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 [ξ, η] 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.