diffraction, macro, apodization

[P3D Peter Abrahams <telscope@xxxxxxxxxx>]

>If diffraction is caused by edges, what if the aperture were a small
>circular gradient on a transparent film where the diaphragm would otherwise
>be, instead of a sharply-defined hole?  Then there wouldn't be an edge per
>se.  Would that eliminate or minimize the diffraction?

This is called apodization and has been the subject of much discussion in
amateur astronomy groups.  Some have tried aperture stops covering around
half the aperture, of window screen material, since at telescopic
magnification any transparent obstruction has to be optically flat.  Some
users claim it works quite well, and they can see more detail on the
planets; and others are sure it is complete nonsense.  It is of undoubted
utility when viewing or measuring a dim companion to a bright star, when
the diffraction pattern of the bright star will obscure the dim one.
Apodization is defined more precisely in optics texts, as suppressing the
rings around the central disc in the out of focus image.
I don't find it in the index of a dozen books on photographic optics.  At
the small apertures at which diffraction becomes a problem, it would be a
challenge to make an apodizer that is optically useable.

_______________________________________
Peter Abrahams   telscope@xxxxxxxxxx
the history of the telescope, the microscope,
    and the prism binocular


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