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Wobbler switching
In case of wobbler switching, the LONGOFF and
LATOFF columns describe the primary dish direction on
sky. Because the secondary mirror switches with additional offsets
between the ON and OFF positions, MRTCAL assumes that the ON phases
always have (0,0) antenna offsets. For simplicity, the OFF phases
offsets are also set to (0,0) (instead of
) but the
CLASS switching section will describe properly the throw when saving
the ON-OFF result, or when saving the ON and OFF separately. Remember
that the wobbler throw is usually along azimuth with a given
projection, while the CLASS offsets are described in equatorial or
galactic system with radio projection: their addition is not
straightforward.
Figure:
Asymmetrical wobbler switching illustration. For each
subscan, the antenna (primary dish) points at
away
from the source, usually in the azimuthal direction (blue
line). This is called antenna noding. During the subscan,
the secondary mirror switches (red line) from the ON position (at
offset 0) to one OFF position (at offset
).
is the wobbler throw. This pattern, which alternates
between OFF1 and OFF2 at each subscan, is called
asymmetrical wobbler switching, as opposed to the
symmetrical wobbler switching (see details in
Fig
).
|
Figure:
Symmetrical wobbler switching illustration. Same as
Fig
, except that the subscans are observed with
symmetry around a given time. Thanks to this, and assuming that
the atmospheric properties drift linearly through time, the
atmospheric variations between the 2 OFF1 subscans and between the
2 OFF2 subscans average at the same level.
|
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Gildas manager
2023-06-01