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