indirectly affected by the new policy: as the cycle progresses, gain sag holes will be created where G130M/1291/FP-POS=3,4 projects Lyα in the detector, potentially affecting wavelength regions of interest in G130M/1222 and G160M. To avoid impacting your 1222 or G160M exposures please consult Tables 3a and 3b below.

The flowchart below is intended to help GOs navigate the impact of the new policy on their programs and help them decide if they can recover their science goal with small modifications to their programs or if they need to submit a request for more orbits to the Telescope Review Board (TTRB), or potentially both. Adding new cenwaves not initially specified in the Phase I is a small change that can be handled through the program’s Contact Scientist.

Each COS Cycle 25 program is assigned a Contact Scientist who will work closely with the PI to answer any questions during the preparation of the phase II. We urge all PIs of approved Cycle 25 COS programs to consider carefully and as soon as possible whether a TTRB request for more orbits is needed before the final Phase II can be submitted. The whole process should be completed in time for the Phase II to be submitted by the deadline of July 20, 2017. The TTRB will expedite requests due to the new COS2025 policy in order to minimize any potential delays.

Wavelength Ranges Affected by Gain Sag on Segment B

The COS2025 strategy allows users to obtain Segment = B data with G130M/1291/FP-POS=3,4. Over time, Lyα airglow will sag these detector regions, which will become un-usable. This means that other settings, projecting light into these detector regions will also be affected. Tables 3a) and 3b) below summarize the wavelength ranges that will be affected for all the gratings. Users should consider carefully whether a wavelength region of interest falls in any of the affected wavelength ranges given in Tables 3a) and 3b).

Wavelength Range Tool

To assist users in preparing their Cycle 25 Phase II proposals we have developed a COS/FUV wavelength range tool that allow users to visualize the COS/FUV wavelength ranges in light of the restrictions on G130M setting usage put in place to maximize the lifetime of the COS/FUV detector.

Example Science Cases: Cycle 24 vs. Cycle 25

The following examples compare Cycle 24 and Cycle 25 program layouts and give an idea of what changes proposers might need to implement due to the new COS2025 policy.

  • Science Case 1: Measure Lyα absorption from IGM at z = 0.1 – 0.2, using QSO spectroscopy
  • Science Case 2: Measure O VI and Lyα absorption from IGM at z = 0.15, using QSO spectroscopy
  • Science Case 3: Measure H, C, N, O, S, and Si in exocomets
  • Science Case 4: Measure gas content and metal abundances in damped Lyα system (DLA) at z = 0.01 using QSO at z = 0.4