| 1 | #include "slalib.h"
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| 2 | #include "slamac.h"
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| 3 | void slaTps2c ( float xi, float eta, float ra, float dec,
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| 4 | float *raz1, float *decz1,
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| 5 | float *raz2, float *decz2, int *n )
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| 6 | /*
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| 7 | ** - - - - - - - - -
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| 8 | ** s l a 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 | ** (single precision)
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| 15 | **
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| 16 | ** Given:
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| 17 | ** xi,eta float tangent plane rectangular coordinates
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| 18 | ** ra,dec float spherical coordinates
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| 19 | **
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| 20 | ** Returned:
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| 21 | ** *raz1,*decz1 float spherical coordinates of TP, solution 1
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| 22 | ** *raz2,*decz2 float 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 | ** Notes:
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| 29 | **
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| 30 | ** 1 The raz1 and raz2 values are returned in the range 0-2pi.
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| 31 | **
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| 32 | ** 2 Cases where there is no solution can only arise near the poles.
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| 33 | ** For example, it is clearly impossible for a star at the pole
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| 34 | ** itself to have a non-zero xi value, and hence it is
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| 35 | ** meaningless to ask where the tangent point would have to be
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| 36 | ** to bring about this combination of xi and dec.
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| 37 | **
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| 38 | ** 3 Also near the poles, cases can arise where there are two useful
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| 39 | ** solutions. The argument n indicates whether the second of the
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| 40 | ** two solutions returned is useful; n=1 indicates only one useful
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| 41 | ** solution, the usual case; under these circumstances, the second
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| 42 | ** solution corresponds to the "over-the-pole" case, and this is
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| 43 | ** reflected in the values of raz2 and decz2 which are returned.
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| 44 | **
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| 45 | ** 4 The decz1 and decz2 values are returned in the range +/-pi, but
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| 46 | ** in the usual, non-pole-crossing, case, the range is +/-pi/2.
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| 47 | **
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| 48 | ** 5 This routine is the spherical equivalent of the routine slaTpv2c.
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| 49 | **
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| 50 | ** Called: slaRanorm
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| 51 | **
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| 52 | ** Last revision: 5 June 1995
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| 53 | **
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| 54 | ** Copyright P.T.Wallace. All rights reserved.
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| 55 | */
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| 56 | {
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| 57 | float x2, y2, sd, cd, sdf, r2, r, s, c;
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| 58 |
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| 59 | x2 = xi * xi;
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| 60 | y2 = eta * eta;
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| 61 | sd = (float) sin ( (double) dec );
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| 62 | cd = (float) cos ( (double) dec );
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| 63 | sdf = sd * (float) sqrt ( (double) ( 1.0f + x2 + y2 ) );
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| 64 | r2 = cd * cd * ( 1.0f + y2 ) - sd * sd * x2;
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| 65 | if ( r2 >= 0.0f ) {
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| 66 | r = (float) sqrt ( (double) r2 );
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| 67 | s = sdf - eta * r;
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| 68 | c = sdf * eta + r;
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| 69 | if ( xi == 0.0f && r == 0.0f ) {
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| 70 | r = 1.0f;
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| 71 | }
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| 72 | *raz1 = slaRanorm ( ra - (float) atan2 ( (double) xi, (double) r ) );
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| 73 | *decz1 = (float) atan2 ( (double) s, (double) c );
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| 74 | r = -r;
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| 75 | s = sdf - eta * r;
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| 76 | c = sdf * eta + r;
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| 77 | *raz2 = slaRanorm ( ra - (float) atan2 ( (double) xi, (double) r ) );
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| 78 | *decz2 = (float) atan2 ( (double) s, (double) c );
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| 79 | *n = ( fabs ( (double) sdf ) < 1.0 ) ? 1 : 2;
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| 80 | } else {
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| 81 | *n = 0;
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| 82 | }
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| 83 | }
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