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SLA_S2TP - Spherical to Tangent Plane   

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 $[\,\xi,\eta\,]$ 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_V2TP is available.


next up previous
Next: SLA_SEP - Angle Between 2 Points on Sphere
Up: SUBPROGRAM SPECIFICATIONS
Previous: SLA_RVLSRK - RV Corrn to Kinematical LSR

SLALIB --- Positional Astronomy Library
Starlink User Note 67
P. T. Wallace
12 October 1999
E-mail:ptw@star.rl.ac.uk