9 replies to this topic

[BillHarris]

In thought that by choosing the year of my Celestron C8 carefully and avoiding a scope made "in the late-1970s and early-1980s" I could avoid the thread mismatch ofvthr visual back and accessories with my 1974 C8. But nope, I'm stuck with that problem.
I have a GSO dual speed Crayford focuser with an SCT thread mount that will only thread onto the back of the C8 a fraction of a turn before it starts to jam. I've not pushed it.
I recall reading threads about that issue. But before wading into dozens of pages, I'll ask if anyone remembers what the solution to that problem was and which thread was useful.
Yes, the scope and visual back machine threads are good and thread smoothly.
Fooey.

--Bill

[CharLakeAstro]

Bill

The proper dimension is 2", 24 TPI Because the older scopes are just slightly larger than spec, but the thread pitch is correct, it can be corrected.

Several people have mitigated the issue by using abrasives to "wear the threads down slightly" to allow newer rear-cell thread accessories to fit.

 

An old thread documenting this is here...

https://www.cloudyni...t/#entry3206650

[Richard O'Neill]

https://www.cloudyni...ifferent-sizes/

[BillHarris]

Thanks, I recall those threads. This is a good start.
Now to get accustomed to the idea of lapping the parts together!!

Tip: Bon Ami cleanser is great for lapping aluminum. It is made from the mineral Feldspar which has a hardness of 5.5, slightly more than aluminum.

[BillHarris]

In an ideal world we'd be able to buy a steel thread-chasing die to clean these threads up. But in our DIY, ATM (Amateur Telescope Maker) world I'd find a used or inexpensive threaded ring, cut a split into it for suppleness, and start lapping with fine abrasive in oil.

--Bill

[apfever]

You are one of the few that thinks in terms of idealized reality. I would caution against too much draw there. The capability and the amount of dedication needed can sloww things down. It would be akin to cutting a nut in half to clamp on a bad stud and turn back off. If one did that it would be good to consider an off diameter cut to some chord line in the right direction, the left and right do make a difference. In the case of a bit of a need instead of desire this fine thread stud was one I had to do, I sure wouldn't for a telescope. It's been in the tray stuff for years since. The cuts are off diameter with one of those fine disk Dremmel type blades. It doesn't take long. About this long to make, and a few blades. 

 

Go minimal thread comfort then toss in a washer/spacer to tighten a T nut accessory. Think of it as an opportunity for a customized optical engineering baffle location. 

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[clamchip]

I would modify the accessories before I modify the port.

Lapping off the anodizing won't be a pretty sight and the lapping grit will be imbedded in the soft

aluminum and continue grinding the male port.

What I would do is use a stiff stainless wire wheel and polish out the female on the accessory or

accessories. Dremel size tools and do it dry, check progress occasionally until a nice fit. 

 

Robert

Edited by clamchip, 17 April 2022 - 10:18 AM.

[BillHarris]

Ap-, Clam-, this SCT thread problem has been around for a while and there are numerous solutions, some dreadful. Thanks for your input!
I suspect this may be a case of pre-CNC tolerances butting heads with CNC tolerances.
The part that has the fitting problem is a Crayford dual-speed focuser which I may or may not use. The Celestron 1-1/4" visual back which was supplied with the scope fits. This is a "sample of one" and I'm evaluating which way to go. I've got a Baader SCT-to-2" eyepiece visual back and some other SCT-to-whatever adapters on order and I'll see what fits and what don't.

2022 and we can't quit being an ATM.

--Bill

[deSitter]

I would modify the accessories before I modify the port.

Lapping off the anodizing won't be a pretty sight and the lapping grit will be imbedded in the soft

aluminum and continue grinding the male port.

What I would do is use a stiff stainless wire wheel and polish out the female on the accessory or

accessories. Dremel size tools and do it dry, check progress occasionally until a nice fit. 

 

Robert

Yes the best idea. I had a focuser that was supposed to fit in a particular tube, but didn't. It was very easy to hollow out the tube a bit and preserve the focuser intact.

 

-drl

[clamchip]

I've never run into this problem myself, but if I do and the male port is large enough so a

female ring is too tight even after polishing it's threads I'd be temped to split the ring with

a saw cut. Carefully clean up the cut so it doesn't scratch the male port anodizing and it

just might work.

 

Robert