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Puzzle about an old eye piece holder


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#1 WyoJoe

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Posted 19 July 2014 - 10:32 AM

I purchased a Meade SCT model 2120 around 1983.

It has not been used much.

I can't recall if this was always the case but I would suspect that I would have noticed this when I first purchased the scope.

I did use it a lot in the first years.

The inside threads on the eye piece holder used to connect to the back plate of the scope are just a bit too large

During a recent star party I realized the holder was hanging on the thread not engaging as threads should and never tightening

Some duct tape wrapped around the external threads of the back plate saved the night 

I also noticed that the inner flange was too small to bottom out on the back plate so even it the threads did engage the inner barrel of the holder would never be tight.

 

I purchased a new holder and it works like a charm but I am baffled as how this can be. I am not looking for a refund the part was very reasonably priced I just cant figure out how this can be. I made a padded case for my lens' and various items and the holder has a spot.

I thought that I may have mixed up the holder with a camera adapter but can't find any thing else that would fit the foam cut out as does the old or new holder. And the old holder will not attach to my old 35mm of a similar vintage.

 

Not a very astronomical topic but if anyone else ran into the issue I would love to hear about it.

Joe

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#2 MistrBadgr

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Posted 19 July 2014 - 11:44 AM

Joe,

 

Just ideas:

 

Is there any way somebody could have switched with you or you switched with someone else and did not know it?

 

I am a professional mechanical engineer by training, though I do mostly water quality and combustion work now.  I am not a metals expert by any means, but have run into some odd things over the last 45 years in the power industry.  There is a mechanism for the softer metals where they can slowly relax with a continuous stress over time and gradually deform.  Some types of brass are good at that.  Many steam condensers have admiralty brass tubes with a different tube sheet material.  The ends of the tubes are rolled to press the metal against the holes in the tube sheets to make them tight.  Over the years, the tube material will gradually relax and cause a leak to start.  The tubing will then be re-rolled to make them tight again.

 

I do not know about aluminum formulas.  If the back was left on the scope, very tight, and was subjected to extremes in temperature where the rear of the scope and the visual back expanded or contracted at different rates, I could see a situation where the ring of the back being stretched if it was held in tension for a long time.  I have no explanation for the back getting longer.

 

I think you would have noticed the loose fitting situation when you first got the scope if the back was in that condition at that time.

 

Possibly one of the long time SCT observers can shed some light on this situation.

 

Bill Steen


Bill Steen, Sky Hunters' Haven Observatory, Broken Arrow, Oklahoma

#3 WyoJoe

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Posted 20 July 2014 - 07:04 AM

Bill

 

I am also an ME PE. If I had to pick a reason I would go with the simplest explanation - the parts getting switched is the most likely. But I can't imagine how or when. And that the switched part looks identical to the photo in the original Meade manual. The construction of the original & new part are similar but not identical. Since one was made in Japan and the new part in USA 30 years apart that is not too much of surprise. Since the ID of the female thread just slips by the OD of the male thread (scope) I wonder if the female was cut oversized and what little wear there is in the black anodized coating was all that was holding it. The holder was never left on the scope other than when in use. But I will keep what you mentioned about brass in mind.  

 

Thanks for taking the time to reply.

 

Joe



#4 MistrBadgr

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Posted 20 July 2014 - 01:28 PM

Hi Joe,

 

What you have run into is definitely a puzzler.  As you said, possibly a small amount of wear was enough.  But, I would think you would have noticed it when you were using the scope before, and not all of a sudden now.  If you ever do come to a firm conclusion, I would be interested in hearing what it is, just out of curiosity.  The situation is weird.

 

Best Regards,

 

Bill Steen


Bill Steen, Sky Hunters' Haven Observatory, Broken Arrow, Oklahoma




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