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Modifying a Towa 339

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#1 wrvond

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Posted 13 November 2019 - 10:51 AM

Towa 339 D=80 F=1200

 

Tenacious' drain pipe mod allows the use of 1.25" oculars and looks virtually indistinguishable from OEM.

IMG 0519

But now TAS has modified his 339 to use a GSO type 2" focuser by 3D printing an extension tube that permanently mounts (with screws) between the OTA and focuser.

https://www.cloudyni...sion-installed/

 

  I've got the bug to do essentially the same.

 

The OTA is approximately 30 inches long as measured with objective cell in place and the printed extension tube is 5.9 inches long, not counting the mating surfaces that fit inside the OTA and outside the focuser flange.

While pondering on where I might get such an extension tube printed, it occurred to me that it kind of makes sense to simply buy a 35.9 inch aluminum or steel tube, paint it and mount the objective and focuser and be done.

 

This morning I measured the I.D. of the OTA as 79.75mm (3.139") and O.D. as 82.0mm (3.228").

Looking at Hastings Irrigation, small miscellaneous tubing, they sell their tubes by O.D. and wall thickness.

The closest I can come to matching my existing tube is 3" O.D. x 0.065" wall thickness (vs. 3.139"x 0.089" wall thickness).

Is that going to be "close enough"?

 

Also, I'm going to need to get the objective end threaded. How to you guys get new OTA's threaded and is there an industry standard for thread pitch?

 

Thanks.

 

 



#2 Diego

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Posted 21 November 2019 - 06:14 AM

Hi, nice telescope.

In my opinion there is too much play between your OTA I.D. and the O.D. of the extension tube 3.139" vs 3.00". If you can't find a metric tube, then a sleeve can be machined to fit the OD of the extension.

Regarding threading, it has to be done on a lathe, there's no way around that. Thread and pitch can be anything you want as long as they match. Optics usually have fine threads.

Good luck with your project!
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