Quote:
Originally Posted by Peter Ward
Yes, as Mark suggested you can run a curved secondary but instead of the energy going into the diffraction spikes, Ceravolo says it goes around the stars and uniformly and hence drops the contrast.
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Peter, I think the energy dissipated out of the Airy disc by spider vanes is pretty infinitesimal. `Contrast ' is a pretty emotive term in optics particularly when anyone talks about loss of it. I'd certainly like to see some hard numbers some day in relation to overall field contrast of curved spiders versus windows.
Most of the energy scattered by a curved vain if spread out evenly would be orders of magnitude less in brightness than a diffraction spike from a straight vane. Windows also have their issues - scatter from dust , condensation , optical surface smoothness and transmissive properties of the glass .
On another note, very few curved spiders I see are correctly designed for even distribution of diffracted energy, which takes the form of 3 vanes , each sub tending a 120 degree arc of a circle intersecting the edge of the secondary and the edge of the primary mirror.
There's a good project for some dedicated imager- to interchange a spider and an optical window and see which wins the shoot-out !