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Need to silicone a secondary mirror

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#1 clusterbuster



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Posted 21 September 2023 - 01:42 PM

Hi All,

 I need to silicone my secondary mirror..

I would like to know a few things..

 What type/brand would you recommend ?? 

And a basic how, to clean the surfaces and apply.

I want to make sure that I do this correctly.


Also, My main mirror (these mirrors just came back from being re-coated),

It is a Zhumell 12", the mirror cell has clips, should I just use the clips or should I silicone the main mirror also ??


 Thanks in advance for all of your suggestions...

Can't wait to get that Z12 out for 1st Light !!


Mark in Tampa


My friend refurbished this Z12, he made a great BASE with BIG TRUNNIONS (GIMBALS), whatever you prefer to call them.. He used made it out of wood wood, we got rid of that particle board !!! 

 I will post a pic of it on this same thread shortly..

Edited by clusterbuster, 21 September 2023 - 02:01 PM.

#2 Larry Patriarca

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Posted 21 September 2023 - 02:53 PM

I'm sure most will flip out at what I did with my Meade 16" Lightbridge mirrors. 


The original secondary holder enclosed the mirror with a metal shroud which was 3.6" diameter for a 3.5" secondary.  When I calculated the necessary size for a 16" F4.5 primary, 2.9" was the required minimum size, so I stole the secondary from an older Orion 12.5" dob that I had parted out.  To mount this secondary, I made a solid aluminum support base that was only 2.5" diameter.  I used foam 2-sided tape to install the secondary to hold a true 2.9" obstruction; the tape held for about 10 years before I removed it last year for recoating at Spectrum. 


Removing the primary for cleaning years ago required removing the whole mirror support base from the bottom of the scope.  Because I had added (2) 4" fans blowing laterally across the mirror, this added wiring made it even more difficult to remove the primary for cleaning.  The primary was held in its mirror cell with several 2" beads of black silicone as well as intrusive mirror clips.  I removed the mirror clips, cut the caulking so I could remove the mirror from the top without having to remove the whole mirror support and then was able to clean the mirror more easily.  I then reset the mirror in its cell without recaulking or using the mirror clips. 


The beads of caulking were still in place (I only sliced them loose from the primary mirror instead of removing them) so they acted as cushioning and relief for thermal expansion.  This worked well until my scope took a nosedive when I wasn't looking and the mirror dumped out the moment it pointed past level.  Fortunately, the primary was not injured but it wasn't something I ever wanted to see happen again. 


So....the mirror clips were originally attached to the mirror cell with 1/4-20 screws threaded into the side of the cell.  With the mirror in place, I transferred the hole locations with a marker, and then (don't flame me too badly fellas) dimpled the marked spots on the glass with a 1/4" drillbit.  I then installed 1/4-20 nylon screws that I ground to a point to match the dimple angle (115 degree drillbit).  The nylon screws are set SNUGLY so as to be enough to restrict movement of the primary, but flexible enough to relieve stress during expansion/contraction of the glass.


This has worked quite well for about 10 years now (foam tape on the secondary, nylon screws on the primary).  

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#3 durangodoug


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Posted 21 September 2023 - 11:11 PM

Here's how I do it:


1) Before getting started

       - Create a jig or determine some other means of aligning the mirror into the desired position on the hub and stay in that position while the silicone cures.

       - Be aware that the mirror surface is quite delicate and can scratch easy, even setting on something as benign as a piece of paper - I like to set up my jig so that the mirror will be mirror side up such that it never has to rest on anything.

       - Find / create three "spacers" to hold the mirror & hub apart by the desired distance while the silicone cures.  I use small lengths (about 1/2 the secondary minor dimension) of a .017" guitar string

       - Get a fresh tube of 100% silicone for the job (yes, it goes bad).  I suspect being fresh is more important than the brand - I did my first secondary some 20 years ago, and it's still holding strong.

       - Get some cotton balls (optional if not intending to clean the mirror surface)


3) Clean the two surfaces with a little soap in some warm water, then just warm water to remove the soap.  If not intending to clean the whole mirror, Q-tips could be used to do a more selective cleaning of the back of the mirror only.

4) Rinse with isopropyl alcohol.  Cotton balls good for gently wiping away drops then letting it evaporate (which happens quickly).  Extra gentle on the mirror surface, if doing that.

5) Holding the mirror by it's sides by hand, apply 3 "dabs" of silicon to the back of the mirror, I think the rule of thumb is 40% of the way from the center to the edge for best support , though I tend to go with somewhat wider spacing, on the theory that I'm seeking a balance between the traditional concern of proper mirror support and, particularly on faster scopes needing with tight collimation requirements, lowering the (albeit tiny) collimation error resulting from the shifting weight on the silicon pads themselves (silicon has a very low Young's modulus, i.e. it's (intentionally) "stretchy"). It's this same reasoning that leads me to use the somewhat smaller .017" gaps than is typically recommended.

6) Put the spacers in position.  With the little pieces of music wire, I put the tip of each piece just inside the silicon pads, which keeps them in place (and they're easy to pull out once cured)

7) Set mirror & hub together per alignment / jig.

8) Place in a relatively warm location in the house, in a place it will not be disturbed for 24 hours (no cheating).

9) Pull out the spacers


As for primary, I've not liked the idea of having clips against my primary mirror, but would also be horrified if it were to fall out should the scope be titled to the horizon.  After the mirror cell & side support is complete, I'll securely attach a couple of short nylon web straps to the mirror cell on each side of the mirror, reaching up along the side of the mirror (but not extending past its surface), and then silicone the straps to the side of the mirror, leaving a bit of slack in them (in an effort to not apply any force to the mirror).

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#4 clusterbuster



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Posted 22 September 2023 - 12:38 AM

Thank You for the information..



#5 Dan_I



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Posted 22 September 2023 - 01:44 AM

I suggest to use aquarium silicone which is very strong.

Clean carefully the back of the mirror and the surface of the holder with acetone.

Not mandatory, but convenient : print a template (on clear acetate) to position the secondary accurately, taking into account the offset.

Take 3 toothpicks of about 1.5mm thickness. Secure their position on the secondary holder using tiny droplets of ordinary glue.

Put the mirror, the holder and the glue in a cool room (about 15°C) without draft and wait a day to be sure that everything is at the room temperature. It is convenient to install the holder on a angled support such that the surface to be glued is horizontal.

Then, acting swiftly, put 3 drops of glue on the holder in between the toothpicks. Press lightly the secondary mirror to ensure that the back is in contact with the 3 toothpicks.

Wait for at least three days to let the glue cure and remove the toothpicks with a slight twist.

Edited by Dan_I, 22 September 2023 - 05:23 AM.

#6 TopherTheME


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Posted 22 September 2023 - 10:04 AM

I would also suggest looking into 3M VHB as a possible solution. Better adhesion strength than silicone and variable thickness foam substrate is available as well. It's held the secondary on my scope for nearly 10 years with no issues. It's also what some scope manufacturers use.

#7 star acres

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Posted 22 September 2023 - 12:49 PM

I wish I knew what Celestron, Zhumell, Sky Watcher, Orion used to glue their mirrors.

#8 durangodoug


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Posted 22 September 2023 - 05:32 PM

Oh and on clip-less mirror retention, I stumbled on this from NHRob in my old thread from the time, which seems like a good idea:


"For the safety straps I am going to use two straps which will velcro to both the mirror side and the cell frame. These will hold loosely. The velcro patches will be glued to the side of the mirror at 2 places, maybe using RTV." - "Makes it easy to remove mirror for cleaning."

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