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March 3, 2023
COS NEWSLETTERS

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In this STAN we discuss the release of Cycle 31 documentation and news of interest to those who may submit Phase I proposals to use COS, including a discussion of G160M use at LP6 and an updated listing of dark rates. We also announce the delivery of a new gain sag table.

Release of Cycle 31 Documentation and Phase I Reminders

Documentation relevant to the preparation of Cycle 31 COS proposals has been released. This includes information pertaining to COS in the HST Call for Proposals and Primer as well as an updated edition of the COS Instrument Handbook.

The default lifetime positions (LP) for each cenwave remain unchanged since Cycle 30. Prospective users are urged to review the increased wavelength calibration overheads for G160M observations at LP6changes in FP-POS requirements at LP6, and the circumstances under which G160M observations may be conducted at LP4 instead.

The COS2025 restrictions on central wavelength use with the G130M grating remain in effect. Users who wish to observe Ly α in zero-redshift targets should use the G130M grating at LP3 instead of the default LP5. 

Proposers who require multiple exposures to be obtained without gaps in time coverage should be aware that, while exposures will be executed in the order specified by the user, they are not guaranteed to be contiguous by default. A sequence of exposures may be split across multiple orbits even if APT's Orbit Planner places them in a single orbit. If exposures must occur without gaps in time coverage, they must be placed in a Sequence Non-Int container.

The requirements discussed above and other special requirements (such as explicit scheduling requirements, waivers of the FP-POS requirements, and ORIENT constraints) must be justified in the Phase I proposal.

 

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Choosing FP-POS for G160M Spectroscopy

The usual requirement to use all four FP-POS may be waived for G160M spectroscopy at LP6; see Section 9.5.1 of the COS Instrument Handbook for details. In particular, the COS team encourages the use of only two FP-POS when a target will be observed with G160M for at most one orbit and the desired S/N is less than or equal to 20.

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 below, but its precise location cannot be predicted in advance due to mechanical uncertainty. If fewer than four FP-POS are desired, a justification must be included in the Phase I proposal.

 

G160M Cenwave Gap Range (Å)
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

 

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New Dark Rates Adopted for ETC 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 31.1 have changed since the previous version. The new dark rates are 2.20E-6 counts/sec/pixel for FUVA (up by 3%) and 2.75E-6 counts/sec/pixel for FUVB (up by 29%). The dark rates for Spectroscopic Target Acquisition are now 3.56E-6 counts/sec/pixel for FUVA (down by 1%) and 4.73E-6 counts/sec/pixel for FUVB (up by 29%). Note that the increased dark rates are expected due to their correlation with increased solar activity.

The NUV dark rate adopted by ETC version 31.1 is 1.03E-3 counts/sec/pixel (down by 18%).

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.

 

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Delivery of a New Gain Sag Table (GSAGTAB)

The COS FUV detector experiences a decrease in sensitivity over time, known as gain sag, as a result of charge extraction from photon events. When the modal gain of a pixel falls below a value of 3, more than 5% of photon events are lost, and a pixel is considered sagged. To mitigate the effects of gain sag in FUV spectra, the CalCOS pipeline uses GSAGTAB reference files to flag and remove sagged pixels from the final spectra. The CalCOS team has recently updated the GSAGTAB reference files for both the normal and blue modes (G130M/1055 and G130M/1096) to omit pixels that have sagged since March 2021 (see Figure 1). Although ~1000 pixels were added to the GSAGTAB files over all active high voltages and both segments, most of these additions were in columns that already included sagged pixels. As a result, there are only a small number of new columns that are omitted in CalCOS processing. The most important changes for users are that the Ly α airglow holes from previous usage at LP4 are now a few columns wider, and new airglow holes from G130M/1291 observations at LP5 affect both LP5 and LP6 observations. The new GSAGTAB files were delivered to HST CRDS on January 19, 2023. Users with FUV data taken after March 2021 are encouraged to download newly processed spectra from MAST.

Two-dimensional gain maps are shown for the currently active high voltage (HV) values for the FUVB segment

Figure 1: Two-dimensional gain maps are shown for the currently active high voltage (HV) values for the FUVB segment. The open cyan circles indicate pixels for which the modal gain has fallen below a value of 3 ("sagged") since March 2021. Note that newly sagged pixels that do not affect active lifetime positions at the HV values shown are omitted from the plot. There are no changes on the FUVA segment that affect users.

 

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