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  #21  
Old 04-05-2022, 03:02 PM
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MattT
Reflecting on Refracting

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Wow JA….go big or go home!

How far from the 200mm tube does the objective sit?

I have an idea for mounting as I have thought on and off, mostly off, about a 9” Istar f9 achro or 10” f11r30, on the same 200mm tube which I have another piece just waiting to be used. Never could get the funds together to buy as Im waiting for the US$ to drop like it did years ago when 90oz cents bought the mighty us$…I digress.

Other question is how long in f ratio, does the 76mm lens make the scope?

I got the mount for such a monster as you know.

Would like to see the beast in real life if possible too?

Interesting project.

Matt
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  #22  
Old 04-05-2022, 04:19 PM
JA
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Originally Posted by MattT View Post
Wow JA….go big or go home!

How far from the 200mm tube does the objective sit?
Hi Matt,

OPTION 1...
The objective is a 270mm ish singlet with an approximately 840mm focal length if I use it as a singlet. So in this case I would need to trim the 200mm tube and add support for the 270mm lens perhaps with a 300mm tube or open truss type addition.

OPTION 2...
If instead I use it in what I understand to be a classical dialyte configuration (A converging objective lens with a diverging lens spaced some distance away in the "light cone" of the converging lens) I calculate I could achieve a 1020mm focal length, using an available 78mm negative lens with approx neagtive1250mm focal length that I have on order. All the focal lengths need to be verified with more precision when I set it up on an optical bench of sorts.

The 200mm tube you so kindly gave me fits the 200mm parallax rings I already had so I am hoping to use that together with a (yet to be acquired) ~300mm aluminium tube grafted or welded on to the 200mm tube to support the 270mm lens in its nude /unmounted state. So that would be ~ 150mm infront of the existing 900mm tube. An of course then mount the large Moonlite refractor focuser on a yet to be machined aluminium disc / donut that could possibly fit inside the 200mm tube (~194mm ID).

OPTION 3...
There is another potentially more interesting way to use the large objective lens and that is for me to make a giant afocal (telecentric) lens. Like the type used to screw on to the front of camera lenses or video cameras to get a typical 1.4 to 2x magnification. With the focal lengths and optics that I have I could make an approximately 2.7 x afocal lens. Using the 270mm Plano Convex 840mm focal length lens and a 125mm Plano-Concave Negative305mm focal length lens that I have and spacing them at approximately 535mm apart so that both lenses have a sort of "common focus" I could generate an output beam 100mm in diameter, which if fed in to another 100mm refractor would increasse its aperture by 2.7 times whilst maintaining its f ratio. So a 100mm f/11 refractor I have would become a 270mm f/11 refractor. Resulting in an equivalent FOV to an ~3000mm focal length - kind of cool. Just don't ask me about chromatic abberation. It would be a trial and error alignment / collimation process. Given the focal lengths and spacings of the lens elements, this option would be ~500mm long and approx 300mm in diameter and sit in front (somehow) of an ~100mm or smaller telescope. The scope and the afocal lens would need to be mounted on a common rail/s to maintain good alignment/ collimation.

Quote:
Originally Posted by MattT View Post
I have an idea for mounting as I have thought on and off, mostly off, about a 9” Istar f9 achro or 10” f11r30, on the same 200mm tube which I have another piece just waiting to be used. Never could get the funds together to buy as Im waiting for the US$ to drop like it did years ago when 90oz cents bought the mighty us$…I digress.

Other question is how long in f ratio, does the 76mm lens make the scope?
It makes it an approx 1020mm focal length (from memory - recheck calcs?) in what I understsand to be known as a classical dialyte configuration so approximately f/3.8

Quote:
Originally Posted by MattT View Post
I got the mount for such a monster as you know.
Your mount is a Beautifully crafted BEAST for sure

Quote:
Originally Posted by MattT View Post
Would like to see the beast in real life if possible too?

Interesting project.
You're more than welcome to see it before / during/ after. Thx again for the 200mm tube. It certainly makes the job easier.

Best
JA

Last edited by JA; 05-05-2022 at 10:37 AM. Reason: typo
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  #23  
Old 04-05-2022, 04:41 PM
JA
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Join Date: Oct 2016
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Originally Posted by ChrisV View Post
That looks mega awesome. This will be interesting to follow!!
Good luck
Thanks Chris _ I'm looking forward to some progress soon, even if it's only acquiring another bit to use or optical bench testing.

Best
JA

Last edited by JA; 04-05-2022 at 04:57 PM.
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