Definitions of COMICS instrument and chop throw PAs
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The two PAs can be specified at the same time to suit the observer's particular needs. Diagrams below illustrate how one can determine these angles in advance to prepare for an observing run.
We consider an elongated object (an oval), which would fit in the rectangular FOV only if its long axis is placed parallel with the long-side of the rectangle (see diagrams). In the diagrams below, chopping on-beam is indicated by an empty oval enclosed inside the FOV. off-beam is shaded and always located outside the on-beam's FOV (i.e. off-chip chop). However, it can easily be adopted to on-chip chop observations by decreasing the chop throw.
This first diagram illustrates when the object is elongated north-south (i.e. Obj_PA = 0 deg). In this case, Inst_PA = 0 deg also. If Chop_PA = 0 deg as well, off-beam will be chopped to the north of the object. If an arbitrary Chop_PA is given, positive value will rotate the original (Chop_PA = 0 deg) position counter-clockwise and negative number clockwise. So if chopping east/west of the object is desired (perhaps there may be some faint extension in the direction of elongation?) Chop_PA should be set either to +90 deg or -90 deg.
Second diagram shows when Obj_PA = +30 deg. To orient the instrument's FOV accordingly, Inst_PA should be set to -30 deg. Chop_PA = 0 deg leaves chop throw in its default direction (i.e. due north). If chopping along the elongation is required, then Chop_PA = +30 deg. If chopping perpendicular to Obj_PA is desired, Chop_PA = -60 deg.
It has probably become obvious by now - in general:
Inst_PA = -Obj_PA
Note: PA increases in a conventional sense - i.e. 0 deg at the equatorial north and increases through east. Inst_PA appears to be in conflict with this convention, however, from the point of view of COMICS, the sky rotates itself to align with the camera FOV, not the other way around! So the normal convention still applies.