-
ACTION:
- From the tangent plane coordinates of a star of known ,
determine the of
the tangent point (single precision)
-
CALL:
CALL sla_TPS2C (XI, ETA, RA, DEC, RAZ1, DECZ1, RAZ2, DECZ2, N)
GIVEN:
XI,ETA | R | tangent plane rectangular coordinates (radians) |
|
RA,DEC | R | spherical coordinates (radians) |
|
RETURNED:
RAZ1,DECZ1 | R | spherical coordinates of tangent point, solution 1 |
|
RAZ2,DECZ2 | R | spherical coordinates of tangent point, solution 2 |
|
N | I | number of solutions: |
|
| | 0 = no solutions returned (note 2) |
|
| | 1 = only the first solution is useful (note 3) |
|
| | 2 = there are two useful solutions (note 3) |
|
-
NOTES:
-
-
(1)
- The RAZ1 and RAZ2 values returned are in the range .
-
(2)
- 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
value, and hence it is meaningless to ask where the tangent point would have to be to
bring about this combination of
and .
-
(3)
- 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.
-
(4)
- The DECZ1 and DECZ2 values returned are in the range ,
but in the ordinary, non-pole-crossing, case, the range is .
-
(5)
- RA, DEC, RAZ1, DECZ1, RAZ2, DECZ2 are all in radians.
-
(6)
- The projection is called the gnomonic projection; the Cartesian coordinates are
called standard coordinates. The latter are in units of the distance from the tangent
plane to the projection point, i.e. radians near the origin.
-
(7)
- When working in rather
than spherical coordinates, the equivalent Cartesian routine sla_TPV2C is available.