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.help dtpv2c Jun99 "Slalib Package"
.nf
SUBROUTINE slDPVC (XI, ETA, V, V01, V02, N)
- - - - - - -
D P V C
- - - - - - -
Given the tangent-plane coordinates of a star and its direction
cosines, determine the direction cosines of the tangent-point.
(double precision)
Given:
XI,ETA d tangent plane coordinates of star
V d(3) direction cosines of star
Returned:
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 = both solutions are useful (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, and hence it is meaningless
to ask where the tangent point would have to be.
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 This routine is the Cartesian equivalent of the routine slDPSC.
P.T.Wallace Starlink 5 June 1995
Copyright (C) 1995 Rutherford Appleton Laboratory
Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.
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