CAM01 (general observation mode) was dedicated to photometric imaging.
It was possible to select either of the wavelength channels (LW or SW),
various pfov's and one or more of the broad and
narrow band fixed filters, as well as sets of discrete CVF positions.
It was a very flexible AOT which allowed up to 64 such observing
configurations within a single Target Deticated Time (TDT).
CAM01 supported staring observations as well
as celestial raster maps having a raster step-size between
2
and 180
. With the raster, one
could not only cover areas larger than the field of view of the camera,
but could also improve the flat-field accuracy using micro-scanning
techniques.
Rastering was recommended
for the detection of weak, confused or extended sources, as well as in
the presence of a structured background. During a raster measurement the
instrument configuration remained unchanged. Rasters could be repeated with
different settings up to the 64 configurations mentioned above. The
orientation of the raster scan was either aligned with the spacecraft axes
(i.e. parallel to the edges of the detector array)
or with reference to the sky N-direction (see the ISO Handbook Volume I,
[40]).
CAM03 (beam-switch) was also dedicated to photometry and allowed the subtraction of one or more nearby reference fields. It also supported the use of different channels, pfov's and filters. The method was recommended for the detection of weak isolated sources, when there was not too much structured background or for observing compact, high-contrast sources, which would suffer from smearing in a raster. Several off-target reference fields were allowed, although this option was rarely used. There was an option to repeat the cycle of on-off measurements.
The default order for performance of beam-switch observations involved first observing the target and then the background position(s). In practice this implementation proved to be ill advised, because a bright source, if observed in the first pointing of the observation, would leave a remnant on the detector which would be carried through the subsequent background measurements, contaminating them and leading to an eventual photometric error when the merged background measurements were subtracted from the on-target measurement (see Section 4.4). Therefore observers often measured the background position first, followed by the on-source position. To do this they had to `mislead' the mission-planning software and give the background position for the on-source position and vice versa. This had the temporary disadvantage of confusing earlier versions of the Off-Line Pipeline Processing (OLP) software, which consequently produced inverted maps, a problem solved, however, in the Legacy ISO Data Archive.