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 WAVECALs, should be used instead.
- For non-CVZ science exposures at the start of an orbit not containing a target acquisition (TA) sequence, the WAVECAL can be inserted into the prior occultation with no impact on the timeline.
- For non-CVZ science exposures at the end of an orbit, the WAVECAL can be inserted into the following occultation with no impact on the timeline.
- For science exposures that immediately follow TA, or do not start or end an orbit, a WAVECAL must be inserted. This can take anywhere from 500 to 900 seconds (8-15 minutes) depending on configuration and observing sequence.
To reduce the impact on the time-on-target, observers may need to decrease the number of FP-POSs observed with each CENWAVE setting. G130M/1055/1096 spectra are much broader in the cross-dispersion direction and do not require as many FP-POSs as the “standard” CENWAVEs to mitigate fixed-pattern noise. Observing strategies for single CENWAVE one-orbit visits have been defined that result in, at most, a 14% reduction of exposure time (7% drop in S/N). For all visits greater than one-orbit, the impact is less than 2% of the total exposure time.
Inclusion of WAVECALs with FUVA turned on only significantly impacts one-orbit visits that observe with both C1055B and C1096B. Due to the relative ratio of throughputs and BOP considerations, it is unlikely that G130M/1055/1096 observations that require FUVA to be turned off would have exposures in the same visit with other CENWAVEs. These exposures will likely also require an NUV TA.
Please contact your Contact Scientist (CS) if you have any questions about the operations described here.
For further information, see the following website for detailed examples and instructions.