From: "Richard F.L.R. Snashall" 
To:  atm{at}shore.net
Reply-To: "Richard F.L.R. Snashall" 




Frank Q wrote:
>>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.
>

I'm surprised that with all the aperture and resolution to burn that some
of these larger Newts seem to have that someone has not tried to put a flat
window with a silkscreened apodization pattern on it. But then again, the
spider will still be there.

        Rick S.

> 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.
>>
>
>
>

--- BBBS/NT v4.00 MP