My point is that assuming the same obstructed area, there is the same total amount of diffraction. It is just not concentrated in the "spikes" with a curved spider. I use a single-stalk secondary holder in my 6" scope. Since the diagonal is only 1" minor-axis and very low mass, I use a rod of much thinner section than most commercial single-stalk rods. I get only two spikes, diametrically opposed. My 4.25" uses a curved spider that is one curved section of sheet metal, with the secondary holder at the mid-point. Neither of these options is particularly good for a larger telescope, with much more massive secondaries supported much further from the tube walls. Wire spiders have gained popularity recently because they obstruct very little area. A curved spider, whatever the number of vanes, is really a type of apodizing mask. Straight-vaned spiders can reduce the intensity of spikes, at the expense of greater total diffraction, by covering them with a thin mask with a non-straight edge, like an undulating wave pattern. Deep-sky objects are so dim that we never notice diffraction effects with large telescopes. It's usually only on bright stars and the planets that it is noticeable, or when imaging. For the average "light bucket", an optimised spider is really no big deal. Schmutz- There you go with those technical terms again. I think Ingalls comments on schmutz in ATM book 2. --- Michael Carnes <MichaelCarnes@earthlink.net> wrote:
There might be more to it than that. I think the total amount of diffraction components is lower because there are only two vanes instead of three or four. The vanes themselves are pretty thin, since much of the rigidity of the structure is provided by the spring tension of the vanes. But certainly there's still schmutz in there, just spread around.
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