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Update a Towa 339?

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#1 krehmkej

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Posted 07 April 2007 - 09:07 AM

I have here a TOWA 339 80mm f15 OTA. I am considering building a new 3.5" tube to eliminate the drawtube and allow use of a modern focuser. The lens is perfect, and the tube has the usual nicks for its age. Question is: 1)Is it a sin to tear this up and rebuild it, and 2) is the lens cell ealily removable from the tube? I have a lathe and can fabricate ant necessary adapter(s)

thanks.

#2 BHunt

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Posted 07 April 2007 - 09:27 AM

You can purchase adapters or make your own .965 to 1 1/4. I would just refurbish the tube.
Bill

#3 mikey cee

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Posted 07 April 2007 - 09:27 AM

James: Do what you feel you must do. First i would assess with a crude scaled ray trace if any vignetting was occurring. If this is a consideration for removing the drawtube. If not and it is just for practical purposes I'd have to ask myself what is to be gained...really? But by all means don't physically cut or drill holes if it can be avoided. And lastly put all original parts safely away for future use. I for one can tell you i wish i hadn't messed with the originality of my 2.4" Manon(repainting it, getting rid of the mount, finderscope and focuser lable). You really don't gain any better images this nice scope is what it is!! Good luck Mike

#4 krehmkej

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Posted 07 April 2007 - 10:58 AM

I'm conflicted for sure. I made a .965>1.25 adapter and used my 1.25 diagonal. Still seemed like looking through a pinhole. I cannot be THAT different than my ETX90 in view. Ray tracing is a good idea. I have just assumed that that little drawtube part was vignetting the image.

#5 trainsktg

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Posted 07 April 2007 - 11:22 AM

Any EP of the same design type, whether 1.25" or 0.965", in the usual 25mm or shorter focal lengths used with these scopes, will have the essentially the same FOV in your scope. I'd ray trace it first, as suggested. My guess is you will find no or little restriction to the light cone due to the draw tube. If it is restrictive, and if you do decide to rebuild the scope, then I also agree that you should keep all of the original parts for later resale, including the tube.

Keith

#6 BHunt

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Posted 07 April 2007 - 11:30 AM

If you are going to change the tube check this out! Seems these people can also cut and possibly thread unless you have your own cnc equipment.
Bill
http://tinyurl.com/3ybfbb

#7 Jae

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Posted 07 April 2007 - 01:39 PM

JWT,

I noticed that there was vignetting as well. There is a thread on it somewhere on this. I'll try to find it.

I basically knocked out the metal stops in the drawtube and lined the tube with flocking paper. Then I either knocked out a baffle or moved it closer to the drawtube. Can't recall.

Then the image got brighter than my 76mm Tasco whereas it was dimmer before. I feel that there was nothing cosmetic that was impacted and only by looking down the tube can you see a difference from original which I'd say was a oops in the design. The lens is razor sharp so the stops were not to enhance a marginal lens.

Jae

#8 Jae

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Posted 07 April 2007 - 01:44 PM

JWK,

Here it is:

hope it helps.



Jae Incredibly long link that makes people scroll left and right.

#9 Don W

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Posted 07 April 2007 - 01:56 PM

Gee whiz. I wish you guys would learn how to make a smaller link. There's a box in the edit page and one of the selections is URL. That allows you to put in a huge link without making everybody scroll left and right to read the thread. You can also use tinyurl.com to make a big link small.

Don Wyman
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#10 Jae

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Posted 07 April 2007 - 03:41 PM

Thanks Don, Sorry, I was wondering how to do that....
Next time ....

Jae


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