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.
Copyright © 2012 Science and Technology Facilities Council.
Copyright © 2014 Cornell University.
Copyright © 2015 Tim Jenness