**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)`

**GIVEN:**-

*V***D(3)**direction cosines of star *V0***D(3)**direction cosines of tangent point

**RETURNED:**-

*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 will be based on the arbitrary assumption that at the tangent point.
- 3.
- The projection is called the
*gnomonic*projection; the Cartesian coordinates 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.

Starlink User Note 67

P. T. Wallace

12 October 1999

E-mail:ptw@star.rl.ac.uk