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Criterion 4" Dynascope

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#1 John Rose

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Posted 17 July 2020 - 06:12 PM

I aquired one of these some time ago. Nice condition for it's age. Except it is missing the secondary. For those who have never seen one of these scopes, The secondary holder is single stalk that is part of the focuser body. A one piece casting if you will. The rectangular secondary having been merely glued to the stalk. After half a century or so the glue failed. Anyone know the proper dimensions and possible source of a replacement? 

 

John Rose



#2 Chuck Hards

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Posted 17 July 2020 - 06:28 PM

The secondary is not in the proper postion.  There is a thread detailing a fix.  You need to lengthen the stalk support, it was designed without considering the secondary thickness.  As a result, the telescope can never be collimated.

 

Search for the thread, someone else may post it, I don't have the time to search.  I own one of these on the equatorial mount,  The pintle mount is junk.  The primaries are not bad (have two).  

 

If you don't care about collimation, just order any elliptical diagonal of similar minor axis.  The thinner, the better.


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#3 Terra Nova

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Posted 17 July 2020 - 06:30 PM

The secondary is not in the proper postion.  There is a thread detailing a fix.  You need to lengthen the stalk support, it was designed without considering the secondary thickness.  As a result, the telescope can never be collimated.

 

Search for the thread, someone else may post it, I don't have the time to search.  I own one of these on the equatorial mount,  The pintle mount is junk.  The primaries are not bad (have two).  

 

If you don't care about collimation, just order any elliptical diagonal of similar minor axis.  The thinner, the better.

This was apparently the product of the same designer that went on to give the world the mighty DX8! lol.gif


Edited by Terra Nova, 18 July 2020 - 04:23 PM.

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#4 Chuck Hards

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Posted 17 July 2020 - 06:33 PM

I just checked my stock secondary, the minor axis is 13/16" precisely.  1/4" thick!

 

Odd size, you will probably be fine with a few mm smaller or larger, either way.  But try for 1/8" or thinner glass.  It won't be a problem with a mirror so small. 



#5 DAVIDG

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Posted 17 July 2020 - 06:34 PM

 John,

     I have restored two of these scopes  

 

https://www.cloudyni...rion-dynascope/

 

https://www.cloudyni...on-rv-4-deluxe/

 

 

    There is  a major manufacturers defect in that  the single stalk that holds the diagonal it  is too short. It places the surface of the original  diagonal too close to the focuser and too far forward. The result is that when you align the optics so they look correct they are not and the primary will be tilted in respect to the optical axis. The result is a fair amount of astigmatism which  ruins the great optical performance that these scopes can have.

    You want to use an elliptical  diagonal that has 1" minor axis  which is better sized than the original rectangular one but it will be thicker then the original so you need to modify the mounting plate on the stalk to get it positioned correctly.

 

             - Dave   


Edited by DAVIDG, 18 July 2020 - 10:03 AM.

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#6 Chuck Hards

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Posted 17 July 2020 - 06:35 PM

Thanks Dave, I hoped you would check-in.  waytogo.gif



#7 Terra Nova

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Posted 18 July 2020 - 09:41 AM

Another reason to miss good old Anchor Optics (the surplus subsidiary of Edmund Optics). I bet they would have had just what you need.

 

:bawling:  



#8 DAVIDG

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Posted 18 July 2020 - 10:02 AM

 Here is one that looks like it is the right size but again you need  to modify the stalk to get the reflective surface in the correct position. 

 

          https://www.ebay.com...l0AAOSwngJd8r7o

 

               - Dave 



#9 John Rose

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Posted 18 July 2020 - 03:00 PM

Thanks for eveyones input. And there I had been merely worrying about getting a new secondary accurately glued into place!  I will watch out for a suitable replacement. Maybe I just need to dig around here harder.  After nearly 30 years of Stellafane swap tables I might even have some thing!    

 

John


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#10 ktran

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Posted 29 July 2020 - 09:46 PM

I went a slightly different route with my Dynascope deluxe. I asked very nicely, and got to rummage through the local astronomy shop's "junk pile" (workshop area), and picked up a bunch: small secondary, inexpensive focuser, some springs, and spider vanes, and proceeded to make my own secondary assembly, with a single extra hole drilled into the scope itself (nothing that can't be hidden by a bit of JB Weld, which is coincidentally an excellent colour match!

 

IMG 5120

 

I couldn't get a single stalk rigid enough to avoid flex, so I fashioned a second one out of epoxy putty and fixed it to some removable tape I lined inside the tube.

 

IMG 5505

 

The result: crisp images with excellent collimation, though the cooldown time on the long narrow tube can be measured with a calendar.

 

dynascope

 

I wanted to preserve my original focuser and secondary stalk, but if anyone has a spare focuser around, I'd love to try DavidG's fix!




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