February 2020 STAN
In this newsletter we provide updates about COS to those who are preparing Phase I proposals for HST Cycle 28. The full complement of COS FUV cenwaves is visualized here.
Upcoming Calibration Improvements to 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 low 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 is improving 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. New reference files are expected to be in place before Cycle 28 begins.
Continued Availability of Cenwaves G140L/800 and G160M/1533
Cenwaves 800 and 1533 were introduced in Cycle 26 and remain available to COS/FUV users. Cenwave 800, observed with the G140L grating, optimizes low-resolution spectroscopy below 1150 Å while extending to 1950 Å. Cenwave 1533, observed with the G160M grating, extends coverage at the short-wavelength end of the grating to overlap with the longest wavelengths covered by G130M/1222. See the March 2019 STAN for additional details.
COS2025 Rules Extend the Lifetime of the COS FUV Detector
The COS2025 rules, designed to extend the lifetime of the COS FUV detector and introduced for Cycle 25, remain in place. These limit the options for placing rest-frame Ly α on the detector; primarily, many G130M cenwaves can no longer be used with detector segment B in operation. Please see this page for further details.
Use of the G285M Grating is Discouraged
Because of its declining throughput, NUV observations with the G285M grating have been discouraged since Cycle 27. The remaining NUV gratings, G185M, G225M, and G230M, continue to be recommended for use. Users interested in medium-resolution spectroscopic coverage between 2500 and 3200 Å are encouraged to use STIS instead.
See Recent STANs for Additional Details
We advise users to consult recent STANs for additional details, including updates to calibration reference files.