Sealant for 130PDS mirror mod

[edarter]

Hi,

Can someone confirm if this silicone sealant is suitable for mounting the primary mirror in the 130PDS? It says its 'neutral' but in what way I'm not sure...

https://www.amazon.co.uk/Everbuild-700TTR-Roofing-Silicone-Sealant/dp/B0012RI3BI/ref=sr_1_11?crid=2F04891P5YQZ0&keywords=neutral%2Bsilicone%2Bsealant&qid=1673458332&sprefix=neutral%2Bsilicone%2Bsealant%2Caps%2C95&sr=8-11&th=1

Thanks
Ed

[CraigT82]

Not sure that wil work too well as the description states it’s for uPVC and wood, and soft metals only.

Ive used this stuff before which works well for sticking mirrors in (also gluing secondaries too) 

V-Tech VT-130 Clear RTV Sealant Adhesive Quality Flexible Silicone: 75gm Gasket https://amzn.eu/d/7Sh7qS1

[alacant]

1 hour ago, edarter said:

Can someone confirm if this silicone sealant is suitable

Hi

I can't, sorry. It seems rather country specific.

Not sure where you are, but here is the Spanish version which we use. It's nothing special and is available at supermarkets; the ones which sell light bulbs and hammers as well as pasta and beer. The main property is that it doesn't continue to harden over time.

Cheers

 

 

[markse68]

2 hours ago, edarter said:

Can someone confirm if this silicone sealant is suitable for mounting the primary mirror in the 130PDS? It says its 'neutral' but in what way I'm not sure...

from what I can find the Neutral cure silicones release alcohol as they cure whereas the other ones give off acetic acid and pong a bit. Not sure if having acetic acid fumes in vicinity of mirror coatings is a good idea so probably the neutral type would be better? Aluminium is supposed to be pretty resistant to acetic acid protected by its oxide layer but not sure I'd trust it. Having said that from a quick search the CN lot are recommending the acetoxy silicones as apparently they stick better without special surface treatments and Dow Corning 732 is recommended. I guess if you let it cure fully with good ventilation the acteic acid fumes shouldn't be a problem

https://www.ebay.co.uk/itm/304502430639

[Craig a]

I used grip bond pro for my 130pds and my Quattro 10inch, that has worked with no issues whatsoever, just ensure the mirror and cell is grease free before you stick 

[Robculm]

Hi Ed, as per Craig, I used the V-Tech VT-130 Clear RTV Sealant (via Amazon). Worked well, but don't use too much. I initially stuck it all the way around which did stress the mirror, had to cut it out (not fun!) and re-stick with 3 'spots' which is now great.

Rob

[edarter]

Thanks all, i'll just order the V-tech stuff then. Does have me wondering now though if aquarium sealant would work. That can't have any acids in it as it would pollute the water and needs to cure reasonably quickly and obviously designed for glass..🤔

[AstroMuni]

1 hour ago, edarter said:

Does have me wondering now though if aquarium sealant would work. That can't have any acids in it as it would pollute the water and needs to cure reasonably quickly and obviously designed for glass.

Thats a good idea. 👍 I would have thought just common bathroom sealant would do the trick, which looks like what Alacant's image suggests.

[Ratlet]

Acetic acid is fairly volatile and won't hang about for too long.  It's been a while since I looked at siloxane chemistry (spent ages researching it about 15 years ago for high temperature use) but the acid cure tends to be be more tolerant of dirty surfaces where as neutral cure is more picky if there is any grease or oils on the surface.  Obviously your surface should be clean before sticking though.

The bigger issue with the acid cure is for a traditional mirror where the coating is on the back of the glass so you'd be sticking the silicone on the mirrored surface as the acid can become trapped and cause the mirror to discolour (if memory serves it can make a traditional mirror turn brown).

 

[edarter]

6 hours ago, Ratlet said:

Acetic acid is fairly volatile and won't hang about for too long.  It's been a while since I looked at siloxane chemistry (spent ages researching it about 15 years ago for high temperature use) but the acid cure tends to be be more tolerant of dirty surfaces where as neutral cure is more picky if there is any grease or oils on the surface.  Obviously your surface should be clean before sticking though.

The bigger issue with the acid cure is for a traditional mirror where the coating is on the back of the glass so you'd be sticking the silicone on the mirrored surface as the acid can become trapped and cause the mirror to discolour (if memory serves it can make a traditional mirror turn brown).

 

I don't think the coating is on the back of the mirror though on the 130PDS?  Not 100% sure though.

[Ratlet]

48 minutes ago, edarter said:

I don't think the coating is on the back of the mirror though on the 130PDS?  Not 100% sure though.

Nah.  It's on the surface.  Traditional mirror do it on the back, newtonians on the front.

 

[bottletopburly]

you want low modulous silicone , Screwfix sell it it more the normal stuff these days it will say on the tube like the Screwfix version 

Edited January 12, 2023 by bottletopburly

[edarter]

2 hours ago, bottletopburly said:

you want low modulous silicone , Screwfix sell it it more the normal stuff these days it will say on the tube like the Screwfix version 

That says highly elastic though, is that what we want to hold the mirror? Would assume that moves more during meridian flip?

[bottletopburly]

9 hours ago, edarter said:

That says highly elastic though, is that what we want to hold the mirror? Would assume that moves more during meridian flip?

Highly unlikely 

[Craig a]

The mirror still needs to be allowed to expand and contract during changes in temperature so the sealant needs to be elastic enough to allow this 

[alacant]

16 hours ago, edarter said:

assume that moves more during meridian flip?

Remember what we're trying to achieve...

1. We're trying to minimise lateral movement of the mirror relative to its cell as the telescope changes angle as it tracks the sky. This prevents the collimation changing.

2. The mirror clips are no longer needed, rendering cleaner stars.

It must NOT be held rigidly, rather as @Craig aquite correctly notes, it should be allowed to expand freely as the temperature changes. Hence the need for sealant which does not harden over time or with temperature.

IF you're uncertain of the suitability of a given sealant, leave a blob of it for a few days or so out in the conditions under which the telescope will operate and check for yourself.

At the same time, let's not overthink this too much; anything is better than using the SW cell as it is and if you do get it wrong, the process is easily reversed.

Cheers and HTH