|COS Instrument Handbook for Cycle 26|
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 800 Å and 1105 Å settings as well as G130M cenwaves that are impacted by the COS 2025 policy; see Section 2.1) unless special circumstances exist. Such circumstances include:
• Sources with unusual spectral energy distributions at FUV wavelengths (bright emission lines or rapidly increasing/decreasing continuum slopes), for which the count rate on one detector segment exceeds the bright-object protection limit, while the other segment is safe for observing. In some cases, this problem may be mitigated by adjusting the central wavelength or FP-POS setting.
• Sources for which the count rate on one detector segment is high but safe, while the other segment has a relatively low count rate. If the science to be done were on the low count-rate segment, operating just that segment would reduce data losses due to dead-time effects and increase the S/N of the resulting spectrum.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 blue mode settings for the G130M (the 1055, 1096, and 1222 CENWAVE settings).If any of the modes are specified for which Segment B spectroscopy is not permitted, 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.