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Orange tube C8 rear cell threads, different sizes?

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40 replies to this topic

#26 Vitez

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Posted 12 February 2021 - 04:50 PM

Hi All,

 

I may have a definitive answer on this subject. I have a Classic Orange C8 from the early 70's as identified by CTS (Celestron Technical Support). I have encountered the same cell thread issue and contacted CTS. 

 

Hopefully, the following information may assist those individuals with older C8's to avoid frustration and possible thread damage if attempting to deepen the threads for a good fit due to the real root cause of this issue for the earliest C8 models. 

 

When CTS told me, in an earlier email, that the visual back threads have remained the same for 50 years, I challenged them due to the issue we have all encountered here. I even sent them a link to this CN thread for additional evidence. Upon further investigation, CTS replied with the following email this morning:

 

Good morning Bill,
Well, if Your two-inch diagonal will not properly fit the thread on the back end of the old C-8 you acquired, this allows me to further refine the date of manufacture. You have one of the very earliest Celestron 8s, I am envious of your find!
The thread pitch on the earliest C-8s manufactured from 1970 to 1973, and possibly 1974, was a little coarser than the now industry-standard 2-inch 24TPI "SCT" thread.
We no longer have the specs on this, but it was something like 2"-18 or 2"-20, so current accessories will cross-thread. A solution would be to have a machine shop measure the thread on the tail of the scope and create a short converter adapter from the old thread to 2"-24 in order to adapt current accessories.
I thought my own 1975 C-8 was one of the older ones in use, but you now have me beat by a year or more.
Good luck!

Celestron Technical Support

 

I did carefully measure my threads and they are indeed 2"-20. Personally, I will investigate having a short adapter ring made out of aluminum to allow me to use my 2-inch eyepiece adapter and other SCT optical accessories I already own for use with my 2 other and more modern SCTs. 

 

I trust this revelation helps anyone encountering this situation by providing a workable solution and avoid additional frustration.



#27 B 26354

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Posted 13 February 2021 - 12:50 AM

Well... in spite of what that tech says about when they think the thread pitch was changed, my C8 was manufactured in the second quarter of 1977 (I bought it new, that summer)... and I encountered the same cross-threading problem when I attempted to attach a 2" visual back to it, one year ago. I have no idea what year they actually changed the thread-pitch from 20TPI to 24TPI, but it was definitely sometime after 1977... not 1975.

 

I ended up following B l a k S t a r's instructions per his post #25 in this thread, and was successful, so I am now able to use my 2" eyepieces on my C8.

 

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#28 TSSClay

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Posted 14 February 2021 - 09:20 AM

Just as an experiment for those that have a question about the thread pitch, a common 1/4-20 or 5/16-24 screw can be used to test for thread pitch. Just place the edge of the screw on the VB threads and see how they match.

Clay
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#29 BillHarris

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Posted 20 April 2022 - 03:12 PM

Blakstar or Vitez:
(last couple of pages)
Now we make progress.
If the 2"-24 standard SCT thread is actually 2"-20 by Celestron's admission, then it stands to reason that Celestron needs to make a bunch of 2"-20 F to 2"-24 M)

#30 BillHarris

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Posted 20 April 2022 - 03:25 PM

Anyways
2"-20 (F) to 2"-24 (M) adapters, if not a set of nice SCT 2" Visual backs for us disadvantaged customers.
I just bought a '76 Sandcast beauty and thought I avoided the "late '70/ early '80s" bad era that I heard about.
Hey, there is a machine shop in Florida that does custom work, Precision Machine or something. What is the name of that place? My google-foo isn't working.

#31 BillHarris

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Posted 20 April 2022 - 08:09 PM

The shop is Precise Parts. This is what I've come up with: Build an adapter that is SCT(F) to SCT(M). The SCT(F) screws onto the odd mismatched threads of the scope, and the SCT(M) becomes the new 2"-24 anodized thread. On the designer, I spec'd C8 SCT threads on the left. On the right, I had to spec a device to fasten to the adapter, so I spec'd a Celestron OAG, which should screw into C8 SCT male threads. Before ordering the work, I'm going to double check with them. If indeed the mismatched threads are 2"-20, I may see if I can get that input thead cut at 20 instead of the SCT 24.

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#32 BillHarris

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Posted 21 April 2022 - 02:46 AM

In the interim:
Found my SAE thread pitch gauge, my 1976 Sandcast Orange Tube C8 is definitely 24tpi ln the rear cell.
Still digging for my micrometer...
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#33 davidmcgo

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Posted 21 April 2022 - 07:44 AM

Same here.  My 1975 and 1976 C8 and C5 are 24 tip.  But some of the older accessories like my C11 reducer plate had undercut the thread depth.

 

It is a tolerance issue or differences in the cutting tool they used to cut the threads on the lathe.  Those need resharpening at times and angles can change a bit, remounting in the tool holder can change position a thou or two.

 

Best wat to remedy is to chase the internal threads on accessories that don’t fit a bit.  Not sure if there are hand tools that could do that.  But just opening them up a couple of thousandths has done it for the newer item I had found that didn’t fit.

 

Dave


Edited by davidmcgo, 21 April 2022 - 07:44 AM.


#34 BillHarris

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Posted 21 April 2022 - 09:01 AM

Dave, it's definitely a conundrum.
Measured the thread OD on that tailpiece and got 1.996, which is a tad on the large side.
"By the book" things should work but they ain't, so we have to innovate.

#35 Chris MN

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Posted 21 April 2022 - 10:24 AM

I'll chime in and confirm these last posts.  I have a mid 70's C8 with those "odd" threads.  Holding up a 3/8"-24 tap to the rear cell threads, the threads match.  Holding up a 1/4"-20 tap to the rear cell threads, the threads DO NOT match.  

 

I have the original visual back which obviously works.  Recently I acquired an old off axis guider for my 80's C8 and much to my surprise, this also fit the mid 70's C8 just fine.  So it has to be in the cut of the threads versus the thread pitch like the other posts have said. 

 

So it's never easy.

 

Chris N

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#36 BillHarris

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Posted 21 April 2022 - 02:28 PM

Whether something fits or not is hit-or-miss.
From Precise Parts I'm having an SCT(F) to SCT(M) adapter fabricated. It will have proper CNC-cut 2"-24 threads on the male side and I'll fit the female side as I can.
This should solve this problem. Estimated delivery is 30 days, I'll report back later on.

--Bill

#37 apfever

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Posted 21 April 2022 - 08:09 PM

I owned the first C8 orange tube for retail, serial number 1.  Tom Johnson kept #1 and #2, eventually giving them away.  I have since sold #1 but I believe the threads were 2"-24.  That would have to be confirmed by the buyer. I know I played with various fittings and certainly would have with the buyer who drove several states to pick it up, but memory is constantly elusive for me, in and out on details.

 

Confirming the pitch (24 or 20) on the first few O.T. would be a nice thing to have for the registry as well as the inevitable ongoing confusion and discussion, that will go on in spite of any confirmation. 

Confirmation of the pitch on the Blue White's focuser would be a good thing. These may be what Celestron was referring to in post #26.  I'd take any current discussion from Celestron about vintage units with a grain of salt to be researched. 


Edited by apfever, 21 April 2022 - 08:17 PM.


#38 BillHarris

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Posted 22 April 2022 - 07:31 AM

Good points, APfever. On my cure for the thread-jamming problem on the late classics, I'm presuming that even my custom adaptor won't thread into the rear of the scope and I'll need to lap the scope threads in. If anyone has an old SCT-threaded ring that I could use for lapping, holler at me via private message; otherwise, I'll place an ad in Classified. --Bill

#39 rnshagam

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Posted 26 December 2024 - 11:07 PM

C8 1980 Hub - VB thread mod 

---SNIP---

I was concerned about the process but found the lapping was working right away after 60 seconds of work. Total lapping time was a mere 5 minutes.

So the old C8 threads with a little lapping work is now compatible with all of the standard post 1980 SCT threaded parts.

 

Referencing this old post #25 from BlakStar, I'm a bit confused.  I recently acquired a vintage orange tube C8, probably from the mid '70s. It appears to have a 24TPI thread, but my thread gauge probably isn't real accurate (from Harbor Freight).   I'd like to modify the flange thread to accept more modern accessories.  I have acquired a Blue Fireball adapter (#T-02) from Agena Astro. < https://agenaastro.c...table-t-02.html>?  Can I use that ring to lap the flange thread?  Will all of the modern SCT adapters then fit after lapping?  Will my original 1-1/4" visual eyepiece adapter I have still fit, too?  I'd hate to go through the process, only to have ruined the rear flange for all eternity!  

Thanks for any info/encouragement


Edited by rnshagam, 26 December 2024 - 11:14 PM.


#40 Astrophoche

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Posted 01 January 2025 - 04:33 PM

Wow. I knew I would find information concerning my current situation here on CN... 

 

I have a 3Q74 Orange C8 SN 322744 (motors 6/74). I have had absolutely no issues hooking anything up to my base until yesterday.  I've hooked up my optical back (original), a third party T48 to M42 camera back (purchased in 2021), an older Meade .63 reducer, a recent vintage 2" Celestron Diagonal from a 2" E-lux eyepiece and filter kit which all fit just fine.

 

Then I tried my brand spanking new Celestron #94175 Corrector/Reducer. I get about a third to a half a rotation before it stops. 

 

OK, so maybe some "lapping" might be what I need for this particular device but I'm a bit concerned that there might be something else wrong with this corrector. At first, before I did a controlled experiment to determine how far I could get this on, I figured it was secure in my blind attachment. I attached my new 2" diag and eyepiece and the thing never made focus. I don't know if I have two problems here with the device and I should return it or "lap away."



#41 Astrophoche

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Posted 30 January 2025 - 12:21 PM

Wow. I knew I would find information concerning my current situation here on CN... 

 

I have a 3Q74 Orange C8 SN 322744 (motors 6/74). I have had absolutely no issues hooking anything up to my base until yesterday.  I've hooked up my optical back (original), a third party T48 to M42 camera back (purchased in 2021), an older Meade .63 reducer, a recent vintage 2" Celestron Diagonal from a 2" E-lux eyepiece and filter kit which all fit just fine.

 

Then I tried my brand spanking new Celestron #94175 Corrector/Reducer. I get about a third to a half a rotation before it stops. 

 

OK, so maybe some "lapping" might be what I need for this particular device but I'm a bit concerned that there might be something else wrong with this corrector. At first, before I did a controlled experiment to determine how far I could get this on, I figured it was secure in my blind attachment. I attached my new 2" diag and eyepiece and the thing never made focus. I don't know if I have two problems here with the device and I should return it or "lap away."

So I borrowed a friend's Meade corrector. And my 2" optics do not focus with it, either. So the corrector seems to be fine on the other issue I thought I might have. Again, it's threads are just fine, but it's an older corrector.  Agena won't take my new Celestron reducer back without charges so I might as well try one of the lapping exercises to "grind 'em down and make them fit."

 

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