Four different observing
modes were available to users, via the so-called `Astronomical Observation
Templates' (AOTs), which allowed to operate the instrument only in a few
standardised ways, giving the observer the choice of the wavelength ranges,
the sampling intervals, and the exposure times. These observing modes
are described in Section 3.2 and
summarised in Figure 3.1.
Two extra observing modes have been implemented later in the mission
to systematically make use of the time
when LWS was not the prime instrument: the parallel mode when another
instrument was used and the serendipity mode when the satellite was slewing
to another target. Unlike the four AOTs, these observing modes were not
available to the users but were used in a systematic way.
Grating mode, in which only the grating was used, provided moderate
spectral
resolving power of about m in the short-wavelength
channels (SW1-SW5) and
m in the long-wavelength
channels (LW1-LW3, LW5) corresponding to a resolving power of
between
and
depending on the wavelengths
being investigated. Because of the multiplexing described in
Section 2.4, usable
data were obtained from all ten detectors simultaneously. Whether or not
these data are useful to the observer depends upon the range of wavelengths
selected.
Fabry-Pérot mode, in which one of the two Fabry-Pérots was used in
combination with the grating, provided high spectral resolving power between
and
. Radiation in different orders of
the Fabry-Pérot falls on detectors other than the prime detector (i.e.
that
selected for the wavelength of interest), and in some cases data from
the non-prime detectors have proven quite useful although the grating
position had not been set to have its peak transmission at the orders
falling on these detectors. Automatic recovery of this non-prime
detector data is foreseen.
In both grating and Fabry-Pérot modes, scans were carried out at one of
two
standard lengths of detector integration ramp, 0.25 s and 0.5 s.
The total integration time per spectral point was then achieved by
varying the number of these ramps. It was originally intended that
spectra be scanned by carrying out a number of integrations
at each setting of the grating or Fabry-Pérot until the total required
integration time had been built up. In order to minimise the low-frequency
end of noise,
a fast-scanning mode was also implemented, in which only one integration
was performed at each position of the grating or Fabry-Pérot. The total
integration time was then built up by repeatedly scanning.
In fact during the mission this method has been recommended for
all observations. Early observations have proven
that in this way one could avoid that all integration ramps
at one wavelength were affected by the same particle hit.