Replace or add to the sky-based tracking offsets.

Optional qualifier arguments:

/add

If this modifier is present the offsets will be added to the existing offsets. Otherwise they will replace them.

Mandatory arguments:

(none)

Optional Arguments:

PointingOffset y

The offset towards the zenith, directed along the great circle that connects the un-offset pointing center and the zenith.

PointingOffset x

The offset along the great circle that is both perpendicular to the above described y axis and passes through the un-offset pointing center. When the telescope is pointing south, x increases towards the west.

Examples:

The following example adds an additional 10 arcseconds to the pointing offset along the x axis and 0.02 degrees to the offset along the y axis.

  sky_offset/add x=0:0:10, y=0.02

The second example shows how to replace any existing sky offsets with new sky offsets. In this case both are being set to zero, to remove all sky offsets.

  sky_offset/set x=0, y=0

Context:

The sky_offset command is designed for use when measuring the primary beams of the individual antennas. It allows one to define a regular grid in x and y over the area of the primary beam and point a source at each of these locations. Unlike the raw telescope-based offsets that are established with the offset command, the sky_offset command maintains the same part of the beam pointing at the source irrespective of the elevation and declination of the source.

Sky based offsets are implemented as offsets along two orthogonal great circles that pass through the un-offset pointing center. The orientation of these great circles is established by defining one of them to be the great circle that joins the un-offset pointing center and the zenith. The offset angle along this great circle is refered to as an x-axis offset. The other great circle is uniquely defined simply by the fact that it must go through the un-offset pointing center at right angles to the x-axis great circle. At zero elevation the second great circle follows the horizon, and it then increases in the same direction as the telescope azimuth.