1 files changed, 127 insertions, 0 deletions
diff --git a/ast/pal/palDs2tp.c b/ast/pal/palDs2tp.c
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+/*
+*+
+*  Name:
+*     palDs2tp
+
+*  Purpose:
+*     Spherical to tangent plane projection
+
+*  Language:
+*     Starlink ANSI C
+
+*  Type of Module:
+*     Library routine
+
+*  Invocation:
+*     palDs2tp( double ra, double dec, double raz, double decz,
+*               double *xi, double *eta, int *j );
+
+*  Arguments:
+*     ra = double (Given)
+*        RA spherical coordinate of point to be projected (radians)
+*     dec = double (Given)
+*        Dec spherical coordinate of point to be projected (radians)
+*     raz = double (Given)
+*        RA spherical coordinate of tangent point (radians)
+*     decz = double (Given)
+*        Dec spherical coordinate of tangent point (radians)
+*     xi = double * (Returned)
+*        First rectangular coordinate on tangent plane (radians)
+*     eta = double * (Returned)
+*        Second rectangular coordinate on tangent plane (radians)
+*     j = int * (Returned)
+*        status: 0 = OK, star on tangent plane
+*                1 = error, star too far from axis
+*                2 = error, antistar on tangent plane
+*                3 = error, antistar too far from axis
+
+*  Description:
+*     Projection of spherical coordinates onto tangent plane:
+*     "gnomonic" projection - "standard coordinates"
+
+*  Authors:
+*     PTW: Pat Wallace (STFC)
+*     TIMJ: Tim Jenness (JAC, Hawaii)
+*     {enter_new_authors_here}
+
+*  History:
+*     2012-02-08 (TIMJ):
+*        Initial version with documentation taken from Fortran SLA
+*        Adapted with permission from the Fortran SLALIB library.
+*     {enter_further_changes_here}
+
+*  Copyright:
+*     Copyright (C) 1996 Rutherford Appleton Laboratory
+*     Copyright (C) 2012 Science and Technology Facilities Council.
+*     All Rights Reserved.
+
+*  Licence:
+*     This program is free software: you can redistribute it and/or
+*     modify it under the terms of the GNU Lesser General Public
+*     License as published by the Free Software Foundation, either
+*     version 3 of the License, or (at your option) any later
+*     version.
+*
+*     This program is distributed in the hope that it will be useful,
+*     but WITHOUT ANY WARRANTY; without even the implied warranty of
+*     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+*     GNU Lesser General Public License for more details.
+*
+*     You should have received a copy of the GNU Lesser General
+*     License along with this program.  If not, see
+*     <http://www.gnu.org/licenses/>.
+
+*  Bugs:
+*     {note_any_bugs_here}
+*-
+*/
+
+#include "pal.h"
+#include <math.h>
+
+void
+palDs2tp ( double ra, double dec, double raz, double decz,
+           double *xi, double *eta, int *j ) {
+
+  const double TINY = 1.0e-6;
+
+  double cdec;
+  double sdec;
+  double radif;
+  double cdecz;
+  double denom;
+  double sdecz;
+  double cradif;
+  double sradif;
+
+  /*  Trig functions */
+  sdecz = sin(decz);
+  sdec = sin(dec);
+  cdecz = cos(decz);
+  cdec = cos(dec);
+  radif = ra - raz;
+  sradif = sin(radif);
+  cradif = cos(radif);
+
+  /*  Reciprocal of star vector length to tangent plane */
+  denom = sdec * sdecz + cdec * cdecz * cradif;
+
+  /*  Handle vectors too far from axis */
+  if (denom > TINY) {
+    *j = 0;
+  } else if (denom >= 0.) {
+    *j = 1;
+    denom = TINY;
+  } else if (denom > -TINY) {
+    *j = 2;
+    denom = -TINY;
+  } else {
+    *j = 3;
+  }
+
+  /*  Compute tangent plane coordinates (even in dubious cases) */
+  *xi = cdec * sradif / denom;
+  *eta = (sdec * cdecz - cdec * sdecz * cradif) / denom;
+
+  return;
+}