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How to remove the primary mirror on a Criterion Dynascope 4?

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#1 ktran

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Posted 02 August 2018 - 11:21 AM

I recently picked up this Dynascope 4. The gentleman selling it said it belonged to his late father, who was an educator and rescued it from a school when it was marked for disposal.

 

43806416391_c5efeffbe2_z.jpg

 

The scope itself is in great shape. I cleaned the secondary, and aligned the primary with it (the secondary isn't adjustable), and quick lunar and planetary views seem pretty good in admittedly hazy summer nighttime conditions in the city. The tube is made of some sort of resin-impregnated cardboard, and the focuser thankfully takes 1.25" eyepieces. I have no idea how old it is. I was hoping it might have been one of the 50s/60s Bakelite tube models. The OTA has a bolt on it to attach to the older-style fixed-point mounts, so this might not have been the mount it originally came with.

 

42903106595_b5aec26635_c.jpg

 

It was probably stored without a dustcap for many years though, and while the primary looks pretty good, the tube innards are pretty dusty. If I eventually try to have a crack at removing the primary mirror to clean it and dust out the tube, how would you suggest I do it? There is no obvious mounting hardware for the mirror cell. My first instinct is to use a rubber strap wrench to try to twist it off the tube. Would this be a good idea? Thanks in advance!


Edited by ktran, 02 August 2018 - 11:22 AM.

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#2 highfnum

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Posted 02 August 2018 - 11:38 AM

i have 4 inch dynascope 

removing is pain in the you know where

 

i take flat head screwdriver and carefully 

push edge of metal down ward going around tube

little by little 

 

mine is pressure fitted


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#3 highfnum

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Posted 02 August 2018 - 11:41 AM

maybe your idea is better

but applying squeeze pressure may make it harder



#4 deepwoods1

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Posted 02 August 2018 - 01:40 PM

Had one, press fit. I twisted while pulling. I could see a little heat might help.


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#5 ktran

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Posted 02 August 2018 - 02:39 PM

Thanks for the suggestions/experiences! I'll try to hold off on removing/cleaning the cell as long as I can, but now at least I have an idea about how to go at it!



#6 deepwoods1

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Posted 02 August 2018 - 06:33 PM

If your goal is to display, leave it alone. Truth is, these are a pain to use. It seems like it’s in good shape. Be happy, don’t worry......



#7 highfnum

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Posted 02 August 2018 - 07:33 PM

you have a upscale version

mine is black tube

20180802_203542_resized_1.jpg


Edited by highfnum, 02 August 2018 - 07:45 PM.

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#8 ktran

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Posted 03 August 2018 - 12:48 PM

If your goal is to display, leave it alone. Truth is, these are a pain to use. It seems like it’s in good shape. Be happy, don’t worry......

Thanks. I will be leaving it alone for the time being, but I do also intend to take it out and use it. I live in a small apartment, in a big, big city, so it won't be a display piece, even though it's certainly in good enough shape to be one!

 

you have a upscale version

mine is black tube

attachicon.gif 20180802_203542_resized_1.jpg

But yours looks to be the legit Bakelite one, and I've a soft spot for things made of bakelite :)



#9 apfever

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Posted 04 August 2018 - 11:31 AM

I have a bit too much bakelite and fiberlite (Meade name, same stuff).
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#10 sdedalus83

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Posted 04 August 2018 - 12:27 PM

I've got a bakelite one: stupid, kneejerk purchase from a nice old man. Be glad you've got a usable mount and nice tube rings.


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#11 DAVIDG

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Posted 04 August 2018 - 12:40 PM

 I restored one a few years back. The way I got the mirror cell off without damaging the tube or the cell was to take a block of wood that I placed up against the tube and the edge of the cell. Then I lightly tapped on the end of the wood and move it around the edge of the cell. It only took a couple  of taps around the edge of the cell and it evenly came off.

   By the way as I discussed in my thread on mine, I discovered that the metal vane that the diagonal is mounted too was made wrong. The surface of the metal plate were the glass diagonal is glued to is were the front surface of the diagonal should be. So I had to cut it off and reposition it in the correct place. If not the reflective surface of the diagonal is too far forward and too close to the focuser. The result is that when you collimate the scope the primary is tilted and you get astigmatism in the image.

https://www.cloudyni...e/#entry4791093

                       - Dave 


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#12 ktran

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Posted 27 August 2018 - 01:43 PM

 I restored one a few years back. The way I got the mirror cell off without damaging the tube or the cell was to take a block of wood that I placed up against the tube and the edge of the cell. Then I lightly tapped on the end of the wood and move it around the edge of the cell. It only took a couple  of taps around the edge of the cell and it evenly came off.

   By the way as I discussed in my thread on mine, I discovered that the metal vane that the diagonal is mounted too was made wrong. The surface of the metal plate were the glass diagonal is glued to is were the front surface of the diagonal should be. So I had to cut it off and reposition it in the correct place. If not the reflective surface of the diagonal is too far forward and too close to the focuser. The result is that when you collimate the scope the primary is tilted and you get astigmatism in the image.

https://www.cloudyni...e/#entry4791093

                       - Dave 

 

Your previous 4" Dynascope posts and threads have been super-helpful while I was researching the scope! Your observations are noted, and if I ever come across a spare focuser assembly, I would give your corrections a go.

 

Otherwise, I did end up trying to use a rubber strap wrench to remove the mirror cell from the tube, and that was fairly effortless and straightforward. Unfortunately, it looks as though the mirror has already been removed and cleanings have been attempted, resulting in some fairly deep scratches. Otherwise, the coating is still good, and I carefully cleaned off the dirt and haze that has accumulated since then (some decades, perhaps!), and I'll just live with the existing scratches.

 

I've still not really been able to take it out to a dark sky site, but from my balcony in the middle of light-polluted Montreal, I can make out equatorial bands on Jupiter quite clearly, though the Cassini division on Saturn proves to be elusive. Mizar A & B were split very easily.

 

The scope came with what appears to be the standard set of 1.25" H 18mm, AR 9mm and AR 7mm eyepieces. I've read that they're not all that bad, really, but I find them to be not quite as nice as my budget Orion plossls.

 

I've got a bakelite one: stupid, kneejerk purchase from a nice old man. Be glad you've got a usable mount and nice tube rings.

The mount's not bad! I do wish it had slow-motion controls though. I might have a go at replacing the old friction washers with new teflon ones.



#13 Jim Curry

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Posted 28 August 2018 - 06:40 AM

This was my first scope. Got all my messier with it and some fainter NGCs too. I cut up and rigged a c clamp for manual slow motion on ra axis. Fun times with friends.
Jim
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#14 ktran

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Posted 10 September 2018 - 09:26 PM

I've spent a bit of time stripping the old grease from the finderscope and focuser, and repacking them with some new white lithium grease. Things seem to be moving much better now. The finderscope, especially, had its focus threads quite seized, and having cleaned and regreased them, it looks about a million times better now that I can actually focus it properly.

 

DavidG's posts and others on restoring 4" Dynascopes have been super-helpful! I am going to, as he suggested, replace the one nylon washer in the mount with two teflon ones, and, as another post suggests, use a small C-clamp to keep the latitude adjustment from slipping and hopefully dampen vibrations at that joint a bit.

 

Some things I'm considering next:

 

- Would even a cheap, modern 5x24 or similar finderscope be an improvement on the original finder? I measured the finder objective, and it's no larger than 18mm or so.

 

- To the surprise of probably no one at all, my scope also shows the "one-sided flare" that results from the badly-designed secondary stalk. I have a few ideas of my own about how to correct it, or I might just replace it entirely with a new spider/secondary, but ideally, I'd like to have a spare one to work on. Does anyone have any of the Deluxe Dynascope 1.25" focuser/secondary assemblies lying around in their junk drawers? :) Because...

 

In the end, it's still a fun scope to use. I had it out to a rural sky site in a dirt road in the middle of a farmer's field on the weekend, and despite its age and shortcomings, it still does pretty well overall with my budget Orion 20mm and 10mm plossls. Higher magnification shows image defects, but with these two eyepieces, I was able to bag the easiest Messier objects -- M31/M32/M110 and M45, in addition to M57 and M27, once the skies got dark and the clouds cleared out of the way.

 

41325578_10156465915470569_4532113391262


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#15 apfever

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Posted 11 September 2018 - 01:05 AM

goodjob.gif   Hey that is one great picture of scopes. Just a nice picture and an actual vintage classic in use.  I did some Astroscan tonight. I can handle that.


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