Space Telescope Science Institute
COS Instrument Handbook Cycle 23
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Cosmic Origins SpectrographInstrument Handbookfor Cycle 23 > Chapter 5: Spectroscopy with COS > 5.6 FUV Single-Segment Observations

5.6
The FUV detector segments are operated and read out independently. For all FUV gratings Segment A detects the longer-wavelength light and Segment B the shorter wavelengths. Normally both segments are used for a science exposure, but there are circumstances in which operating with one detector segment at the nominal high voltage and the other effectively turned off may be beneficial. The SEGMENT optional parameter allows this choice. STScI strongly recommends use of both segments (the default for all but the G140L 1105 setting) unless special circumstances exist. Such circumstances include
The optional parameter SEGMENT specifies which segment of the FUV detector to use for an observation. A value of BOTH will activate both segments. This is the default setting. If SEGMENT=A is selected, only Segment A of the detector will be activated for photon detection, and the spectrum will contain data from only the long-wavelength half of the detector. If SEGMENT=B is selected, only the short-wavelength Segment B of the detector will be activated and used to generate data. Wavelength and flat-field calibration procedures remain the same for a particular segment whether or not the other segment is operating. Observers should take care to set SEGMENT appropriately when using the new blue mode settings for the G130M (the 1055, 1096, and 1222 CENWAVE settings).
If grating G140L is specified with the 1105 wavelength setting, then the value defaults to SEGMENT=A. Switching from two-segment to single-segment operation (or back again) incurs a substantial overhead time; see Table 9.5.

Cosmic Origins SpectrographInstrument Handbookfor Cycle 23 > Chapter 5: Spectroscopy with COS > 5.6 FUV Single-Segment Observations

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