/*
*+
* 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
* {enter_further_changes_here}
* Copyright:
* 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 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 General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301
* USA.
* Bugs:
* {note_any_bugs_here}
*-
*/
#include "pal.h"
#include
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;
}