SLA_DTPV2C

Plate centre from $ξ, η$ and $x, y, z$

ACTION:: From the tangent plane coordinates of a star of known direction cosines, determine the direction cosines of the tangent point (double precision)
CALL:: CALL sla_DTPV2C (XI, ETA, V, V01, V02, N)

XI,ETA	D	tangent plane coordinates of star (radians)

V	D(3)	direction cosines of star

V01	D(3)	direction cosines of tangent point, solution 1

V02	D(3)	direction cosines 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 vector V must be of unit length or the result will be wrong.
(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 XI value.
(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 can be regarded as valid if the vector V02 is interpreted as the “over-the-pole” case.
(4): The projection is called the gnomonic projection; the Cartesian coordinates $[ξ, η]$ 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.
(5): This routine is the Cartesian equivalent of the routine sla_DTPS2C.