|COS Instrument Handbook for Cycle 24|
For any COS exposure, including target-acquisition exposures, any change of spectral elements incurs an overhead. A transition from FUV to NUV requires movement of OSM1 to the NCM1 position, followed by a possible OSM2 movement. On the other hand, a transition from NUV to FUV requires only the movement of OSM1 from NCM1 to the desired FUV grating. Table 9.3 gives the times required for movement between OSM1 spectral elements and Table 9.4 gives the times for movement between OSM2 spectral elements.All COS visits start with OSM1 at the G130M position (with central wavelength 1309 Å) and OSM2 at the G185M position (with central wavelength 1850 Å). These gratings are highlighted in Table 9.3 and Table 9.4. OSM1 and OSM2 move sequentially, so the total overhead is the sum of the two separate overheads. The time required to move from one optical element to another is independent of the initial and final central-wavelength and FP-POS settings.