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Dobsonian mirror removal mystery

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#1 JJDreese

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Posted 12 April 2024 - 09:21 PM

I'm trying to fix a broken mirror float system under a 14.5" mirror on a Sky Designs truss Dobsonian.

 

I need to remove the mirror to get to it.  In theory, I could lift the mirror up and out, but the hole in the top is just slightly larger than the mirror diameter (about 0.25 inches).

 

It seems more logical to remove the top shell of the box, so I removed the lower ring of screws, but that bottom platform won't fall out (gently of course).

 

I have to be careful because I don't know what is poking the back of the mirror.

 

I've tried to gently nudge it with a rubber mallet. Pry it with screw driver.  I don't see any more screws.

 

Note: This doesn't have the sling mount, but has four nylon bolts that center it and four wood hold-down tabs.

 

On the left side (the side with a trunion) is a stack of what looks like linoleum pieces. They are about 0.5 inches away from the mirror.

 

Does anybody know or remember what the standard method was for removing the mirror?  I'd like to avoid laying the box on the side and manhandling it out.

 

Note: when I get it out, I'd like to add hinges and a couple of lock bolts.

 

John

 

Attached Thumbnails

  • BigDob.jpg
  • BoxBottom.jpg
  • NoScrews.jpg


#2 Daveatvt01

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Posted 12 April 2024 - 09:44 PM

I have a 17.5” with a sling. The mirror comes out of the top hole at least once a month, sometimes more. Just slip your hands underneath the mirror to support it from the bottom, tilt it, and carefully lift it out. (After you twist the clips out of the way, of course)


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#3 Asbytec

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Posted 12 April 2024 - 10:01 PM

I had a Sky Designs 18", and the mirror lifted nicely out of the box. Never had a problem. I am sure that is true in your case, as well. 


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#4 Jon Isaacs

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Posted 12 April 2024 - 10:32 PM

I have a 17.5” with a sling. The mirror comes out of the top hole at least once a month, sometimes more. Just slip your hands underneath the mirror to support it from the bottom, tilt it, and carefully lift it out. (After you twist the clips out of the way, of course)

 

:waytogo:

 

This is how I do it.

 

Jon


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#5 JJDreese

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Posted 12 April 2024 - 11:11 PM

Thank you guys!  Also, many thanks to Shawn Simonson.  The secret was to lift and tilt (slightly) to make room for my fat fingers.

 

I pulled it out, revealing the black pleather circle that the floats are stapled too. Gotta admit, it's a clever way to keep them from spinning or moving.

 

I'm not a fan of the button-top dowels that support the mirror - those seem like fairly sharp points of support.  I'm going to replace it with thick felt circles (my 12 inch Dob has that).

 

Any reason to use pointy dowels instead of felt circles?

 

The collimation screws are pushing on embedded metal disks - they look like electric box knockouts.  Was that the common interface surface?

 

John

Attached Thumbnails

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#6 JJDreese

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Posted 12 April 2024 - 11:20 PM

Planned upgrades:

1) Replace the button-top dowel supports with rounded felt pads

2) Paint the four hold down clips and their support towers black.

3) Leave box unpainted.

4) Replace strut tube hex nuts with wingnuts.

5) Replace secondary screws with 8 knurled thumb screws.

 

Should I do anything to the float plate metal circles?  Perhaps put a "dent" in the place where the collimation screws are hitting?

 

Thoughts?

 

John



#7 Asbytec

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Posted 13 April 2024 - 03:30 AM

 

Thoughts?

 

John

Add a fan large enough to fit easily between the flotation pads. And/or a side blowing fan. Whichever design you prefer. 

 

I dunno, I might consider painting the interior of the mirror box black. Up to you. 


Edited by Asbytec, 13 April 2024 - 03:35 AM.

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#8 Daveatvt01

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Posted 13 April 2024 - 07:10 PM

Felt pads are less ideal than some other materials- they have a tendency to not be as slippery as needed. I added a center fan underneath to mine as well as side blowing fans. We have major temperature swings here in AZ. Questions about improving/modifying the mirror cell could be asked too in the ATM forum if you don’t get the replies you need here.
Some mirror support info here: http://www.loptics.c...rorsupport.html
​Here’s a thread on my 17.5” if it’s helpful in any way (it’s not quite up to date): https://www.cloudyni...ight-questions/


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#9 JJDreese

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Posted 13 April 2024 - 10:45 PM

Felt pads are less ideal than some other materials- they have a tendency to not be as slippery as needed. I added a center fan underneath to mine as well as side blowing fans. We have major temperature swings here in AZ. Questions about improving/modifying the mirror cell could be asked too in the ATM forum if you don’t get the replies you need here.
Some mirror support info here: http://www.loptics.c...rorsupport.html
​Here’s a thread on my 17.5” if it’s helpful in any way (it’s not quite up to date): https://www.cloudyni...ight-questions/

Thank you!  The Sky Design info here (as I'm finding out) is very detailed and helpful.

 

John



#10 Asbytec

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Posted 13 April 2024 - 11:21 PM

Thank you!  The Sky Design info here (as I'm finding out) is very detailed and helpful.

 

John

I agree with Dave, too. I've read felt can be sticky and not allow the mirror to settle and relax. Some argue a tiny bit of resistance or tension on the back of the mirror can actually deform the mirror's surface. Especially large thin mirrors. Try some Teflon pads. The wood buttons probably work fine, too, despite any aversion we might have for pointy things poking the back of our mirror. 


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#11 JJDreese

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Posted 14 April 2024 - 11:00 AM

I guess I mis-spoke with the usage of felt.  It's more of a hard felt (probably nylon) about 1/8 inch thick, often used as a bumper for furniture.  It's much slipperier than typical felt.

 

John



#12 JJDreese

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Posted 14 April 2024 - 03:14 PM

BIGGEST MYSTERY:  What are the three white strips on the left?  As far as I can tell, they are thick white linoleum, stapled to the left wall.  NOTE: They are not on the bottom, but the left side (the wall that hosts a trunion).

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  • MirrorBox.jpg


#13 Daveatvt01

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Posted 14 April 2024 - 05:35 PM

As long as you are replacing parts, why not use hard plastic like in the linked article from earlier? If you do go with felt pads and the mirror sticks a little on the pads, a gentle shake of the scope or light kick will free it again. Sometimes I do that if I see some astigmatism. Mine came with felt pads and next time I have it apart I plan on replacing them with plastic.
On the subject of astigmatism, the original (I think) secondary on my scope had a lot of astigmatism, likely from being siliconed to the wooden stalk. A replacement secondary fixed that. 
Does your scope star test ok?

Also what was the original problem you were trying to fix?

I have no guesses for the linoleum.


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#14 JJDreese

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Posted 15 April 2024 - 11:24 PM

As long as you are replacing parts, why not use hard plastic like in the linked article from earlier? If you do go with felt pads and the mirror sticks a little on the pads, a gentle shake of the scope or light kick will free it again. Sometimes I do that if I see some astigmatism. Mine came with felt pads and next time I have it apart I plan on replacing them with plastic.
On the subject of astigmatism, the original (I think) secondary on my scope had a lot of astigmatism, likely from being siliconed to the wooden stalk. A replacement secondary fixed that. 
Does your scope star test ok?

Also what was the original problem you were trying to fix?

I have no guesses for the linoleum.

It's funny you mention the stickiness.  The problem this one was having was that it would "float" above the collimation screws.  Essentially, the collimation screws wouldn't have any effect on the mirror until you shook the box.  The four nylon bolts were not overly tight.  After some investigation, it looks like the nylon bolts themselves were binding up on the frosted sides of the mirror.  I put in some sheets of milk jug plastic between the nylon bolts and the mirror edge and that seems to have fixed the problem for the most part.  I tested it on the Orion Nebula and the Moon and it was tremendously sharp.  I will do a star test tomorrow evening.



#15 Asbytec

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Posted 16 April 2024 - 12:40 AM

Actually, you kinda want to shake the box so the mirror settles before collimation.
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#16 grao

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Posted 16 April 2024 - 11:24 AM

Enormously useful thread:

 

https://www.cloudyni...rebuild-thread/

 

There are references to a swift kick being useful for collimation - was not sure if it was a joke, but after reading the references to shaking the box, maybe that was part of the process...:-)

 

I have a question - I have a 20 inch version with the original UTA and a primitive dowel rod secondary mount. Seems to work just fine, so am reluctant to fiddle with it.

 

Anybody upgraded theirs and was it worth the effort or should I leave well enough alone?


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#17 grao

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Posted 16 April 2024 - 11:26 AM

Also useful:

 

https://www.cloudyni...gns-dobsonians/


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#18 JJDreese

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Posted 17 April 2024 - 01:22 AM

Enormously useful thread:

 

https://www.cloudyni...rebuild-thread/

 

There are references to a swift kick being useful for collimation - was not sure if it was a joke, but after reading the references to shaking the box, maybe that was part of the process...:-)

 

I have a question - I have a 20 inch version with the original UTA and a primitive dowel rod secondary mount. Seems to work just fine, so am reluctant to fiddle with it.

 

Anybody upgraded theirs and was it worth the effort or should I leave well enough alone?

That broomstick secondary was quite the challenge. I can collimate most Dobsonians in about 5 minutes, but this one took me nearly 2 hours of fiddling.  I ended up replacing all the screws with knurled thumbscrews.  It doesn't help that the spider vane mount angles have zero relation to the focuser angle.




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