Overheads

The original LWS Observers Manual did not contain an estimate of the overheads that should be taken into account when calculating observing times. The exact calculation of these overheads is very complicated. An estimate however can be made using the following overhead times.

1. Overheads for the setup and switchoff of the instrument and its subsystems.
   • Grating: 18 seconds overhead at the start of the observation and 20 seconds at the end of the observation.
   • FP: 18 seconds at the start of the observation, 38 seconds for every FP that is used, and 20 seconds at the end of the observation.
2. Overheads for the internal calibration of the instrument using the illuminators (illuminator flashes).
   • Grating: at least two opaque illuminator flashes (one at the start of the observation and one at the end). Each opaque illuminator flash takes 154 seconds (so total 308 sec). In addition if the observation is longer than 20 minutes, an open illuminator flash is done every 20 minutes. Each of these open illuminator flashes takes 112 seconds.
   • FP: at least 2 illuminator flashes at the start and end of the observation. Each one of these takes 126 seconds (so total 252 seconds). Like the grating mode, if the observation is longer than 20 minutes, an illuminator flash is performed every 20 minutes. Each of these illuminator flashes again takes 126 seconds.
3. Overheads for raster scans: see the ISO Observers Manual, section 12.3, pages 32 to 35.