SLA_S2TP - Spherical to Tangent Plane

ACTION:

Projection of spherical coordinates onto the tangent plane (single precision).

CALL:

CALL sla_S2TP (RA, DEC, RAZ, DECZ, XI, ETA, J)

GIVEN:

RA,DEC	R	spherical coordinates of star (radians)
RAZ,DECZ	R	spherical coordinates of tangent point (radians)

RETURNED:

XI,ETA	R	tangent plane coordinates (radians)
J	I	status:
		0 = OK, star on tangent plane
		1 = error, star too far from axis
		2 = error, antistar on tangent plane
		3 = error, antistar too far from axis

NOTES:

1. The projection is called the gnomonic projection; the Cartesian coordinates $[ \xi,\eta ]$ are called standard coordinates. The latter are in units of the distance from the tangent plane to the projection point, i.e. radians near the origin.
2. When working in $[ x,y,z ]$ rather than spherical coordinates, the equivalent Cartesian routine sla_V2TP is available.