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#include "erfa.h"
int eraTpxes(double a, double b, double a0, double b0,
double *xi, double *eta)
/*
** - - - - - - - - -
** 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) 2013-2021, NumFOCUS Foundation.
** Derived, with permission, from the SOFA library. See notes at end of file.
*/
{
const double TINY = 1e-6;
int j;
double sb0, sb, cb0, cb, da, sda, cda, d;
/* Functions of the spherical coordinates. */
sb0 = sin(b0);
sb = sin(b);
cb0 = cos(b0);
cb = cos(b);
da = a - a0;
sda = sin(da);
cda = cos(da);
/* Reciprocal of star vector length to tangent plane. */
d = sb*sb0 + cb*cb0*cda;
/* Check for error cases. */
if ( d > TINY ) {
j = 0;
} else if ( d >= 0.0 ) {
j = 1;
d = TINY;
} else if ( d > -TINY ) {
j = 2;
d = -TINY;
} else {
j = 3;
}
/* Return the tangent plane coordinates (even in dubious cases). */
*xi = cb*sda / d;
*eta = (sb*cb0 - cb*sb0*cda) / d;
/* Return the status. */
return j;
/* Finished. */
}
/*----------------------------------------------------------------------
**
**
** Copyright (C) 2013-2021, NumFOCUS Foundation.
** All rights reserved.
**
** This library is derived, with permission, from the International
** Astronomical Union's "Standards of Fundamental Astronomy" library,
** available from http://www.iausofa.org.
**
** The ERFA version is intended to retain identical functionality to
** the SOFA library, but made distinct through different function and
** file names, as set out in the SOFA license conditions. The SOFA
** original has a role as a reference standard for the IAU and IERS,
** and consequently redistribution is permitted only in its unaltered
** state. The ERFA version is not subject to this restriction and
** therefore can be included in distributions which do not support the
** concept of "read only" software.
**
** Although the intent is to replicate the SOFA API (other than
** replacement of prefix names) and results (with the exception of
** bugs; any that are discovered will be fixed), SOFA is not
** responsible for any errors found in this version of the library.
**
** If you wish to acknowledge the SOFA heritage, please acknowledge
** that you are using a library derived from SOFA, rather than SOFA
** itself.
**
**
** TERMS AND CONDITIONS
**
** Redistribution and use in source and binary forms, with or without
** modification, are permitted provided that the following conditions
** are met:
**
** 1 Redistributions of source code must retain the above copyright
** notice, this list of conditions and the following disclaimer.
**
** 2 Redistributions in binary form must reproduce the above copyright
** notice, this list of conditions and the following disclaimer in
** the documentation and/or other materials provided with the
** distribution.
**
** 3 Neither the name of the Standards Of Fundamental Astronomy Board,
** the International Astronomical Union nor the names of its
** contributors may be used to endorse or promote products derived
** from this software without specific prior written permission.
**
** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
** "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
** LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
** FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
** COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
** INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
** BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
** LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
** CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
** LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
** ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
** POSSIBILITY OF SUCH DAMAGE.
**
*/