There are three different chopper modes available for single pointing photometry. The decision to chop or stare and the selection of the chopper mode will depend on the structure of the source and the nearby sky background. The different chopper modes are illustrated in Fig. 6.
Rectangular -
In this mode, the satellite points to the position between the source
and an off-source position, and the chopper moves alternately between these
two positions. The source is always in the positive beam in spacecraft
Y-direction. Chopper throws (peak to peak) between 30
and 330 in
steps of 30 can be selected.
There is the additional constraint
that the chopper throw has to be greater or equal to the selected
aperture diameter
or the array side length (to avoid overlap of the
beams) and less or equal than 360 minus the aperture diameter or
array side length (to avoid beam vignetting). For instance, PHT-P observations
with the
10 aperture allow the use of all possible throws, namely 30,
60, 90, 
, 330; observation with the 79 aperture
allows the use of the following throws: 90, 120, 270; and
observation with the 180 aperture allows only the 180 throw. For
C100 the throws of 150, 180 and 210 are allowed, and for
C200 only a throw of 180 is possible.
Triangular -
Chopping occurs to two positions on either side of the source. The
satellite points to the source position, and the chopper moves from one
off-source position to the source and than to the other off-source
position and finally back to the source, and so
forth. In this way, the source is observed twice during each cycle and each
of the off-source positions once.
Chopper throws (distance between on-source and off-source postions)
between 15 and 150 in steps of 15 can be selected. There
is the additional constraint that the chopper throw has to be greater
or equal to the selected aperture diameter or the array side length (to
avoid overlap of the beams) and less or equal than 180 minus half
the aperture diameter or half of the array side length (to avoid beam
vignetting). For instance, observing with the 10 aperture allows the
use of the full set of throws, namely 15, 30, 
, 150;
observing with the 79 aperture allows the use of the following
throws: 90, 105, 120. The 120 aperture is the largest
one to be used for the triangular chopping mode.
C200 may not be used with this chopper
mode, and for C100 the only throw is the 150 one.
Saw-Tooth -
As in the triangular mode there are two off-source positions, and
the satellite points
to the source. However, after having moved from one off-source position
to the source and then to the other off-source position, the chopper
moves back to the first off-source position. During the transition
between the two off-source positions the source is inside the beam
for a very short time. Here the same chopper throw limitations apply as
for triangular chopping.
The different chopper modes determine the number and locations of reference positions observed with respect to the on-source position (cf. Fig. 6). If the sky background is expected to be comparable to the source flux, or stronger, at least one reference position should be observed.
By chopping, the background can be measured with one telescope pointing. Otherwise, an additional pointing must be made, which means an additional observation using the same AOT on the background position. Depending on the structure of the background flux, e.g. flat background, infrared Cirrus, or with strong gradients, one of the different chopper modes should be selected. The three different chopper modes are illustrated in Fig. 6.
