Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

5.6 FUV Gap Coverage and Single Segment Usage

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The FUV detector contains two segments whose active areas are separated by a gap approximately 9 mm wide. The optical image of the spectrum is continuous, but the wavelengths that fall on the gap are not recorded. The area between the two segments of the FUV detector causes a 14.3 Å gap in the wavelength coverage for the G130M grating, and 18.1 Å for G160M. Depending upon the science requirements of the observation, these wavelengths can be brought onto the active area of the detector by choosing one of the alternate central wavelength settings. Each FUV "M" grating has five central wavelength settings and the G140L has two. Wavelengths that fall on the gap with one of the settings are visible with at least one of the other settings.

The 9 mm gap between the FUV segments corresponds to about 1500 FUV pixels. Note that one step of OSM1 motion (such as occurs between individual FP-POS settings) gives rise to a displacement of 240 pixels. Thus just executing a full set of FP-POS settings is not sufficient to provide full wavelength coverage of the gap.

Single Segment Usage

The COS FUV detector consists of two distinct segments which are, at the lowest commanding level, operated and read out independently. Normally, both detector segments are utilized for a science exposure, however, there are circumstances where 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 usage of both segments (the default for all but the G140L 1105 Å setting) unless very special circumstances exist. Such circumstances include, but are not limited to:

• Sources with unusual spectral energy distributions at FUV wavelengths (bright emission lines or rapidly increasing/decreasing continuum slopes), where the count rate on one detector segment may exceed the bright object protection limit, but the other segment would be safe for observing.
• Other sources with unusual spectral energy distributions, where the count rate on one detector segment would be high but safe, and the other segment would have a relatively low count rate. In this case, if the science to be done were on the low count-rate segment, operating just that segment may result in a substantially reduced dead-time correction.

Wavelength and flat-field calibration procedures will remain the same for a particular segment whether the other segment is operating or not.

The Optional Parameter SEGMENT (=BOTH (default), A, or B) specifies which segment of the FUV detector to use for an observation. A value of BOTH will activate both segments. If 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 B is selected, only the short-wavelength segment B of the detector will be activated and used to generate data.

Please note: if grating G140L is specified with the 1105 Å wavelength setting, then the value must be SEGMENT=A. Bear in mind that segment A detects the longer wavelength light and segment B the shortest wavelengths, and this is true for all FUV settings. Switching from two-segment to single-segment operation (or back again) incurs a substantial overhead time; see Table 9.4.