Spherical to Tangent Plane
CALL sla_S2TP (RA, DEC, RAZ, DECZ, XI, ETA, J)
RA,DEC  R  spherical coordinates of star (radians) 


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