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Due to gain sag, the Lifetime Position (LP) on the FUV detector at which spectra are obtained changes from time to time. At the beginning of Cycle 30, LP6 will become the default for G160M spectroscopy. The Cycle 30 COS Instrument Handbook discusses the increased wavelength calibration overheads at LP6 due to the introduction of SPLIT Wavecals and the circumstances under which G160M observations may be conducted at LP4 instead. It also discusses changes to the FP-POS requirements at LP6, allowing fewer than four FP-POS to be used when the S/N goal is ≤ 25. Scientists who are proposing the use of the G160M grating in Cycle 30 are strongly encouraged to read this material. Note that use of LP4 for G160M observations must be requested and justified in the Phase I proposal.
The COS2025 rules, designed to extend the lifetime of the COS FUV detector, are of particular interest to scientists who want the full two-segment wavelength coverage offered by the G130M grating or who want to observe Ly α in zero-redshift targets. The COS2025 rules will continue to apply in Cycle 30. The rules limit the locations where damaging Ly α airglow can fall on detector segment FUVB: 1) G130M cenwaves greater than 1291 cannot be used when segment B is on, and 2) G130M/1291 is limited to two FP-POS when both segments are on. Please see the COS2025 page for further details. A consequence of these rules is that, for zero-redshift astronomical targets, Ly α falls on gain-sagged regions of the detector at the default LP5. Users who wish to observe Ly α in such targets should instead use LP3. This option must be requested and justified in the Phase I proposal.
All special requirements must be requested and justified in the "Special Requirements" section of Phase I proposals. A list of such requirements appears in the Call for Proposals. For COS, the most common requirements are explicit scheduling requirements (e.g., for planetary transits), waivers of the FP-POS requirements, requests to use non-default lifetime positions, and ORIENT constraints.
ORIENT constraints are commonly used for extended sources (i.e., sources with FWHM > 0.6"). Observing these at different position angles can result in changes in the cross-dispersion and dispersion profiles on the COS detectors. Such changes can lead to differences in spectral resolution and flux calibration that may prove problematic when combining spectra across visits or performing science on separate spectra taken at different position angles. This can be mitigated by implementing an ORIENT constraint across visits within APT. As with other special requirements, these must be requested and justified in the Phase I proposal.