which adhesive for secondary

[AlentejoSkies]

Hi,

Gradually putting my Darkstar back together after a bit of maintenance...

The secondary was originally held in place with epoxy around its edge, still strong after all these years. Had to take it off though to recoat, and now I wondered about using silicone to give a bit more flexibility, but with such a small contact area I am worried it could come loose with catastrophic results. So actually I guess it will be epoxy again and I have answered my own question. Unless anyone with experience tells me otherwise?

Douglas

[Peter Drew]

I wouldn't use epoxy, good chance that it would distort the mirror.  A good rim of silicone adhesive would be better, once set pierce the silicone bead at some point to allow the expansion of air trapped under the mirror.

[toxic]

i would use a stiff adhesive you dont wat any movement at all, use a contact adhesive like evostick and apply it to both surfaces and wait till its tacky before putting together.

 

[AlentejoSkies]

@Chris, do you mean there should be no movement so the collimation doesn't shift?

[Skipper Billy]

I wouldn't use silicone anywhere near a mirror or mild steel parts - it contains a strong acid which will eat many things it comes into contact with including most metals.

 

[AlentejoSkies]

Regarding the acid in silicone - I would not put it in a sealed box to cure, for sure. But I think the acetic acid is gone after a few days. It would be in direct contact with the back surface of the secondary, and the plastic of the secondary holder.

(I did once leave an open tube of silicone in a sealed tool box for a year, made a real mess... everything heavily corroded and smelling of fish & chips.)

[Skipper Billy]

If it was originally stuck with epoxy by the manufacturer then I would continue to use epoxy.

We are boat builders and I can show you aluminium hatch frames that have been sealed to the deck with silicone (not by us ;-) ) that are massively eroded by the silicone.

 

 

[Peter Drew]

The secondary mirror attached to its aluminium holder on my 17" Dob shows no sign of deterioration after 20 years no problems to date., some silicone canisters do say not suitable for aluminium. I use Dow Corning brand where possible.

[101nut]

2 hours ago, AlentejoSkies said:

Regarding the acid in silicone - I would not put it in a sealed box to cure, for sure. But I think the acetic acid is gone after a few days. It would be in direct contact with the back surface of the secondary, and the plastic of the secondary holder.

 

Use the correct materials and it will be fine - neutral curing silicone is made for this type of sensitive job. Dow, 3M and many others often have it in small tubes for specialist jobs like fish tanks. For example this supplier lists various products and this would probably suit your purpose. You want a high modulus type for sufficient rigidity with a little bit of movement.

AndyG

[mapstar]

I think someone posted this last time from RS components.

http://uk.rs-online.com/web/p/silicone-sealants/4236758/?origin=null|fp&cm_sp=featureproducts-_-FeaturedProductsContent-_-4236758

I've bonded mirrors on before and used glazing sealant designed for aquariums. 

[AlentejoSkies]

Neutral curing silicone it is then. Thanks a lot!

[Charic]

As I read through the thread, my initial thought was to use something like marine silicone, the type the fish tanks are held together with, as they  have to sustain a great weight/pressure when the tank is full, and should be non-toxic, and possibly non-corrosive to metals, as some tanks  are still metal framed?  101nut  has already  mentioned fish tanks. 

[Horwig]

If I can ad my experience here. I used silicone on my secondary, but it was from a tube which I had already opened for another job some time before. The mirror slid off the cured silicone!

Lesson learnt, always use brand new silicone, not some leftover from another job.

 

Huw

[Uplooker]

Not that I have performed this myself, but of all the threads associated with this task, I have gleaned that people use three dobs of the silicone normally used for vivariums / fish tanks