era.tpxes.md

eraTpxes

rv = ERFA.tpxes(a, b, a0, b0)

In the tangent plane projection, given celestial spherical coordinates for a star and the tangent point, solve for the star's rectangular coordinates in the tangent plane.

Given:

   a,b       double  star's spherical coordinates
   a0,b0     double  tangent point's spherical coordinates

Returned:

   *xi,*eta  double  rectangular coordinates of star image (Note 2)

Returned (function value):

             int     status:  0 = OK
                              1 = star too far from axis
                              2 = antistar on tangent plane
                              3 = antistar too far from axis

Notes:

1. The tangent plane projection is also called the "gnomonic projection" and the "central projection".

2. The eta axis points due north in the adopted coordinate system. If the spherical coordinates are observed (RA,Dec), the tangent plane coordinates (xi,eta) are conventionally called the "standard coordinates". For right-handed spherical coordinates, (xi,eta) are also right-handed. The units of (xi,eta) are, effectively, radians at the tangent point.

3. All angular arguments are in radians.

4) This function is a member of the following set:

       spherical      vector         solve for

     > eraTpxes <    eraTpxev         xi,eta
       eraTpsts      eraTpstv          star
       eraTpors      eraTporv         origin

References:

Calabretta M.R. & Greisen, E.W., 2002, "Representations of celestial coordinates in FITS", Astron.Astrophys. 395, 1077

Green, R.M., "Spherical Astronomy", Cambridge University Press, 1987, Chapter 13.

This revision: 2018 January 2

Copyright (C) 2013-2021, NumFOCUS Foundation. Derived, with permission, from the SOFA library.