40 replies to this topic

[Joe O]

Did Celestron make changes to the thread pitch of the 2" rear cell on the C8 from the early 80's to today? While shopping for a new diagonal, I have read about problems with conflicting threads.

Joe O

[Littlegreenman]

Yes, but the change occurred in the early or mid 1970's. I'm 99% sure any C8 made in the 1980's would have no problem with a modern diagonal.

It has been discussed here on CN. Here is a link, and within that thread is another link with more info:

Thread on C8 visual back thread issue

My first scope was a mid 1970's C8 I acquired almost 20 years ago. A Tele Vue 2" visual back adapter would only thread a bit, and then it jammed. An early William Optics Crayford focuser worked just fine. Some early Celestron visual backs--I forget, maybe the difference was on the scope, has a gap to pop in un-threaded filters; the visual back held in on by pressure. On one orange C8 (I've gone through too many) a visual back from that came with one scope when used on another scope--the details are lost in the fog of time--threaded on just fine but not far enough. The result was the un-threaded part of the visual back was free to spin around.

I made it a policy of mine that any visual back that worked with any Celestron scope was mated to that scope, "till death do you part."

Littlegreenman

[Joe O]

Hey, thanks. Good read.
I did a search. I guess I didn't go back far enough.

Joe O

[wfj]

I have an off brand focal reducer field flattener that screws on my C-8 just fine, but doesn't a C5 a few years earlier. Yet I can exchange visual backs with no problem.

Go figure ...

[PiSigma]

Despite what is in those threads about this being an old wives tale I just ran into the problem with my 1976 vintage C8 and the Celestron 2" Giant Star Diagonal. I have two - one screws on fine and on the other the nut only turns about 1/2 turn and binds.

I don't know for sure if this is meaningful but I've noticed that the one that fits (on the left) has coarser serrations on the nut than the one on the right, which doesn't fit.

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[PiSigma]

And all of my other accessories, which do fit, have the same coarser serrations. The Multiple Ocular Holder serrations are a bit different than the rest in that they don't have the 'flats' between the serrations but are 'peaked'. They are still a coarse serration though.

If this is a true way to tell what fits and what doesn't then perhaps, when picking vintage accessories for these older C8s, you can compare serrations with a part that you know fits your model.

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[apfever]

Perhaps this will give a definitive solution to the issue of the Celestron SCT threads. The answer is yes and no!

I have an early 80's Orange C8 , a 90's black C8, and an early to mid 2000's grey tube C14. These are my three references for the male threads that came on each OTA.

I also have two celestron visual accessories. One multi occular turret from the early orange C8, and a 1.25" visual back from the 90's black tube C8. I have no original fitting from the C14 threads. These are my two references for the female threads on the accessories.

I measured all of the treads, male and female, with an excellent caliper. I measured all the threads in 3 directions of rotation EACH. I also tried each of the two accessory on each of the 3 scopes for a total of 6 fittings.

The male threads on the OTAs get smaller as they get newer.
Orange C8 threads OD = 1.992"
Black C8 threads OD = 1.990"
C14 threads = 1.987"

The threads on the fittings get smaller as they get newer.
Orange C8 turret = 1.974"
Black C8 visual = 1.959"
The difference on the ID of these two female threads is .015" or almost 1/64". Also the visual back for the black C8 had the most variance in out of roundness by .006" and you'd never know it in use.

Both of the fittings will fit on each telescope. The smallest fitting (black C8 visual back)) fits on the largest OTA (orange C8) fine without binding. It is a very solid fit.
The largest fitting (C8 turret) fits on the smallest OTA threads (C14)and holds. The threads lock well before engaing a full thread cut with less than one full turn, and will not pull off. This is still noticably loose after fully threading just up to the flange stop. There is no pitch issue and the turret holds securely to the C14 without concern though the fitting ring is sloppy on the C14.

Even though they are given the same thread designation of 2"-24, the earlier ones are larger by true specs and the newer ones are smaller. Think about it, this explains why every one only has issues with the newer fittings on the older scopes and never the other way around. This also explains why the grinding compound technique works since it is only a matter of thousandths of an inch on the same thread designations.

If I had a very new fitting from the C14 that is any smaller that the black tube visual back, it might have difficulty with the orance C8. If new OTA's get any smaller on the male threads, I might doubt the security of a larger older fitting ring.

[PiSigma]

Great research Neil. I suspected this was an issue with the maximum and minimum diameters of the threads.

[wfj]

Seconded. The focal reducer I mentioned was new, and the C5 I mentioned was badged "Celestron Pacific", thus older than my C8.

Thank you Neil.

[apfever]

Added confirmation.

I've acquired a lot of C8 fittings, Have a 70's vintage C8 now (permanent keeper), and just sold my newer C14 OTA but did use it for checking.

As far as I can tell, the thread designations are the SAME. Same specs! Perhaps the earliest versions of the C8 did have a different spec, I don't know. I have consistantly run into the same situation, with various combinations, where the older fittings (female)are loose on the newer threads (male), and the newer fittings are tighter on the older threads. My current accessories are vintage and are notoriously looser on the newer fittings. All the newer fittings did fit on the older OTA's, however they were snugger and I did have a new 1.25" visual back that was pretty darn tight on my earlier velvet tone C8. I sold that visual back with a slightly newer orange peel C8.

I believe that the specs have been the same, but the newer ones have been on the small side of specs and the older ones on the larger side of specs. I have NO doubts the threads are the same specs, except for possibly the very earliest issue sand cast or blue/white which I've never owned.

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[orion61]

I like the 70's flashlight

[JamesL]

I found that the threads on my old orange c8 were not deep enough. Modern accessories only screw on a few threads and stop.
Otherwise the thread pitch is the same as modern accessories.
Found an old post here for the solution and it worked perfectly.
Get some grinding compound (at the auto parts store) and apply to threads and work the piece back and fourth until it screws on all the way.
Of coarse you should plug the hole so none gets into the scope and clean it from the threads when done. This takes about 15 minutes.
The only difference will be is that the threads are now shiny silver looking instead if black.
Now everything will fit just fine as you have made the threads deeper.

[Littlegreenman]

I think this thread gives adequate answers to the questions. This issue has been brought up here and elsewhere; a Google for Celestron-visual-back- pitch-cloudynights, or on The Yahoo SCT forum (if Yahoo forums still exist) will eventually get you even more information.
I had an Orange tube, most modern accessories and modern visual backs did not thread in all the way. The newer visual backs would thread in enough to stay on, but not enough to press the tube against the back end of the scope. So the diagonal in the tube could just spin around.
One exception was an early Crayford SCT visual back, branded William Optics, but it was sold under another brand. It worked fine.

LGM

[bremms]

Found the same issue with my 74 sandcast. 2" GSO diagonal fit, but later C8 back was super snug.

[Bill Griffith]

Great thread

Neil's measurements do confirm the intention of same size not the same tolerance. The tighter the tolerance the more expensive the part. I would guess over the years many different vendors were in play. Different anodizing techniques probably assisted in interference fitting.

As James mentioned a lapping compound on the tight can usually ones will solve the interference.

Clover Compound can be purchased various fine grit sizes and works quite well.

Side note, lapping in RA gears and declination sets is also a plus with the Clover. They will feel like butter and accomplish minimal backlash

Bill

[TCW]

I suspect the real issue is thread pitch. The binding describes by other posters is almost certainly caused by different thread pitch.

[BVH]

An old thread revival.  I bought the Celestron Focal Reducer for my 1976 or maybe a year earlier C-8 and it will not thread on beyond about 30 degrees without abruptly stopping.  I read the posts above about the slight O.D. differences but with the 2" x 24 TPI being the same - which I concur with as measured on my scope.  I'm a hobby machinist and have a PM 12-36 metal lathe in my garage and know threads fairly well.  The adapter stopped so abruptly and would go no further with reasonable pressure so I'm not going to play with it.  Also, my original 1.25" star diagonal would not go in as far as it needs to.  It contacts the F.R. lens about 3/8" from bottom.  Oh well.

[apfever]

An old thread revival.  I bought the Celestron Focal Reducer for my 1976 or maybe a year earlier C-8 and it will not thread on beyond about 30 degrees without abruptly stopping.  I read the posts above about the slight O.D. differences but with the 2" x 24 TPI being the same - which I concur with as measured on my scope.  I'm a hobby machinist and have a PM 12-36 metal lathe in my garage and know threads fairly well.  The adapter stopped so abruptly and would go no further with reasonable pressure so I'm not going to play with it.  Also, my original 1.25" star diagonal would not go in as far as it needs to.  It contacts the F.R. lens about 3/8" from bottom.  Oh well.

I suspect the real issue is thread pitch. The binding describes by other posters is almost certainly caused by different thread pitch.

The issue is not a thread pitch. They have the same pitch spec as measured. The diameter of the threads is an issue as measured. ALSO never mentioned, the percentage of cut could be an issue as well but not as likely. If the male scope threads are close to 100% cut and the female fitting is at 75% cut, then there could be an issue. I haven't examined the threads with a magnifyer to check the amount of flat on the threads or gullies. This all reference later sand cast on up. I've never had a tri-color or blue-white to see if there are actual thread spec differences in diameter or pitch.

Edited by apfever, 16 November 2017 - 09:58 AM.

[rolo]

The tricolors have a different thread pitch and the same issue as the scope mentioned above.

[B l a k S t a r]

Linking my 2019 questions and measurements to old threads I'm finding now.

https://www.cloudyni...spec/?p=9815479

 

So far the evidence and measurements point strongly to a standard 2-24" thread but a Class 3A I wager as its higher tolerance. The newer units (post 1980 far as I've found) list as 2-24" on Celestron web site support FAQ and they state the older units were not the same. 

 

Celestron FAQ:
https://www.celestro...496fcd7ed&_ss=r

 

I imagine the newer size is a 2-24" class 2A which has a looser tolerance range which has an interference range condition when interfaced with the 3A fit class. Now, I'm inferring here as these are not UN series sizes but there's no reason for the size in question to deviate from standard mechanical thread size and tolerance designations. 

 

I'll report back with my results after I've lapped the old thread.

Edited by B l a k S t a r, 18 December 2019 - 01:40 PM.

[apfever]

Hi Brian,

 

I've been stating for years that the early and later threads are the same., but on opposite sides of specs. Your comment is the first I've heard of 3A and 2A classifications. I don't know about tolerances, but the early threads are BIGGER and the later threads are _smaller.  Mismatch issues always go the same way. Old female threads on new male are sloppy and new female threads on old male are tight, all pointing to older threads being bigger or possibly having a different percentage cut. I've compared and swapped more fittings on more orange tubes than I'll ever remember or care to bother doing again for test. 

[John Higbee]

Despite what is in those threads about this being an old wives tale I just ran into the problem with my 1976 vintage C8 and the Celestron 2" Giant Star Diagonal. I have two - one screws on fine and on the other the nut only turns about 1/2 turn and binds.

I don't know for sure if this is meaningful but I've noticed that the one that fits (on the left) has coarser serrations on the nut than the one on the right, which doesn't fit.

Ditto for my 1976 C8...the 1.25" Celestron eyepiece turret starts to thread on, then binds.  John 

[B l a k S t a r]

Hi Brian,

 

I've been stating for years that the early and later threads are the same., but on opposite sides of specs. Your comment is the first I've heard of 3A and 2A classifications. I don't know about tolerances, but the early threads are BIGGER and the later threads are _smaller.  Mismatch issues always go the same way. Old female threads on new male are sloppy and new female threads on old male are tight, all pointing to older threads being bigger or possibly having a different percentage cut. I've compared and swapped more fittings on more orange tubes than I'll ever remember or care to bother doing again for test. 

I had a bit of a time trying to sort this out and I think your on the mark. Unfortunately I can't seem to find any specs on line for the thread series - or indeed what thread series it actually is. I was a mechanical designer for 30+ years so I've specified quite a few threads over the years but mainly UN (Unified National), ISO and some SAE and pipe threads etc.  That's where my knowledge of thread fit class tolerances comes from. Classes 1,2&3 are successively higher tolerance A is external, B is internal thread. Hence 10-24 UNF-3B is a Unified National Fine #10 24 threads per inch class 3 toleranced internal (B) bore thread spec.  Camera and telescope threads are a bit outside my experience and familiarity but I've seen some with optical and laser table rigs, usually ISO and German gear.

 

So my take is this: company manufactures product with good quality and competition and other forces drive prices down and compromises are made. So the more expensive thread going to a cheaper to produce class thread is one of many ways things go. Said companies' manufacturing specs are a guarded secret and not up for disclosure. We all know the rest. 

 

If you research this you will see solutions presented by many folks that state "use this product it worked for me". That's fine to a point but not always true for every case.  I'm left frustrated because if I knew what the darned thing actually is then I can deal with it with much greater confidence of success. 

 

Just the the way it is in competition land. I'll get around to picking up some lapping compound in the near future. Thanks for chiming in - I appreciate it!

 

 

Edited by B l a k S t a r, 19 December 2019 - 02:39 PM.

[apfever]

Try lapping lightly and examining the results on both threads frequently. See if you can determine where the newer female threads and older male threads are taking the lap. Most of these fittings are anodized black so that should make it easy to see where the lapping is taking place. The grey on the scope may not be so forgiving but careful examination should reveal where the male threads are taking the lap.  

I think this information will go a long way in determining what happened in the thread change. 

A finer lapping compound might help to start, or use a finer compound after getting a few threads going.

[B l a k S t a r]

C8 1980 Hub - VB thread mod

 

Contacted Celestron by email and the tech department confirmed the old visual back threads on the C8 are 2-22.

Maximum engagement was only 1-2 threads, not providing a proper seat for the 2019 VB. Note the thread wear below.

 

 

I got some silicon carbide 90 grit lapping grains and mixed it with a dab of bicycle grease for the slurry.

I applied the slurry to the external thread and began to slowly turn by hand using a 2019 2-22 collar from a 1-1/4" VB. Note the collars pristine thread.

 

 

I could feel immediate progress but took my time working the collar on the hub by hand and then gently gripping with channel lock pliers.  That worked very well with 1/16 turn advancement on every 90° ccw to cw lapping stroke.

 

 

Lapping slurry below.

 

 

Wiped the threads down and cleaned with alcohol and cotton. You can see all of the anodizing from both parts was removed over the entire thread form.

 

 

 

The 1-1/4" VB easily engaged fully to allow the flange face to firmly seat. There is no noticeable play between the threaded hub & collar.

 

 

My 2" VB adapter for my TeleVue diagonal only allowed 1/8 of a turn and now engages 3.5 turns and is a slightly tight fit. This shows the C8 boss is now engaging with a new off the shelf standard part. I may lap the boss with the collar one more time with 180 grit to improve the 2" adapter fit.

 

 

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.

 

Edited by B l a k S t a r, 20 January 2020 - 05:16 PM.