SLA_DV2TP

Direction Cosines to Tangent Plane

ACTION:: Given the direction cosines of a star and of the tangent point, determine the star’s tangent-plane coordinates (double precision).
CALL:: CALL sla_DV2TP (V, V0, XI, ETA, J)

V	D(3)	direction cosines of star

V0	D(3)	direction cosines of tangent point

XI,ETA	D	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): If vector V0 is not of unit length, or if vector V is of zero length, the results will be wrong.
(2): If V0 points at a pole, the returned $ξ, η$ will be based on the arbitrary assumption that $α = 0$ at the tangent point.
(3): The projection is called the gnomonic projection; the Cartesian coordinates $[ξ, η]$ 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.
(4): This routine is the Cartesian equivalent of the routine sla_DS2TP.