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).
The reduction software MOPSIC offers powerful planning tools to optimize the mapping parameters including the rotated secondary (AEO in MOPSIC, Fig. 2).