Py ERFA:docs

- Index
- Modules

### Navigation

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

# tpstv

erfa.tpstv(xi, eta, v0)

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

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

  Notes

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

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

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

      Given:
         xi,eta  double     rectangular coordinates of star image (Note 2)
         v0      double[3]  tangent point's direction cosines

      Returned:
         v       double[3]  star's direction cosines

      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 complete the star vector in the (xi,eta)
         based triad and normalize it, then rotate the triad to put the
         tangent point at the pole with the x-axis aligned to zero
         longitude.  Writing (a0,b0) for the celestial spherical
         coordinates of the tangent point, the sequence of rotations is
         (b-pi/2) around the x-axis followed by (-a-pi/2) around the
         z-axis.

      4) If vector v0 is not of unit length, the returned vector v will
         be wrong.

      5) If vector v0 points at a pole, the returned vector v 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.