5.10 Wavelength Settings and Ranges

Table 5.3 and Table 5.4 show the wavelength ranges recorded on the detectors for each valid combination of grating and central-wavelength setting at the nominal FP-POS=3 position (Section 5.8.2). The wavelength ranges spanned at other FP-POS settings may be estimated using the FP-POS step values provided in Chapter 13. Note, however, that uncertainties in the positioning of the Optics Select Mechanisms (Section 3.1.3) correspond to about half of an FP-POS step. These wavelength ranges are subject to change as the instrumental calibration evolves. The most recent measurements are available from the COS website.

To assist users in preparing their Phase II proposals for Cycle 25 and later COS observations, a COS/FUV Wavelength Tool has been developed which allows users to visualize how the wavelength ranges covered by the allowed COS FUV settings (i.e., settings in supported mode) overlap with those of settings that have been withdrawn (i.e., not in supported mode) to preserve the lifetimes of the COS FUV detectors.  Unlike Table 5.3, the FUV wavelength tool shows estimated coverage for each FP-POS as well as the locations of narrow regions that lack coverage due to Ly α gain sag. These narrow regions are also listed in Tables 3a and 3b on the COS2025 page.