**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 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 rather than spherical coordinates, the equivalent Cartesian routine sla_V2TP is available.

Starlink User Note 67

P. T. Wallace

12 October 1999

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