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Secondary mirror removal mystery

Dob Mirror Making Optics
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#1 JJDreese

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Posted 12 May 2021 - 12:44 AM

I bought a used 12" Zhumell Dob in good shape*  for a really good price. The primary mirror needs recoated (I've done all the tricks and it's gotta be done). I'm going to have the secondary done at the same time, but...

 

I can't figure out how to remove the secondary from the holder.  It's in a plastic enclosure with a clip that seemingly holds it in. But when I remove the clip, the mirror is held in place still pretty rigidly.

 

I would try prying it, but I don't want to damage it.

 

Thoughts?

Attached Thumbnails

  • Secondary.jpg


#2 Woj2007

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Posted 12 May 2021 - 05:36 AM

Hi,

 

The secondary mirror - depending on the batch - is either glued with a double-sided tape (easier to remove) or with a black silicone (harder to remove).

 

If you want to remove it, you have to try - very carefully - to pry up the mirror  by putting some thin screwdriver down the holder (on the screw side) and bending it up.

 

I did remove such a secondary once, however I slightly damaged the mirror (chipped part of the glass on the back).

 

If anything happens, you could always buy a new one instead of recoating :) 

 

https://agenaastro.c...rror-70-mm.html

 

Or even exchange the whole set:

 

https://www.teleskop...ng-and-ATM.html

 

Or maybe (my recommendation) it is time to upgrade the secondary for some nice Antares?

 

http://antaresoptics.com/SEMirrors.php

 

 

 

 

I bought a used 12" Zhumell Dob in good shape*  for a really good price. The primary mirror needs recoated (I've done all the tricks and it's gotta be done). I'm going to have the secondary done at the same time, but...

 

I can't figure out how to remove the secondary from the holder.  It's in a plastic enclosure with a clip that seemingly holds it in. But when I remove the clip, the mirror is held in place still pretty rigidly.

 

I would try prying it, but I don't want to damage it.

 

Thoughts?


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#3 SteveG

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Posted 12 May 2021 - 02:37 PM

I bought a used 12" Zhumell Dob in good shape*  for a really good price. The primary mirror needs recoated (I've done all the tricks and it's gotta be done). I'm going to have the secondary done at the same time, but...

 

I can't figure out how to remove the secondary from the holder.  It's in a plastic enclosure with a clip that seemingly holds it in. But when I remove the clip, the mirror is held in place still pretty rigidly.

 

I would try prying it, but I don't want to damage it.

 

Thoughts?

As noted, it's held in there with double stick tape. Try a hair dryer to warm it up, then pry it out.

 

FWIW, I tool my holder (like yours) and ground away the outer sides so that I was left with a flat, smooth surface. I then glued the secondary back on with 3 small globs of silicon. This eliminates the clip. You don't want double stick tape, as it can deform the mirror.


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#4 Woj2007

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Posted 12 May 2021 - 03:36 PM

I also got rid of the holder collar and made it flat, gluing the bare secondary to it with 3 globs of silicon.

SM.jpg

 

The tantalizing :) question is, do you really need to recoat your secondary? Secondaries are SiO2 protected, and they last long. If you see speckles, maybe it would be just enough to clean it. 


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#5 JJDreese

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Posted 12 May 2021 - 11:03 PM

I also got rid of the holder collar and made it flat, gluing the bare secondary to it with 3 globs of silicon.

attachicon.gifSM.jpg

 

The tantalizing smile.gif question is, do you really need to recoat your secondary? Secondaries are SiO2 protected, and they last long. If you see speckles, maybe it would be just enough to clean it. 

 

Good question.  I'm getting the primary recoated and they do the secondary for free, so I figured I'd have them done at the same time.



#6 Ranger Tim

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Posted 16 May 2021 - 01:15 AM

Sometimes you can saw through the silicone or tape with fishing line. I have used the “Spiderwire” brand of braided line with good results.


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#7 Jon Isaacs

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Posted 16 May 2021 - 03:53 AM

As noted, it's held in there with double stick tape. Try a hair dryer to warm it up, then pry it out.

 

FWIW, I tool my holder (like yours) and ground away the outer sides so that I was left with a flat, smooth surface. I then glued the secondary back on with 3 small globs of silicon. This eliminates the clip. You don't want double stick tape, as it can deform the mirror.

 

I don't think it's held by tape. Look at the photo, you are not looking at the backside of the mirror, youre looking at the holder and the clip that holds the mirror in place.

 

My 10 inch GSO looks just like that.

 

If I remember correctly, the key is to remove the center bolt so the secondary holder is free. I believe that bolt hole is a through hole and provides access to the backside of the secondary so you can push on it to remove it.

 

Jon


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#8 Woj2007

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Posted 16 May 2021 - 04:12 AM

I don't think it's held by tape.

I dismantled two such GSO secondaries in the past.

The one from my local retailer was glued with black silicon, the other one from TS was held by tape.


Edited by Woj2007, 16 May 2021 - 04:13 AM.


#9 Jon Isaacs

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Posted 16 May 2021 - 11:29 AM

I dismantled two such GSO secondaries in the past.

The one from my local retailer was glued with black silicon, the other one from TS was held by tape.

 

In addition to the clip?

 

Jon



#10 SteveG

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Posted 16 May 2021 - 11:56 AM

Mine looked exactly like post #1 and was held in place by tape, along with the clip.



#11 Sol Robbins

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Posted 16 May 2021 - 01:14 PM

Years ago I had a 10" with the same secondary holder. That sleeve that extends from the secondary holder and also holds that secondary clip is very tight. So tight that it can induce astigmatism by pinching the secondary mirror. 

 

I used a hair dryer to heat it up for quite a while. I could slowly and gradually get the mirror out. I took my time with this.

 

Once the secondary mirror was out, I removed that sleeve with a small razor saw and sanded it flush with the surface that had that double sided tape used to stick the secondary mirror on. Once all that was done, I siliconed that secondary back onto the holder.


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#12 Woj2007

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Posted 16 May 2021 - 04:30 PM

In addition to the clip?

 

Jon

Yes :)

In the silicon version, the silicon was spread all over like butter.

What is more, the clip was tightening the mirror so much that it scratched the coating a bit on the touch point. For transportation overseas this kind of securing makes sense, but for observations it can cause some astigmatism.


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#13 Jon Isaacs

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Posted 16 May 2021 - 05:06 PM

I can only say the secondary on my 10 inch was neither glued nor taped.

 

I bought it in 2003 when it was a year old. I found the secondary was pinched, removed it, relieved the plastic so it was not pinched. I reinstalled it and 18 years later, it's still going strong.

 

Jon


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#14 SteveG

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Posted 18 May 2021 - 02:12 PM

Years ago I had a 10" with the same secondary holder. That sleeve that extends from the secondary holder and also holds that secondary clip is very tight. So tight that it can induce astigmatism by pinching the secondary mirror. 

 

I used a hair dryer to heat it up for quite a while. I could slowly and gradually get the mirror out. I took my time with this.

 

Once the secondary mirror was out, I removed that sleeve with a small razor saw and sanded it flush with the surface that had that double sided tape used to stick the secondary mirror on. Once all that was done, I siliconed that secondary back onto the holder.

Exactly the same as I did. Nothing holding it in place now but the 3 globs of silicone. I did this at least 10 years ago.




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