5.6 FUV Single-Segment Observations

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.  A value of BOTH will activate both segments.  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 for the G130M/1055, 1096, and 1222 CENWAVE settings.

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 COS2035 policy; see Section 5.12.1) 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.

For example, targets that are too bright to observe at longer wavelengths with the COS G130M grating may be observable on Segment B with the 1055 and 1096 settings by turning off Segment A, which covers longer wavelengths.  However, in this case no usable TAGFLASH wavelength calibration lamp spectrum is recorded, because no visible wavelength calibration lines fall on Segment B. As a result, the spectrum observed on Segment B cannot be corrected for mechanism drift or zero-point offsets.  In such cases, for each CENWAVE and FP-POS position, a separate GO-wavecal exposure should be taken with Segment A turned on immediately adjacent to the Segment B science observations. This Segment A wavecal will allow the zero-point offset of the wavelength scale of Segment B to be corrected to within 10 pixels. Details can be found in Section 5.7.5 "Inserting Wavecals in Observations with G130M/1055 and G130M/1096, SEGMENT=B."  Example observing scenarios are in Section 9.7 "Examples of Orbit Estimates."  

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.