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 800 Å and 1105 Å settings and those impacted by the COS 2025 policy
, both detector segments are powered on by default. To turn one of them off, set SEGMENT
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 CENWAVE
s, 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 TAGFLASH
s are not available and WAVECAL
exposures with FUVA turned ON must be inserted into the observing sequence adjacent to each CENWAVE
setting used. As a result, in these cases FP-POS=ALL
should not be used. Individual FP-POS
science exposures, and their associated WAVECAL
s, should be used instead. A detailed description of how to insert these WAVECAL
s can be found at the June 2013 COS STAN.
Overheads associated with cenwaves new to Cycle 26 (Section 2.2
) have not been determined as of publication, but they are expected to be similar to those of the other cenwaves in their respective gratings.