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The SkyFrame Model

A SkyFrame is, of course, a Frame ([*]) and also a Mapping ([*]), so it inherits all the properties and behaviour of these two ancestral classes. When used as a Mapping, a SkyFrame implements a unit transformation, exactly like a basic Frame ([*]) or a UnitMap, so this aspect of its behaviour is not of great importance.

When used as a Frame, however, a SkyFrame represents a 2-dimensional spherical coordinate system, in which the shortest distance between two points is a great circle. A SkyFrame therefore always has exactly two axes which represent the longitude and latitude of a coordinate system residing on the celestial sphere. Many such coordinate systems can be represented by a SkyFrame, as we will see shortly.

When it is first created, a SkyFrame's axes are always in the order (longitude, latitude) but this can be changed, if required, by using the AST_PERMAXES routine ([*]). The order of the axes can be determined at any time using the LatAxis and LonAxis attributes. A SkyFrame's coordinate values are always stored as angles in (double precision) radians, regardless of the setting of the Unit attribute.



next up previous
Next: Creating a SkyFrame
Up: Celestial Coordinate Systems (SkyFrames)
Previous: Celestial Coordinate Systems (SkyFrames)

AST A Library for Handling World Coordinate Systems in Astronomy
Starlink User Note 210
R.F. Warren-Smith & D.S. Berry
30th April 2003
E-mail:ussc@star.rl.ac.uk

Copyright (C) 2003 Central Laboratory of the Research Councils