Last Update 27 03/ 2001
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Face poles on the spherical projection are projected further to a plane (usually a sheet of paper or a screen) tangent to it by the gnomic projection.
Initially the figure shows a sphere with radius CO=73 units, centred in C, oriented with the South pole coincident with the origin O on the plane represented by the white line.
When angle oca=0o the gnomic projection of the face pole (named A) is zero units far from the origin O. In that case the crystal face pole is 0o from the South pole. If the green square on the bottom of the figure is displaced by a push and drag mouse action, different oca angles may be selected with the respective OA distance displayed. The represented plane size may support pole face projections less than approximately 74,7o from the South pole. Shrinking the radius co with a push and drag mouse action on the green square on the right side of the figure the limit of the face pole angle can be extended to 87,8o.
Bibliography
1. Stout, G.H. and Jensen, L.H., X-ray Structure Determination, Macmillan, London, 1972.
2. Woolfson, M.M., X-ray Crystallography, Cambridge University Press, London, 1970.
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