>
> >I was looking at the Novak catalog and saw a curved vane secondary
> >holder. Are there special mathamatics involved in designing one of these
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> >or is it just any old curved vane will do.
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>
> In a word. Yes. Then No. Sky & Tel had some articles that
> analysed this to death.
Yes,
This article was sometime in 1986. I studied it very carefully as I was building 8" binos at the time and was concerned about multiple diffraction spikes with the two misaligned spiders. The curved spider design with the most effective removal of diffraction spikes was the design using three vanes touching the hub at the usual 120 degrees. Each vane is a semi-circle; the spider looks a bit like a three armed spiral galaxy. I made the vanes thick enough so that there is no vibration ( as you can't put them under tension ).
In practice there are no visible diffraction spikes, only a soft halo around the brightest stars which looks no different than slighly dusty optics. Planetary detail is still good. I like this kind of spider because I find that diffraction spikes spoil the illusion that you are looking at the object floating in space through a spaceship pothole window, an illusion which is very strong in binoculars.
Mark