COS Instrument Handbook for Cycle 24

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9.5	Science Exposure Overheads

Science-exposure overheads are dominated by the time required to move OSM1 and OSM2 and to read out the on-board memory buffer at the end of each exposure. While the Phase II overheads computed by APT may be less than the values presented below, it is important to plan Phase I proposals using the conservative overheads given below to ensure adequate time for each exposure.

The full overhead calculation for science exposures depends upon a number of factors including generic exposure set-ups (which depend on the detector and observing mode), whether an aperture change is required, whether a grating change is required, whether the central wavelength setting for the grating is changed, and the directional sense of any required motion to implement an FP-POS change. Table 9.5 lists these additional overheads.

When moving to a new grating, you may specify any combination of central wavelength and FP-POS setting with no additional overhead penalty. As a result, the FP-POS sequence 1,2,3,4 is more efficient than 3,4,1,2, because no backward motion is required.

Table 9.5: Science Exposure Overhead Times

Overhead times (s)	FUV		NUV
	TIME-TAG	ACCUM	TIME-TAG	ACCUM
Exposure set-up	71	79	36	38
Grating change	see Table 9.3		see Table 9.4
Central wavelength change	72		75
FP-POS forward1	3		3
FP-POS backward1	70		70
PSA − BOA Change	8		8
WCA − BOA Change	10		10
SEGMENT reconfiguration	330		N/A
Memory readout2	116	1082	116	482

1

“Forward” refers to the preferred direction of motion of OSM1 or OSM2 and “backward” to the opposite direction. The preferred direction is toward larger FP-POS values.

2

ACCUM mode readout overheads can be hidden within subsequent exposures under certain circumstances, but the rules are complex. Use these values as safe upper limits for proposals.

To estimate the overhead for an exposure, round the desired exposure time up to the next whole second and add the generic exposure setup overhead from Table 9.5. If a grating change has occurred from the previous exposure, add the appropriate values from Table 9.3 and/or Table 9.4. If a central wavelength change is made, add the appropriate value from Table 9.5. If an FP-POS movement is made, add the appropriate value for motion in the preferred direction (toward larger FP-POS) or non-preferred direction. Note that all dispersed-light target-acquisition exposures are obtained with FP-POS=3. For all FUV observations except the G140L 1105 å setting, both detector segments are powered on by default. To turn one of them off, set SEGMENT to A or B and add the associated overhead. Lastly, add the appropriate detector memory readout overhead.

Due to the 100x difference in sensitivity between the COS FUVA and FUVB segments when observing with the G130M/1055 and 1096 CENWAVEs, it is expected that many observers will need to turn off FUVA when observing bright targets. (We refer to these SEGMENT=B observations here as either C1055B or C1096B). Only those observers using these two configurations are affected by this issue.

Under these conditions the zero point of the wavelength solution cannot be determined because the MgF2 window on the PtNe lamps (WAVECAL) blocks light below ~ 1180å (all WAVECAL light falls on FUVA). This results in a degradation of the resolution when FP-POS are combined by calcos and decreases the archival value of the COS data. In these cases, normal TAGFLASHs are not available and WAVECAL exposures with FUVA turned ON must be inserted into the observing sequence adjacent to each CENWAVE/FP-POS setting used. As a result, in these cases FP-POS=ALL should not be used. Individual FP-POS science exposures, and their associated