In general, a ``map projection'' describes the relation between
2 spherical coordinates, longitude and latitude
,
5on the
celestial sphere, and 2 Cartesian coordinates
and
, which in
radio astronomy and the NCS we often call ``position offsets''.
Up to now, , only the ``radio'' projection
is supported, for which:
If we want to observe several positions on the sky at or near the
source position as specified with SOURCE,
we often do this by requesting position offsets
in the map projection. Also, the resulting data, e.g., images,
are usually stored and displayed as a function of and
.
For most observations, parameters and options of the observing mode are sufficient to specify the position offsets:
The PAKO commands for most Observing Modes expect
fixed offsets (or start- and end-offsets for OTFMAP) as parameters.
These can be either
in the radio projection, specified with the option:
/SYSTEM projection
or in the true angle horizon system (see below), specified with the option:
/SYSTEM trueHorizon
NOTES. For POINTING, the OTF offsets are always in system trueHorizon, and are specified implicitly though the angular length of the subscans.