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 x=0, y=0

Context:

To measure the primary beam of an antenna, one points the telescope at a point source, then inserts tracking offsets that move the telescope off source to one vertex after another of a regular orthogonal grid of positions centered on the position of the point source. At each of these positions, the point source samples a different part of the primary beam of the antenna. Because azimuth and elevation are not orthogonal, except when one is pointing at a source on the horizon, and because the distance on the sky that corresponds to a given azimuth offset, depends on the elevation, one can't use azimuth and elevation tracking offsets to establish this grid. Hence the sky_offset command was written to provide a way to establish a rectangular coordinate system of offsets on the sky, centered on the source that is being tracked. These offsets are specified along two orthogonal great circles that intersect at the pre-offset pointing center. The y-axis great circle follows the line of increasing elevation, and the x-axis great circle is perpendicular to this.

A simple way to think of these offsets, is to imagine lying on one's back, flat on the deck of the telescope, with one's feet pointed towards the lowest part of the deck, the top of one's head pointed towards the highest part of the deck, and one's arms stretched out perpendicularly from one's sides, flat against the deck. One's feet would then point at a sky offset of x=0,y=-90 degrees, one's head would be pointed at a sky offset of x=0,y=90, one's right hand would point at a sky offset of x=90,y=0 degrees, one's left hand would be pointed at a sky offset of x=-90,y=0, and one's face would be looking along the bore-site at x=0,y=0.