March 5, 2021

About This Article

In this STAN we provide updates about COS to those who are preparing Phase I proposals for HST Cycle 29.

FUV Spectroscopy at Lifetime Position 5 for the G130M Grating and Lifetime Position 3 for the G140L Grating

The COS FUV detector is susceptible to gain sag, a reduction in the ability of the detector to convert incoming photons into electrons. One strategy for mitigating this is to occasionally change the location along the cross-dispersion direction where spectra are recorded on the detector, the lifetime position (LP). Since Cycle 25, LP4 has been in use except for the G130M cenwaves 1055 and 1096, which are at LP2. Beginning in Cycle 29, G130M cenwaves 1291 and longer will use a new LP5, while G130M cenwaves 1055, 1096, and 1222 will remain at their current LPs. G140L cenwaves will use LP3. Changes in LP yield minor changes in sensitivity, resolution, and overheads. The calibration of LP5 (and of G140L/800 at LP3) is expected to be complete before Cycle 29 begins. G160M cenwaves will initially remain at LP4, but G160M exposures longer than approximately half an orbit are expected to move from LP4 to LP6 in Cycle 30.


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Continued Enforcement of the COS2025 Rules

The COS2025 rules, designed to extend the lifetime of the COS FUV detector, will continue to apply at LP5. These limit the options for placing rest-frame Ly α on the detector: 1) G130M cenwaves greater than 1291 can no longer be used with detector segment B in operation, and 2) G130M/1291 is limited to two FP-POS when both segments are in use. Please see COS2025 for further details.


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NUV Grating G285M is Available but Unsupported

Because of its declining throughput, NUV observations with the G285M grating are Available but Unsupported. See Section 5.11 of the COS Instrument Handbook for further explanation of Available but Unsupported status. The remaining NUV gratings, G185M, G225M, and G230L, continue to be recommended for use. Users interested in medium-resolution spectroscopic coverage between 2500 and 3200 Å are encouraged to use STIS instead.


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Improvements to the Calibration of FUV Cenwaves G130M/1055 and G130M/1096

Cenwaves 1055 and 1096, observed with the FUV G130M grating, are known as the blue modes. Extending to shorter wavelengths than the original G130M modes, they provide coverage from 899 Å to 1196 Å and from 940 Å to 1236 Å, respectively, with a 15 Å gap in the middle due to the physical gap between detector segments. The resolving power offered by these modes peaks at 11,000-12,000 near the lowest wavelength ends of their ranges and falls to 2,000-4,000 at the longest wavelengths covered. This is smaller than the resolving power offered by G130M at longer wavelengths but significantly larger than that offered by G140L over these wavelengths. The COS team has improved the flux and wavelength calibrations of these modes, including the time dependence of the flux calibration, to bring their accuracy in line with those of the other G130M modes.


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See Recent STANs for Additional Details

We advise users to consult recent STANs for additional details, including updates to calibration reference files not discussed here.


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HST Help Desk 

Please contact the HST Help Desk with any questions.