Protostar 3 vane versus curved spider

I am putting together an 8" f/8 scope that will be mainly used for planet/lunar viewing. I was all set to order the Protostar 3 vane spider and then heard about the Protostar curved spider and paralysis by analysis has set in. Has anyone compared these two spiders? It seems to be a trade off between the greater stability of the 3-vane versus spreading the diffraction around (to undectable levels?) for a more asthetically pleasing view. Thanks.

Dave Adams

Dave,

My first serious telescope was an 8" f/8, and it gave me some fabulous views of the Moon and planets (and deep sky) over the years. It had a traditional 4 vane spider with thicker vanes than Protostar uses. I suspect you would do just fine with either choice. I note that a friend of mine, who has been doing this as long as I have but has probably tried more Newtonian variations, opted for a curved spider in his latest creation.

Clear skies, Alan

If you were able to use material of the same
thickness, then the curved spider would win.
But usually it ends up that the curved spider
needs alot more thickness to get same stability.
I've compared side-by-side 8" F6 scopes with
thin (0.02") 4 vane and thick (0.1") curved
spiders. While the curved one had no
spikes, there was instead a noticable smear
of light all around the star or planet that
was absent on the 4 vane -- overall
it seemed to scatter more light than the 4 vane.

Dave, how big of a secondary mirror are you using? You're likely using a 1.3" m.a., so it will be quite light and the curved spider ought to have no problems supporting it. Regarding diffraction, given vanes with the same thickness, the curved vane (single semicircle) ought to have no more total diffraction than the three vane, possibly less. But as others have mentioned if the former has thicker vanes then it may reduce the benefit. FWIW, Portaball uses Protostar curved vane spiders in their scopes.

Decisions, decisions...

Best regards,
Doug Scobel

Thanks for all the input. I spoke with Bryan at Protostar and felt comfortable with the curved vane. He said they have tried different designs over the last 2 years and are confident they have a good design that addresses the stability issue while using thin vane material. The curved spider uses the same thickness material as the 3 vane spider so overall diffraction should be less. The vanes are wider at the base to help with stability. I might not have gotten this correct but there is a plate mounted somewhere on the vanes that acts as damper. Since this scope will be used mostly for planet and lunar viewing, I opted to order it in the 1" size along with a new secondary. I hope I don't find the vignetting objectionable but I don't think I will be using this scope very often for wide field viewing.

Dave Adams