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Author Topic: Filter Design.....Following On  (Read 1497 times)

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Offline stealthtech

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Filter Design.....Following On
« on: 16:47:46, 25 May, 2012 »
Cheers for the input in Brians' image thread, Si. Your explanation was kind of what I was thinking, but didn't explain too well.

I'd imagined that, for example, a 2" off centre aperture, although avoiding the central obstruction, would still only grab the same ammount of photons as a standard 2" scope. Obviously the larger the aperture the better. This should be balanced against how much light any obstructions will restrict.....surely in a Mak there can't be that much of an obstruction to reduce the light grabbing capacity by a significant ammount over the rest of the aperture (is there?). That's was my thinking, does it sound logical  :canadian: :crazy:?

Hmmmm, an idea.....it might be interesting to do a comparison when I've made the filter for the Mak. Mask the largest aperture I can off centre and do a direct comparison between the two, to see how it affects things in practice.

Martyn

P.S. I'm sitting here aching to get out and give it a whirl.....Bern's had a run on the solar film and is momentarily out of stock. I keep looking outside, seeing the sun and it's doing my head in. You know the weather is going to turn next week when it arrives  :wounded:
Martyn
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Offline dm-mas

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Re: Filter Design.....Following On
« Reply #1 on: 18:07:18, 28 May, 2012 »
In theory (mine at least.  ;) ) a 2" off axis mask will give you the same resolution as a 2" scope, but with the focal length of the scope it's on the front of - so, effectively increasing the F-Ratio of the scope.
Cheaper & safer for visual, but not particularly necessary for white light imaging.
Some H-Alpha filters need an F-Ratio of f10 or f15 to work properly (e.g. Daystar's) So stopping down the aperture is an alternative to barlowing up to achieve the necessary F-Ratio.
Personally I always go for the best resolution I can get - hence my full aperture filter for my 12" LX200!  :D Although it doesn't get used much as the seeing conditions you need at that scale don't come along very often during the daytime.  :(

David.
EQ6 pro & home made pier, with various combinations of the following... 8" & 12" LX200 OTA's, Orion ED80, TS65 quadruplet & Meade AR6 150mm refractors,Orion optics 6" F4 & GSO 8" F4 newts, Coronado Maxscope 60, DMK21 & 41, QHY8, modded 350D, Atik 314L. And not enough space to keep it all in!!!

Offline colinsk

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Re: Filter Design.....Following On
« Reply #2 on: 15:03:42, 20 March, 2016 »
The other advantage of an off axis mask is the smaller aperture is less sensitive to seeing effects.

Offline MikeDunn

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Re: Filter Design.....Following On
« Reply #3 on: 15:45:24, 20 March, 2016 »
 :whistle: seeing as the OP's not been around for 2.5 years ...

Seems to be a spate of zombie threads recently.
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