Concrete RTV mirror cell.

[marcopolo]

Hello all. There's no shortage of crazyness here so I was wondering. In a simple RTV cell arrangement you have the mirror collomation plate that the mirror is RTV'd to usually from 12-18mm Ply or similar, MDF etc...

usually for scope mirrors in the 6-10in region

I was wondering. Has anyone ever tried making a concrete disc for this, probalby reinforced with some steel mesh?

One of the reasons i'm masking is that the thermal coefficients of expansion are not that dissimilar. I realise adequately thick RTV will take up that difference anyway. But i'm just curious.

Also you could just embed the collimation bolts in the concrete.

thanks,

Marco. 

[marcopolo]

...the tumblewheel blew silently across the field of view as the other astronomers contemplated the sheer stupidity of the question........

[saac]

I haven't seen it done Marco but there's no reason I can see that it would not work. The question is what benefit are you hoping to get from using concrete/cement?  I would have thought that a wooden construction would be easier to fabricate.  You would certainly be adding weight to the mirror assembly which depending on your design may be a benefit.  I don't think your suggestion is totally crazy and certainly from thermal stability perspective has merit. Be interested to hear your thoughts on why you are considering it. 

Jim

[Astrobits]

Interesting idea. I have looked up the thermal expansion of wood and concrete and some of the numbers are as follows:

Concrete  between 7 and 13 x 10^-6

wood ( pine)   5 x 10^-6

So concrete is about twice as bad as wood for thermal stability. However, another consideration is the change in dimensions with humidity where the wood is between 4 and 8% depending on the direction of the grain, i.e. about 10000 times as great as the thermal expansion. I could not find an equivalent number for concrete but would expect it to be much lower.

Concrete needs bulk to be strong, try breaking a slab a few mm thick, so the mass will need to be allowed for in the balancing and the overall weight of the telescope.

Nigel

Edited November 19, 2022 by Astrobits

[marcopolo]

Hi all,

Ordinarily i'd make a 6, 9 point cell out of Aluminium etc....standard route for most of my Dobsonians. However this scope is a 1930's refurb  - 7 .5in F8 Newtonian on a VERY heavy GEM. (200+ Kg all in). The existing mirror cell if I can call it that is a heavy brass backplate holding the mirror in with shims and no support so to speak. I'm adding the mirror collimation plate to this arrangement, keeping the heavy brass scope backplate that is presently serving as the mirror cell.

So that all said the weight will make little or no difference. I have some 9mm ply I could double up (18mm) and RTV to that.

It was simply to try something new/different. I might just give it a go simply because I can easy 3D print a mould for the concrete, drop in some threaded m4-m5 threaded rod  or some steel mesh as concrete reinforcement. Embed the M8 collimation bolts (1mm fine thread pitch) in the concrete which is the other advantage.

I literally have all the ingredients lying around my workshop to do this.

Honestly not much more logic than that...just a bit of fun. I can't see it failing too easily. 

 

Thanks,

Marco.

[saac]

I think It's a nice project, I'd certainly give it a go - nothing to lose really. I guess you would want your concrete mix to be fine to produce a smooth finish (maybe polish it). Possibly seal or paint to keep dust down!

Jim 

[marcopolo]

Yep, good idea. I’ll definitely float the surfaces to a polish a few times during drying then seal with something…

[marcopolo]

Thats it all farbicated. I ened up just using firbreglass in the mould. Concrete was not happy for a few re a treat. 

[saac]

Looks good Marco, sealing it was definitely worth it. Have you installed it yet - any feedback on first light?

Jim 

[marcopolo]

Hi, so i've had first light with it installed (will try get a photo uploaded soon) There's no astigmatism present in the unfocused Airy disc, nice sharp images. No change in collimation when the scope is pointed at different altitudes. No  loss of collimation ove rtime (over 3 weeks now) - so i'm actually extremely happy with the arrangement. TBH the whole fabrication was a bit of a faff but it does seem to be successful.