Best Silicone for secondary

[Horwig]

I'm having another go at fixing my secondary, I think I badly over-siliconed it first time around, very noticable astigmatism. Any thoughts on which Silicone adhesive to use, anybody used this one for instance?

https://www.sealantsonline.co.uk/product.php?product=HAN2417&category=00020001&utm_source=google&utm_medium=products&gclid=EAIaIQobChMI_OnB0PnD1wIVYbftCh1zHw2nEAQYAyABEgJPLfD_BwE

 

Huw

[Astrobits]

I use any Silicone that specifies it's use for bonding to glass. They usually mention greenhouse applications. So the one you link to should be satisfactory.

The secondary only needs one blob in the centre of the mirror, extending to no more than half the minor axis diameter, assuming that you have a solid base, and not a cut tube for the holder. Make sure that the final thickness of the silicone is approx 3mm thick to prevent astigmatism.

Nigel

[estwing]

not sure which make but I was told to use clear as coloured loses its strength quicker..

[estwing]

mine has 3 blobs like a triangle formation...and put a matchstick behind, keeping a gap so when the time comes to take off a knife can slide round the back

 

[Horwig]

3 minutes ago, estwing said:

not sure which make but I was told to use clear as coloured loses its strength quicker..

I had the same advice for using any tube adhesive from a tiler, clear is much stronger than white

[Peter Drew]

I've put my faith in Dow Corning silicone for this purpose and it's what I usually recommend. However their seems to be a move towards using double sided adhesive products. Whatever you use, make sure there is a big enough gap to get a cutting tool in for when you eventually need to remove it.  

[Horwig]

2 hours ago, Peter Drew said:

I've put my faith in Dow Corning silicone for this purpose 

Read a lot recently about double sided tape, but I'm going to go with the Dow Corning Glass adhesive 817. My local Travis Perkins seems to have it in stock. This time I'll use less spread and more thickness than what I used for the least fixing, let's see if that improves matters.

 

Huw

[NGC 1502]

 

Hi Huw.  Really hoping  you don’t mind me mentioning, and not at all trying to be picky, but you did mention double sided tape. Best to use  double sided sticky foam tape, it’s about 2mm thick, far less likely to cause any distortion of the secondary than the much thinner double sided sticky tape.  Posted for clarification for any readers.

I know you are using silicone, that’s my choice too.

Cheers, Ed.