Advice needed on modding an 80mm F5.9 scope

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DJD
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Advice needed on modding an 80mm F5.9 scope

Post by DJD » Wed Aug 27, 2014 5:49 am

Dear All,
I have a WO GT81 F5.9 scope that works well as a telephoto and with a front-mounted SM40 etalon.
I would like to exploit the full aperture though. I have a PST mod back-end that I use with my Tal 100
which also works well.
The easiest way ( and most expensive ) way forward is to get a larger front mounted etalon. Sadly,
secondhand units are hard to come by here in UK.
I have attempted to mock-up a mod via barlows and my PST mod back-end, but getting
focus is problematical and my atempts have failed spectacularly so far. I have seen the thread on glasspath correctors to give enough focus but I don't want to throw more money away chasing a potential solution if it is not possible for this scope ( without major surgery).
Has anyone modded a GT81 ?
Best wishes,
David
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swisswalter
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Re: Advice needed on modding an 80mm F5.9 scope

Post by swisswalter » Thu Aug 28, 2014 4:17 am

Hi David

good luck with the mod. May be a pic or two can help
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Re: Advice needed on modding an 80mm F5.9 scope

Post by marktownley » Thu Aug 28, 2014 8:10 am

I've modded a 70mm f6 scope, which is the same principle effectively, it works well.

There are a couple of things you need to do here:

First get a 40mm diameter -120mm focal length negative achromat, Edmunds optics sells these. Secondly get a cheapy 2" barlow lens, ditch the glassware from it and replace with the 40/-120, you will need to make a 20mm aperture for this which is easy enough to do in a piece of foil / card etc. This then acts as a collimating lens, and stopped down to 20mm aperture means it is running at f6 to match the f-ratio of your scope.

Then remove the collimating lens from the PST etalon assembly and keep safe somewhere as you will need to replace this if you want to run at f10 again with the Tal.

Then screw the barlow holder with EO collimator into the PST nosepiece. This extends the position of the collimator up the tube and you will be able to achieve focus.

Voila! Will work nicely :)
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Re: Advice needed on modding an 80mm F5.9 scope

Post by DJD » Thu Aug 28, 2014 9:11 am

I knew you would have done something like this Mark :D .
Couple of questions. When you say screw the barlow into the PST nosepiece, what do you mean.
I have the AOK PST etalon adapters with the 2" filter front threads. Is the lens prior to the etalon a
standard thread ? Second question, is there a specific distance for the negative achromat of this prescription to be placed prior to the etalon?
I was working towards leaving the PST F10 etalon assembly alone and trying to match the F6 cone of the objective to F10 cone via spacings with available two inch barlows/telextenders.
Thanks,
David
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Re: Advice needed on modding an 80mm F5.9 scope

Post by marktownley » Thu Aug 28, 2014 9:29 am

Yes, just screw onto the front of the AOK swiss nosepiece onto its 2" filter threads. There's no specific distance you need to be working to regards the negative achro infront of the etalon.
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