AOT SWS01 performs for each of three apertures two long scans (one up
and one down), at rates which are too high to obtain the nominal resolution of
the SWS. To avoid complications from undersampling and/or imperfect flat
fielding, the spectra are artificially blurred, so that each individual
detector can produce a sufficiently sampled spectrum. The blurring is
achieved by stepping the grating scanners 8 or 16 times per integration
interval. Scanning in two directions will help to recognize and reduce
memory effects in the detectors.
AOT SWS02 is designed to obtain high-quality reliable grating scans of
single spectral lines. It makes a finely sampled scan of the line and
its surroundings. The scan length is fixed, at an amount comparable to
the length of a detector array (some 8 resolution elements), so that
each detector in the array will provide a well-sampled line profile.
This is a robust strategy, which helps to deal with many contingency
situations. For the detection of weak lines against strong continua this
is the best grating AOT (AOTs SWS06 and SWS07 make fine grating scans
only in their slowest, most sensitive modes).
Each up-and-down scan is preceded by a 20-second dark-current
measurement (for the quickest scans this amounts to an overhead of 20%).
To guard against responsivity drifts (detector memory effects
), the scans are always made in two directions.
AOT SWS06 is designed as a flexible AOT to observe arbitrary wavelength
intervals, as long as they do not cross AOT-band boundaries. The scan
rate depends on the required measurement sensitivity: the quickest scans
use step sizes of about one resolution element. This means that the
spectrum recorded by a single detector can be undersampled and that flat
fielding is required to combine the outputs of
all 12 detectors to produce a reliable spectrum.
To monitor responsivity or baseline drifts, long scans are preceded,
followed and possibly interrupted by reference scans around a fixed
wavelength, selectable by the observer. In addition, dark-current
measurements are made before the first reference scan and after the last
one. The strategy of taking as many reference spectra as are needed to
monitor detector drifts and to use dark-current measurements only
sparingly is probably reasonable for long measurements. The reason to
opt for reference scans instead of dark-current measurements is that it
is expected that the detectors will be less upset by a switch to a
nearby wavelength than by a switch to total darkness.
AOT SWS06 is not recommended for quick measurements of single lines,
because
(i) the sampling is coarser than necessary for quick measurements
of single lines; and
(ii) the scanned range can be larger than necessary for quick
measurements of single lines.
AOT SWS07 will perform Fabry-Pérot scans. The strategy is to make at least three scans in the same direction (from longer wavelengths to shorter ones). Scanning in two directions is less attractive because of hysteresis in the F-P drive. The LW grating is kept in tune with the F-P wavelength, to minimize leakage from the adjacent F-P orders.
AOT SWS07 allows simultaneous use of the SW grating section over the
2.38-7 
m micron range. Use of the SW grating in the range 7-13.6 m
(through aperture 3) is not carried out simultaneously, and any observations
requested in this range will occur before the aperture 3 FP observations. The
lack of merging at these wavelengths is due to a slight aperture mis-alignment
between the SW grating and the FP. The SW measurements in SWS07 are carried
out essentially as in AOT SWS06.
Because it is expected that the F-P scans will tend to last much longer than
the SW scans, the merge philosophy differs from that of the pure grating AOTs:
the phasing of the measurements is independent for the two spectrometer
sections. The two spectrometer sections can not be used completely independently
(measurements have to use the same aperture ). The F-P
measurements will have to be interrupted from time to time because of the needs
of the grating measurements, and vice versa.
AOT SWS07 is preferable to AOT SWS02 of course only if extra spectral
resolution is required. Note that line-to-continuum ratios
may be 20 times larger with the F-P than with the grating, depending on
the intrinsic width of the line.