Keywords: pako

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#### Map Projections and Offsets.

In general, a ``map projection'' describes the relation between 2 spherical coordinates, longitude and latitude , 1on 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:

where and are the source coordinates specified with `SOURCE`. 2Note that this is the same system of offsets as in ``OBS'' of the old control system.

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:

- for `TRACK` and `VLBI` and are fixed during the complete scan;
- for `ONOFF` and change from subscan to subscan;
- for `OTFMAP` and change continuously or ``on-the-fly'' (OTF) during the OTF subscans.

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.

Next: Global Offsets. Up: Coordinate Systems, Map Projections, Previous: Coordinate Systems, Map Projections, Contents Index