Re: ATM 3-Vane or 4-Vane Spiders

[PK6811S@ACAD.DRAKE.EDU]

>Sorry, even with no vanes at all there will be spikes caused by the edge of
>the aperture.  However, because the aperture edge is curved, the spikes are
>spread uniformly around each star image.  These edge spikes, as well as the
>spikes from spider vanes, might be suppressed if there could be a gradual
>blended shading around the obstructions.  Of course, there may be other
>undesireable consequences.

Repeat after me:

DIFFRACTION IS NOT AN EDGE EFFECT
DIFFRACTION IS NOT AN EDGE EFFECT
DIFFRACTION IS NOT AN EDGE EFFECT

What we call 'diffraction' is the observable effect of obstructing part
of a wave front.  Whether the edge is 'blended', 'graduated', 'matted'
or whatever is irrelevant.  The obstruction creates the effect.  The
bigger the obstruction the bigger the observed effect, most schemes to
soften the edge make the obstruction bigger and therefore do not improve
the situation.

On the other hand a 'spike' is observed when the effect is concentrated
in a narrow area.  It is possible to spread the effect out and remove
spikes by making the obstruction in some other shape than a straight
line.  This will usually make the obstruction bigger but may make
the image more pleasing to the eye - if one objects to spikes :-)

Paul Kline
pk6811s@acad.drake.edu