We now offer a new bolometer mapping mode at the 30m telescope using the rotated chopping secondary. In this observing mode the scanning and the chopping direction can be rotated with respect to the azimuth direction.
For mapping using the chopping secondary it is highly desirable to scan sources along their smallest spatial extent. So far this requirement limited the LST range when a source could be optimally observed while scanning in azimuth direction only. For some sources this condition is never fulfilled.
The rotation of the chopping secondary and the scanning direction
eliminates this limitation (see Fig.1). Currently we offer a
rotation of the chopping secondary in the range of
.
We hope to extend this range soon to
. The known
misalignment between the subreflector hyperboloid axis and the
subreflector rotation axis is mainly corrected at the time when
the subreflector is rotated. Nevertheless, it is still recommended
to execute a pointing scan just after the rotation in order to
correct any pending offset.
The rotation of the chopping/scanning direction with respect to
azimuth implies that the two chopper phases scan through different
air masses which may lead to baseline drifts. From our experience
this effect can efficiently be removed in the data processing at a
level of mJy in good weather conditions (see Fig.1).
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The reduction software MOPSIC offers powerful planning tools to optimize the mapping parameters including the rotated secondary (AEO in MOPSIC, Fig. 2).
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