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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.
- 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.
- 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.
- Overheads for raster scans: see the ISO Observers Manual,
section 12.3, pages 32 to 35.
N.R. Trams, P.E. Clegg and B.M. Swinyard
Wed Aug 7 11:47:53 MET DST 1996