STScI Analysis Newsletter (STAN)
Cosmic Origins Spectrograph (COS)
In this STAN we inform Cycle 25 proposers who are preparing to submit a Phase II of changes to COS/FUV operations.
- COS2025: Strategy to Extend Lifetime of COS/FUV Detector Places Restrictions on G130M Settings Available
- New COS/FUV G130M/1223 Cenwave Available
- New COS/FUV PEAKXD Target Acquisition Algorithm
With the move of COS/FUV to Lifetime Position 4 on Oct 2 2017, new restrictions on the G130M observing modes are being put in place to maximize the long term performance of COS. As these changes were adopted at the May 2017 STUC meeting, proposers did not have the opportunity to consider them when writing their Phase I proposals. Observers are requested to start work on their Phase II proposals promptly and to familiarize themselves with these changes. Observations with the G140L grating and with the COS/NUV channel are unaffected by these new policies. While no restrictions are being placed on G160M, some small wavelength ranges will be un-usable due to gain sag; this applies also to some wavelength regions in G130M/1222/1223/SegmentB. Detailed information about the changes is available at: http://www.stsci.edu/hst/cos/cos2025/. Cycle 25 PIs who have questions about these policies should email their Contact Scientists.
With APT25.2 a new G130M/1223 cenwave has been made available to Cycle 25 proposers. This new cenwave is similar to the existing G130M/1222 cenwave, but has spectral resolution optimized for Segment A, while G130M/1222 is optimized for Segment B. Segment A covers the wavelength range of 1226Å to 1360Å and segment B covers 1071Å to 1207Å. Both of these cenwaves place Lyα airglow in the segment gap, thus extending the lifetime of the COS/FUV detector. Additionally, G130M/1223/SegmentA covers the gap created by Lyα airglow at G130M/1291. More information about the new G130M/1223 cenwave is available at: http://www.stsci.edu/hst/cos/cos2025/1223_cenwave.
With the move to COS/FUV Lifetime Position 4 (LP4) on Oct 2, 2017, all FUV target acquisitions will take place at LP4 and will use a new ACQ/PEAKXD algorithm. This applies to all Cycle 25 programs, as well as to any un-executed programs from previous Cycles that use dispersed light target acquisition (expected to be very few at most - in that case the CS will work with the PI to implement any changes if needed).
The LP4 ACQ/PEAKXD procedure employs the ACQ/PEAKD algorithm, but is executed in the cross-dispersion (XD) direction. Three optional parameters, NUM-POS, STEP-SIZE, and CENTER have been added to ACQ/PEAKXD in APT25.2 to support this new algorithm. In this algorithm, a linear scan of NUM-POS dwell points, spaced by STEP-SIZE is performed in the cross-dispersion direction. Like the ACQ/PEAKD algorithm, the ACQ/PEAKXD centering algorithm is specified by the CENTER keyword. All LP4 ACQ/PEAKXD optional parameters will have default values that are appropriate for centering a point source with coordinates known to 0.4” or better. In particular, the default value for NUM-POS has been changed from 1 to 3. Targets with less certain coordinates will continue to need to be immediately preceded by an ACQ/SEARCH, and all ACQ/PEAKXDs should be followed immediately by an ACQ/PEAKD.
Please note that the restrictions to G130M settings described in COS2025: Strategy to Extend Lifetime of COS/FUV Detector Places Restrictions on G130M Settings Available apply to FUV dispersed light target acquisitions as well. Users are reminded that they should use the target acquisition exposure times provided by the ETC for Segment A only for all G130M cenwaves supported in TA, except 1291, for which the exposure time for both Segments should be used.
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