Careful consideration must be given to chopped observations of extended objects like e.g. galaxies or bipolar outflows which are sensitive to the direction of chopping. The optimum direction of the chopper throw should be assessed. For example, the observation may be compromised if the reference beam coincides with a nearby object (see Fig. 10 of IOM3.1).
ISOPHOT offers the possibility to define a region around the source which is forbidden for the chopper off-position. Note that specification of any avoidance region should include consideration of the size of the selected aperture/array by the observer. This is not done by the logic.
The chopper angle is defined by the angle (counted anti-clockwise) between
North and the line through the source position and the off-position.
The chopper avoidance region is determined by the segment on the sky enclosed
by the chopper avoidance start angle ( 
) and chopper avoidance
end angle ( 
). Both angles are counted from North through East.
In case of triangular and saw-tooth modes the chopper avoidance region at
the other side of the target enclosed by ( 
) and
( 
) is included.
The observer should be aware that the chopper angle is determined by the
orientation of the spacecraft. Since the spacecraft Z-direction
is always pointing to the Sun and the chopper can only move in Y-direction,
the chopper angle depends on the position of the spacecraft w.r.t. the Sun
and the position of the target on the sky. A target in the ecliptic pole
can have all ( 
) chopper angles over a period of a full year, whereas a
target close to the ecliptic plane has only a limited chop direction. It
is recommended to check the program IRSKY to investigate whether a requested
avoidance angle is feasible.