
ACTION:
 Given the tangentplane coordinates of a star and the direction cosines of the
tangent point, determine the direction cosines of the star (double precision).

CALL:
CALL sla_DTP2V (XI, ETA, V0, V)
GIVEN:
XI,ETA  D  tangent plane coordinates of star (radians) 

V0  D(3)  direction cosines of tangent point 

RETURNED:
V  D(3)  direction cosines of star 


NOTES:


(1)
 If vector V0 is not of unit length, the returned vector V will be wrong.

(2)
 If vector V0 points at a pole, the returned vector V will be based on the arbitrary
assumption that $\alpha =0$
at the tangent point.

(3)
 The projection is called the gnomonic projection; the Cartesian coordinates $\left[\phantom{\rule{0.3em}{0ex}}\xi ,\eta \phantom{\rule{0.3em}{0ex}}\right]$ 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_DTP2S.