Py ERFA:docs

- Index
- Modules

### Navigation

- next »
- « previous |
- PyERFA v2.0.1.5 »
- erfa Package »
- tpxev

# tpxev

erfa.tpxev(v, v0)

  In the tangent plane projection, given celestial direction cosines for a star and the tangent point, solve for the star’s rectangular coordinates in the tangent plane.

  Parameters:
    v double array
    v0 double array
  Returns:
    xi double array
    eta double array

  Notes

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

      - - - - - - - - -
       e r a T p x e v
      - - - - - - - - -

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

      Given:
         v         double[3]  direction cosines of star (Note 4)
         v0        double[3]  direction cosines of tangent point (Note 4)

      Returned:
         *xi,*eta  double     tangent plane coordinates of star

      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 direction cosines represent observed (RA,Dec), the tangent
         plane coordinates (xi,eta) are conventionally called the
         "standard coordinates".  If the direction cosines 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) The method used is to extend the star vector to the tangent
         plane and then rotate the triad so that (x,y) becomes (xi,eta).
         Writing (a,b) for the celestial spherical coordinates of the
         star, the sequence of rotations is (a+pi/2) around the z-axis
         followed by (pi/2-b) around the x-axis.

      4) If vector v0 is not of unit length, or if vector v is of zero
         length, the results will be wrong.

      5) If v0 points at a pole, the returned (xi,eta) will be based on
         the arbitrary assumption that the longitude coordinate of the
         tangent point is zero.

      6) 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.