From: "Frank Q" 
To: "Paul Cousoulis" ,
        
Reply-To: "Frank Q" 


>
> I don't understand. Taper what? The tube opening or the mirror itself?
>
> Paul

Hi Paul

You taper the transmission characteristics of the objective. For a mirror,
you have 100% reflectivity in the centre and taper the reflectivity to a
very low value as you get closer to the edge.

For a lens, it would be clear in the centre and become opaque as you move
towards the edge.

The chief cause of the rings in the diffraction pattern is the sharp edge
of the objective where the transmission/reflection goes instantaneously
from 100% to 0%. Making this transition more gradual removes the high
frequency ringing in the diffraction pattern, broadens the central maximum
and decreases the overall brightness of the image.

Cheers

Frank Q



----- Original Message -----
From: "Paul Cousoulis" 
To: 
Sent: Wednesday, March 05, 2003 6:42 AM Subject: Re: ATM Diffraction due to
secondary sizing


>
> Richard Schwartz wrote:
> >
> > And to carry your argument further, a refractor with NO central
obstruction
> > might do even better.   But even this can be improved on by APODIZING:
> > instead of a sharp chop off of the aperture at the edge, have a gradual
> > tapering.  That will take light out of the first ring and put it into
the
> > center (while making the center slightly larger).  The result should be
> > improved ability to see low contrast dark features on a bright
background at
> > the highest possible powers.

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