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Old 02-12-2019, 04:18 PM
sfarndell (Scott)
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sfarndell is offline
Join Date: Nov 2007
Location: Melbourne
Posts: 8
It's always going to be a tradeoff, but ultimately a well-balanced scope can be any reasonable weight without being difficult to operate (there are some large scopes in excess of 200kg which are not motorised and work well).

Is weight the factor? Do you have a lot of stairs? This could also impact use if you have to make multiple trips up and down to fetch/store parts.

With the UTA it's unlikely to be heavy enough to be much of an issue and it comes at the expense of larger bearings (which can be made quite light by removing bulk) but you'll have shorter truss poles. The bearings don't have to be permanently attached so the extra bulk unlikely to be an issue.

As to single vs double ring: I've built a 16" ultralight dob with a single ring UTA then remade it in a more traditional way with a double ring as I had too much vibration/issues. Double ring UTA is much easier overall (IMHO) as it's easier to attach the spider (I use a wire spider), baffles and the focuser. It also reduces vibration attributable to these components due to how they attach on the double ring UTA. As an added benefit, you also don't have to constantly worry as much about scratching your secondary when you dismantle, etc. Extra weight can be saved as you can also cut a lot of wood out of the rings to reduce weight without reducing structural integrity (same thing for your mirror box, but you'll need to add some sort of dust cover/protection).

I think your major issues lie in your mirror and rocker boxes. Have you considered a flex rocker? They can be made relatively light, but quite stiff, provided you don't mind cutting a lot of holes! Should be a lot lighter than 13kg!

Def some room to shave weight on your mirror box. How heavy is your mirror? Can you post pics of your current setup?

On Carbon fiber. I'm in the process of making a 14" binodob and had the *great* idea to make the upper assembly from CF. It's expensive. Very very expensive. You need a vacuum pump, gauge and catch pot as a minimum, plus consumables. CF cloth is around $30/m but you will require multiple layers over a core material to make it stiff enough so it'll use a lot. Cheap core was $70 for a 2400 x 1200 x 20mm sheet. AA grade Marine ply not much more expensive, easier to work and more readily available.

My only output so far is a 10mm thick of CF which I hope to turn into helical focusers. Unless I stuff it up, which is also consuming a lot of thinking time.

CF requires a lot more effort and much more difficult to work with than plywood so I'd recommend you use plywood and then skin it in CF if you want the look, but at the cost of extra weight.

That being said, I'd love to see a CF build

Hope that helps

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