Probably the best documents are the papers by Mark Calabretta and Eric Greisen. It is this draft that the Coordinate classes are based upon. The design has changed since the initial draft. At that time, the Coordinate classes may also be revised. In the recent versions, this paper has split into three, and the projection parameters have been reworked into the PV matrix.
Type Hold all the known types of celestial projections.
Construct a projection which needs no parameters. SIN is unique in that it can be created with 0 or 2 parameters. Construct a projection which needs parameters. The parameter vector must be the length of the required number of parameters.