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New Celestron Reducer not fitting on 70's C8

Accessories Astrophotography Celestron Classic Equipment SCT
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#1 Astrophoche

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Posted 30 January 2025 - 12:20 PM

From another thread on threads. 

 

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."

 

Update: 

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, the Meade's threads are just fine, but it's an older corrector.  Agena won't take my new Celestron reducer back without charges and any other new one would have the issue, so I might as well try one of the lapping exercises to "grind 'em down and make them fit." 

 

But before I do.... Anyone else done this before? With only the telescope and the reducer to do the lapping? I'm sure, but my ability to do digital searches is sadly lacking. Thanks in advance!



#2 photoracer18

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Posted 30 January 2025 - 02:35 PM

There was a story that early C8 did not have the universal Celestron SCT thread that later Celestrons and Meades  did but I have never seen one like that and my first one was a 1976 model. So I put the story as an urban legend. I know Russian Maks had a different thread before switching to the Celestron SCT one. And of course Criterion stayed with a non-Celestron thread for the Dynamax SCTs. A third of the way on usually means the thread pitch is off. Almost exactly what you get when you try to put something like a T-ring thread on a Pentax/Praktica M42 screwmount because one is 42mm x 1mm pitch and the other is 42mm x 0.75mm pitch.



#3 rnshagam

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Posted 30 January 2025 - 03:29 PM

To follow up on my previous posting <https://www.cloudyni...zes/?p=13873744>  I did go ahead and lap the rear flange threads of my vintage Orange Tube C8 against the Agena Blue Fireball adapter (#T-02) using some fine carborundum mixed in with some Super Lube grease.  I was able to get the adapter to screw all the way on.  And the visual back that came with the OTA still fits on the rear flange threads.  Of course, I don't know if a fresh adapter will  fit without further lapping since the threads on the adapter also got lapped. 

 

By the way, the original poster said it took him about five minutes of lapping.  I spent about an hour of cautious work and earned a blister on my thumb! Your mileage may vary.  




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