COS Instrument Handbook for Cycle 24

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9.3	Spectral Element Movement Overheads

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.

Table 9.3: Overhead Times for Motions Between OSM1 Spectral Elements

Movement times (s) from	to G140L	to G130M	to G160M	to NCM1
G140L	—	158	200	115
G130M	164	—	112	116
G160M	206	116	—	159
NCM1	121	109	154	—

Table 9.4: Overhead Times for Motions Between OSM2 Spectral Elements

Movement times (s) from	to G230L	to G185M	to G225M	to G285M	to MIRRORA	to MIRRORB
G230L	—	209	140	176	105	99
G185M	204	—	136	102	169	175
G225M	135	141	—	108	100	106
G285M	170	107	103	—	136	142
MIRRORA	100	174	105	141	—	71
MIRRORB	94	181	112	147	77	—