Starting in Cycle 21 the spectral resolutions of the FUV G130M 1055 and 1096 central wavelengths settings were substantially increased above the values offered during earlier cycles and, when combined with the 1222 setting first offered in Cycle 20, this now allows resolutions of 8000 to 12,000 to be obtained (for lifetime position 2) at any wavelength between 900 and 1150 Å
(see Figure 5.5
). Users should note, however, that for each of these modes the focus values have been set to optimize the resolution over a limited part of their wavelength range, and it will be necessary to use multiple settings to get the maximum resolution over this full range. At longer wavelengths the resolutions offered by any of these settings will be inferior to those available with the original complement of G130M central wavelength settings (2191, 1300, 1309, 1318, and 1327).
Users should also note that 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 there is no usable TAGFLASH wavelength calibration lamp spectrum recorded because there are no visible wavelength calibration lines that 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 “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.
Several minutes are required to reconfigure the COS FUV detector each time the segments are powered on or off. However, with careful planning much of this overhead can often be hidden in the occultations or other overheads. Additional details and examples of observing scenarios can be found in Chapter 5
and in Section 9.7
“Examples of Orbit Estimates”.