| 1 | #include "slalib.h"
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| 2 | #include "slamac.h"
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| 3 | void slaDtps2c ( double xi, double eta, double ra, double dec,
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| 4 | double *raz1, double *decz1,
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| 5 | double *raz2, double *decz2, int *n )
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| 6 | /*
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| 7 | ** - - - - - - - - - -
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| 8 | ** s l a D t p s 2 c
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| 9 | ** - - - - - - - - - -
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| 10 | **
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| 11 | ** From the tangent plane coordinates of a star of known RA,Dec,
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| 12 | ** determine the RA,Dec of the tangent point.
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| 13 | **
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| 14 | ** (double precision)
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| 15 | **
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| 16 | ** Given:
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| 17 | ** xi,eta double tangent plane rectangular coordinates
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| 18 | ** ra,dec double spherical coordinates
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| 19 | **
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| 20 | ** Returned:
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| 21 | ** *raz1,*decz1 double spherical coordinates of TP, solution 1
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| 22 | ** *raz2,*decz2 double spherical coordinates of TP, solution 2
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| 23 | ** *n int number of solutions:
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| 24 | ** 0 = no solutions returned (note 2)
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| 25 | ** 1 = only the first solution is useful (note 3)
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| 26 | ** 2 = both solutions are useful (note 3)
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| 27 | **
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| 28 | **
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| 29 | ** Notes:
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| 30 | **
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| 31 | ** 1 The raz1 and raz2 values are returned in the range 0-2pi.
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| 32 | **
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| 33 | ** 2 Cases where there is no solution can only arise near the poles.
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| 34 | ** For example, it is clearly impossible for a star at the pole
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| 35 | ** itself to have a non-zero xi value, and hence it is
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| 36 | ** meaningless to ask where the tangent point would have to be
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| 37 | ** to bring about this combination of xi and dec.
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| 38 | **
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| 39 | ** 3 Also near the poles, cases can arise where there are two useful
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| 40 | ** solutions. The argument n indicates whether the second of the
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| 41 | ** two solutions returned is useful; n=1 indicates only one useful
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| 42 | ** solution, the usual case; under these circumstances, the second
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| 43 | ** solution corresponds to the "over-the-pole" case, and this is
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| 44 | ** reflected in the values of raz2 and decz2 which are returned.
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| 45 | **
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| 46 | ** 4 The decz1 and decz2 values are returned in the range +/-pi, but
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| 47 | ** in the usual, non-pole-crossing, case, the range is +/-pi/2.
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| 48 | **
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| 49 | ** 5 This routine is the spherical equivalent of the routine slaDtpv2c.
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| 50 | **
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| 51 | ** Called: slaDranrm
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| 52 | **
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| 53 | ** Last revision: 5 June 1995
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| 54 | **
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| 55 | ** Copyright P.T.Wallace. All rights reserved.
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| 56 | */
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| 57 | {
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| 58 | double x2, y2, sd, cd, sdf, r2, r, s, c;
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| 59 |
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| 60 | x2 = xi * xi;
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| 61 | y2 = eta * eta;
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| 62 | sd = sin ( dec );
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| 63 | cd = cos ( dec );
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| 64 | sdf = sd * sqrt ( 1.0 + x2 + y2 );
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| 65 | r2 = cd * cd * ( 1.0 + y2 ) - sd * sd * x2;
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| 66 | if ( r2 >= 0.0 ) {
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| 67 | r = sqrt ( r2 );
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| 68 | s = sdf - eta * r;
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| 69 | c = sdf * eta + r;
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| 70 | if ( xi == 0.0 && r == 0.0 ) {
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| 71 | r = 1.0;
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| 72 | }
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| 73 | *raz1 = slaDranrm ( ra - atan2 ( xi, r ) );
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| 74 | *decz1 = atan2 ( s, c );
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| 75 | r = -r;
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| 76 | s = sdf - eta * r;
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| 77 | c = sdf * eta + r;
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| 78 | *raz2 = slaDranrm ( ra - atan2 ( xi, r ) );
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| 79 | *decz2 = atan2 ( s, c );
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| 80 | *n = ( fabs ( sdf ) < 1.0 ) ? 1 : 2;
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| 81 | } else {
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| 82 | *n = 0;
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| 83 | }
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| 84 | }
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