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# tpsts

erfa.tpsts(xi, eta, a0, b0)

  In the tangent plane projection, given the star’s rectangular coordinates and the spherical coordinates of the tangent point, solve for the spherical coordinates of the star.

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

  Notes

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

      - - - - - - - - -
       e r a T p s t s
      - - - - - - - - -

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

      Given:
         xi,eta    double  rectangular coordinates of star image (Note 2)
         a0,b0     double  tangent point's spherical coordinates

      Returned:
         *a,*b     double  star's spherical coordinates

      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".  If the spherical coordinates are with
         respect to a right-handed triad, (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

      Called:
         eraAnp       normalize angle into range 0 to 2pi

      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.  See notes at end of file.