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The treatment of overheads for EXES differs from that of most other SOFIA instruments. Instead of on-source times, users are required to specify "wall clock" times in SPT and SSPOT, which is the on-source time plus all overheads except those related to acquisition and instrument set-up. The overheads include time on "empty" sky in the off-slit nodding and mapping modes, as well as read-out and other telescope and instrument inefficiencies. The ETC and the Figures in this manual give the clock times needed. The figures in this manual only give clock times. SPT and SSPOT will add an additional 15 minutes for peak-up, wavelength optimization, flux calibration, and flat field overheads in the High_Medium, Medium, and Low configurations, and 20 minutes in High_Low. In all configurations, an extra 3 minutes of peak-up time is needed for the narrow (1.4") slit.
If no peak-up is necessary (e.g., after a wavelength change on the same target, if the source is extended, or if the continuum emission is too weak), the overheads can be reduced by using the 'alternative overhead' option in SPT and the "No Peak-Up" option in SSPOT. Overheads can also be reduced if multiple sky positions are observed in the same wavelength setting. In this case, click the "No Wavelength Setup" button in SSPOT. Note however that the time on a given target on a single flight is limited to 90-180 minutes, so full overheads may be needed again once the sum of AOR times exceeds 90 minutes. Conversely, if a single observation takes more than 90 minutes, it may need to be split into multiple AORs, each with full overheads. Please consult the EXES and SOFIA staff in these cases.