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 spherical coordinates of star (radians)

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

RETURNED:

XI,ETA 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 $[\,\xi,\eta\,]$ 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 $[\,x,y,z\,]$ rather than spherical coordinates, the equivalent Cartesian routine sla_DV2TP is available.