### SLA_DS2TP

Spherical to Tangent Plane

ACTION:
Projection of spherical coordinates onto the tangent plane (double precision).
CALL:
CALL sla_DS2TP (RA, DEC, RAZ, DECZ, XI, ETA, J)
##### GIVEN:
 RA,DEC D spherical coordinates of star (radians) RAZ,DECZ D spherical coordinates of tangent point (radians)

##### 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)
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.
(2)
When working in $\left[\phantom{\rule{0.3em}{0ex}}x,y,z\phantom{\rule{0.3em}{0ex}}\right]$ rather than spherical coordinates, the equivalent Cartesian routine sla_DV2TP is available.