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# tpxes

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.

  Parameters:
    a double array
    b double array
    a0 double array
    b0 double array
  Returns:
    xi double array
    eta double array

  Notes

  Wraps ERFA function `eraTpxes`. The ERFA documentation is:

      - - - - - - - - -
       e r a T p x e s
      - - - - - - - - -

      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) 2013-2023, NumFOCUS Foundation.
      Derived, with permission, from the SOFA library.  See notes at end of file.