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I'm a bit like Brendan - I look at it from an imaging perspective. Building an OTA as well as a mount for deep sky imaging will be quite a challenge.
Here's my 2 cents worth:
- I'd go for the regular secondary support - not wire.
- A truss starts to make sense with a 300mm primary, but you are not totally beyond the realm of a tube. The other nice option would be a light but strong composite tube. You could do both of these yourself, I'm guessing the truss would be the cheaper option - so maybe that's the way to go.
- Plan in advance the full image train (camera, coma corrector, focuser and auto guiding etc) - this will determine the distance from the secondary to the focal plane and also the secondary size. Then it's easy to work back and figure out all the other dimensions from that.
If you're looking to do it on the cheap, you shouldn't have any problem picking up lots of useful parts second hand - either here or on astromart. Lots of people have replaced GSO focusers, secondaries, spiders and even primaries. Maybe you could even pick up an old-school mount and refurbish it?
James
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