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Subroutine: IRCO_PROJECT.

Purpose: Project points on celestial sphere onto a plane.

File: irco_project.shl.

Author: 85 Oct 01 Albrecht de Jonge.

Use:
call IRCO_PROJECT (
PRID, I integer,
XYZ, I doubleprecision array (3,N),
X, Y, O doubleprecision array (n),
N I integer.
)
PRID a projections system number (see IRCO_PRNAME),
XYZ 3-dimensional coordinates on the sphere,
X, Y the corresponding coordinates on the plane,
N number of points to be projected.
Description:
Assume the coordinates xyz give coordinates of points on the unit
the unit sphere, in a right-handed system of x,y and z axes.

All points will be mapped on to a plane with a X-Y system.
The origin of the plane system is the projection centre,
and corresponds to xyz = (1,0,0).
If the plane is plotted conventionally
(plane X-axis horizontal left to right, plane Y-axis upwards),
the picture close to the origin corresponds to a view of the sky
looking outwards along the celestial +x-axis, with the
celestial y-axis upwards and the celestial z-axis to the right.

No scale factor is inserted, close to the origin a small angle D
on the sky in radians corresponds to a distance D in the plane.

All these projection methods have the following property:
suppose xyz = ( a, b, c ) and X, Y = p, q ;
if abs(b) << 1, abs(c) << 1, then 0 < 1 - a << 1
and, approximately, p = c and q = b.

Note that traditionally in astronomy, the x-axis is increasing to
the left due to the fact that we are looking form the inside out.
In these routines this fact is NOT incorporated.

References:
IRCO_PRNAME.
Updates:
90 Apr 24 Sjag Steensma; documentation only,
89 Jan 09 Timo Prusti, unification of versions,
86 Jan 21 Albrecht de Jonge, renamed,
86 Jan 21 Do Kester, one more projection.
900921 DK, change in plate definition rotates xyz