palDtps2c
palDtps2c
- Determine RA,Dec of tangent point from coordinates
- Description:
- From the tangent plane coordinates of a star of known RA,Dec,
determine the RA,Dec of the tangent point.
- Invocation:
- palDtps2c( double xi, double eta, double ra, double dec,
double
raz1, double decz1,
double
raz2, double decz2, int
n);
- Arguments:
-
-
xi = double (Given)
-
First rectangular coordinate on tangent plane (radians)
-
eta = double (Given)
-
Second rectangular coordinate on tangent plane (radians)
-
ra = double (Given)
-
RA spherical coordinate of star (radians)
-
dec = double (Given)
-
Dec spherical coordinate of star (radians)
-
raz1 = double
(Returned)
-
RA spherical coordinate of tangent point, solution 1 (radians)
-
decz1 = double
(Returned)
-
Dec spherical coordinate of tangent point, solution 1 (radians)
-
raz2 = double
(Returned)
-
RA spherical coordinate of tangent point, solution 2 (radians)
-
decz2 = double
(Returned)
-
Dec spherical coordinate of tangent point, solution 2 (radians)
-
n = int
(Returned)
-
number of solutions: 0 = no solutions returned (note 2)
1 = only the first solution is useful (note 3)
2 = both solutions are useful (note 3)
- Notes:
- The RAZ1 and RAZ2 values are returned in the range 0-2pi.
- Cases where there is no solution can only arise near the poles.
For example, it is clearly impossible for a star at the pole
itself to have a non-zero XI value, and hence it is
meaningless to ask where the tangent point would have to be
to bring about this combination of XI and DEC.
- Also near the poles, cases can arise where there are two useful
solutions. The argument N indicates whether the second of the
two solutions returned is useful. N=1 indicates only one useful
solution, the usual case; under these circumstances, the second
solution corresponds to the "over-the-pole" case, and this is
reflected in the values of RAZ2 and DECZ2 which are returned.
- The DECZ1 and DECZ2 values are returned in the range
/-pi, but
in the usual, non-pole-crossing, case, the range is
/-pi/2.
- This routine is the spherical equivalent of the routine sla_DTPV2C.