Dobsonian design questions

[casstony]

I've probably been thinking a little harder than I should about this but I'm curious whether these design changes could work on a Lightbridge, particularly the 12" lightbridge.

Could you use 4 inch shorter truss tubes to allow lower power use of a binoviewer? Would this require a larger secondary mirror?

If the above is achievable the scope would be 4" shorter for seated viewing and a focuser extension tube (or different focuser) could be used for mono mode.

[janoskiss]

Unless you get a 2" binoviewer with a huge clear aperture, you must put correcting optics (i.e., barlow, OCA) that slows down the light cone (i.e., increases magnification) between the focuser and binoviewer when you use the BV with a fast scope. This is needed not only to move the focal plane farther out of the focuser, but also to allow proper illumination of the focal plane at the EP's field stop. This is because when you have say a BV with a generous 26mm clear aperture and an optical path length of 120mm, it has the same effect as placing your EP at the end of a 120mm length of 26mm diameter tube. If the lightcone is too fast (steep) it just physically wont fit in that long thin tube for all or most points in the focal plane. At f/5, your fully illuminated field will be only about 2mm in diameter. For a 12" lightbridge with a FL of 1500mm, that is just 4.6 arc minutes or about 0.08 degree.

In short, it's not worth it. You are much better off with a good OCA.

[rmcpb]

What Steve said. That is a HUGE mod and would need the whole top end of the scope redesigned otherwise you will be losing light by the bucket load.

[casstony]

Thanks for the answers guys.

[74tuc]

Nice Newtonian design program available here:

http://home.att.net/~dale.keller/atm/newtonians/newtsoft/newtsoft.htm

Test your mods using this program. Yes! it is a big mod.