March 2024 STAN

 In this STAN, we discuss items of interest for those who are preparing Phase I proposals for Cycle 32. We also inform users about lower-than-expected sensitivities in specific FUV modes, attributable to the solar cycle, and announce updated detector dark rates. 

Release of Cycle 32 Documentation and Phase I Reminders

Documentation relevant to the preparation of Cycle 32 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 LP6, changes 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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Certain COS FUV modes have slightly lower sensitivity than estimated in the Phase I ETC v32.1

The operation of the COS FUV modes can be affected by the solar cycle, influencing the rate at which their sensitivity changes. However, predicting the magnitude of these changes is challenging. The expected sensitivity of a given mode is predicted for the mid-point of each observing cycle. Recent analysis of the COS time dependent sensitivity has revealed that sensitivity is dropping somewhat faster than expected, resulting in ETC calculations that are underestimating exposure times for a given SNR by < 10%, with most <3%. While this is still within the quoted accuracy levels of the ETC, estimations for faint objects in particular could be impacted if a large exposure time is needed and could thus affect the number of HST orbits required to meet a specific science case. Users are encouraged to increase their estimated exposure time in their Phase I proposal, if needed, and point to this STAN as justification.  A majority of users will not have to do this. The table below gives the detailed differences for all COS FUV modes or wavelength regions where the estimated sensitivity difference exceeds 3%. Updated sensitivities will be delivered for ETC v32.2 to support Cycle 32 Phase II preparations.

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New Dark Rates Adopted for ETC v32.1

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 32.1 have changed since version 31.2. The new dark rates are 3.39E-6 counts/sec/pixel for FUVA (down by 11%) and 4.21E-6 counts/sec/pixel for FUVB (down by 17%). The dark rates for Spectroscopic Target Acquisition are now 6.23E-6 counts/sec/pixel for FUVA (down by 12%) and 7.01E-6 counts/sec/pixel for FUVB (down by 11%). Note that the variable dark rates are expected due to their correlation with increased solar activity.

The NUV dark rate adopted by ETC version 32.1 is 1.25E-3 counts/sec/pixel (unchanged).

PIs are strongly encouraged to use the latest version of the ETC for determining exposure times for use in their Phase I proposals.

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