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RE: [ATM] side obstruction
Jonathan,
I put some simulation output in:
http://www.atmlist.net/contrib/steve_koehler-at-securecomputing-dot-com/lavoie/
lavoie1.png shows what parts of the aperture are obstructed. I added a spider,
as well. I'm not sure if you have a spider, but I wanted to compare the
strength of the spikes between it and the focuser obstruction.
lavoie2.png shows a star test of this aperture with a perfect optical figure.
lavoie3.png shows an MTF graph, showing how the spider and focuser obstruction
affect the transfer of various image frequencies. I compare it to a 26%
obstruction (yours), and to a 15% obstruction. I don't know what kind of scope
you have, but you would get a lot of improvement if you could reduce the
obstruction size!
lavoie4.png shows a focused star image with the gamma factor turned way up so
that the spikes are visible. The diagonal spikes are due to the focuser
obstruction. I believe that the spike becomes one sided if you defocus a little
either side of focus.
I haven't tried modifying the focuser obstruction, either to curve it or to
soften the sharp edge. I suppose you could use a little window screen to soften
the edge a bit, but it's effect is really pretty small, as shown in the MTF, so
it's probably not worth it.
By the way, I use my scope with a filter slide that protrudes all the time about
as much as your focuser protrusion, perhaps a little less. I don't notice a big
problem in use, although the 45 degree diffraction spike is always visible in
focus on bright stars.
-- Steve Koehler
steve_koehler@securecomputing.com
> -----Original Message-----
> From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf
> Of Jonathan Lavoie
> Sent: Tuesday, April 19, 2005 11:50 AM
> To: atm@atmlist.net
> Subject: [ATM] side obstruction
>
> Hello all,
>
> What is the diffraction effect of obstruction on the side of the lightpath
> compared to central obstruction? Let's say my focuser tube would obstruct
> 20mm deep by 51mm wide in a 406mm aperture scope with a 107mm central
> obstruction. Would I get any visible effect?
>
> Jonathan
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