Options for infinite objective optical chain in SM1

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ray_parkhurst
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Options for infinite objective optical chain in SM1

Post by ray_parkhurst »

Having found that Mitty objectives fit in SM1 mechanics, and after finding an SM1 fine focus helicoid, I'm thinking of ways to implement an SM1 infinite optical chain. Unfortunately, I can't seem to find a 200mm 1" achromat to work with, but I do have a 100mm, and this brings up a few questions regarding infinite chain implementations...

There seem to be several variables:
- distance from objective to tube lens
- focal length of tube lens
- distance from tube lens to image plane
- other components (eg teleconverter) to affect image

I'm thinking of two possible implementations:

1... Objective -> helicoid -> 100mm achromat -> 100mm extension -> 2x teleconverter
2... Objective -> 100mm achromat -> 100mm extension -> 2x teleconverter

#1 uses the helicoid, or movement of entire system, to focus. Focusing changes the distance between objective and achromat, but small changes here should not have much effect.

#2 moves both the objective and achromat. They work in opposition, but I believe due to the magnification this would work effectively the same as #1

Is there an advantage / disadvantage to either of these configurations? Is there a better configuration?

What is the disadvantage to using a 100mm tube lens with a 2x teleconverter to magnify the image, compared with a 200mm tube lens? I like the idea of being able to remove the teleconverter to halve the magnification. Maybe use teleconverter for FF and remove it for APS-C.

So there are some thoughts. Would appreciate some insight into these configs and options.

Scarodactyl
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Re: Options for infinite objective optical chain in SM1

Post by Scarodactyl »

Edmund sells a 25mm diameter 200mm achromat. I have set one up to use as a tube lens on my stereo microscope and it works well in that context.
Both focus methods are used on microscopes, though moving the objective independent of the tube lens is I think a bit out of fashion. AO ised to do it but for currebt offerings I can only recall it as an option on stereo microscopes and macroscopes. Maybe on inverted scopes?

ray_parkhurst
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Re: Options for infinite objective optical chain in SM1

Post by ray_parkhurst »

Scarodactyl wrote:
Tue May 24, 2022 6:52 pm
Edmund sells a 25mm diameter 200mm achromat. I have set one up to use as a tube lens on my stereo microscope and it works well in that context.
Both focus methods are used on microscopes, though moving the objective independent of the tube lens is I think a bit out of fashion. AO ised to do it but for currebt offerings I can only recall it as an option on stereo microscopes and macroscopes. Maybe on inverted scopes?
I see two 25mm dia, 200mm fl achromats at Edmund:

https://www.edmundoptics.com/p/25mm-dia ... lens/2697/
https://www.edmundoptics.com/p/25mm-dia ... lens/7735/

Which one do you use?

blekenbleu
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Re: Options for infinite objective optical chain in SM1

Post by blekenbleu »

ray_parkhurst wrote:
Tue May 24, 2022 4:58 pm
1... Objective -> helicoid -> 100mm achromat -> 100mm extension -> 2x teleconverter
2... Objective -> 100mm achromat -> 100mm extension -> 2x teleconverter

#1 uses the helicoid, or movement of entire system, to focus. Focusing changes the distance between objective and achromat, but small changes here should not have much effect.
That is basically how AO infinity microscopes work, moving only objectives to focus, exploiting infinity space to hold oculars etc rigidly.
#2 moves both the objective and achromat. They work in opposition, but I believe due to the magnification this would work effectively the same as #1
That is basically how other microscopes move the stage to focus.

Instead of using achromats + extension + teleconverter,
consider an appropriate autofocus zoom telephoto camera lens.
With a camera body that supports focus bracketing,
this enables focus stacking with no additional rails, stepper motors and controllers.

In this configuration, Nikon's trinocular T head has a lens at the bottom
which converts finite objectives to infinity space around its binocular prisms.
Removing its trinocular chimney also removes what is effectively a tube lens,
making infinity rays available to the (EF-S 55-250mm f/4-5.6) telephoto focused near infinity:
Image
Metaphot, Optiphot 1, 66; AO 10, 120, and EPIStar 2571
https://blekenbleu.github.io/microscope

ray_parkhurst
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Re: Options for infinite objective optical chain in SM1

Post by ray_parkhurst »

blekenbleu wrote:
Wed May 25, 2022 3:19 am
Instead of using achromats + extension + teleconverter,
consider an appropriate autofocus zoom telephoto camera lens.
With a camera body that supports focus bracketing,
this enables focus stacking with no additional rails, stepper motors and controllers.

In this configuration, Nikon's trinocular T head has a lens at the bottom
which converts finite objectives to infinity space around its binocular prisms.
Removing its trinocular chimney also removes what is effectively a tube lens,
making infinity rays available to the (EF-S 55-250mm f/4-5.6) telephoto focused near infinity:
I've considered building such a system but have never moved that far toward infinity. I'm attempting to integrate infinite objectives with my finite system, using a bellows for focusing the tube lens, and stepper rail for moving the whole system. This new SM1 compatibility with M26 just brought up a few new options to do this work, so I thought I'd bring it up with the community. If I ever build a new, infinite system, I'd likely use some AF method as you've described.

Scarodactyl
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Re: Options for infinite objective optical chain in SM1

Post by Scarodactyl »

Focusing with the lens isn't really ideal from an optical perspective, since you are pushing your objective out of spec by asking it to focus away from infinity (or at a different distance in the case of the Nikon setup above). How much you can get away with (and how much throw you can get) will vary with what objective you're using.

I was wrong, the one I got was a thorlabs AC254-200-A. I missed out on an eBay listing for the EO equivalent.

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