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EyeSage wrote:

(excellent comments snipped for brevity)
On the other hand, for imaging, some people like the spikes seen in the images of brighter stars.

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I'd fall into that category.

In my experience, curved spider vanes (and by extension, scalloped ones) have the effect, as EyeSage has stated, of spreading diffraction... this reduces contrast somewhat, as the light that would have gone into diffraction spikes is now spread across the entire field of view. Combined with curved support's tendency towards instability, it would seem that, short of obtaining an optical window to support the diagonal mirror, the thinnest-possible straight vanes are the best (and most widely-used) solution, providing both the best stability and highest contrast.

Edmund Scientific used vanes only .005" thick in its tensioned-vane diagonal supports of the late 1970's, and I can attest to the sharpness of the diffraction spikes this design provides, as well as wonderful contrast.

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what i was thinking of was something along the lines of what is found here:

http://www.eclipsechaser.com/eclink/image/astrogd.htm

just ctrl+f and search for "scallop" and it'll take to you an image to which i am referring (just past halfway down the page).

as EyeSage pointed out, another quick reference to it can be found in Jeffery Charles' "Practical Astrophotography"... page 103 to be exact. Again, it strikes me that if it works as I understand it, it would provide the benefits of a curved spider w/o the compromise in increased obstruction area nor decrease rigidity... perhaps i should just experiment with it... it certainly would be interesting.

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there was a thread about vanes [dont remember when, cant find it]

and someone did a lot of different tests of vane thickness and shape infront of a refractor.

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All the bother about vane thickness is moot if one considers this simple geometrical property; the field of view is not a point. If the vanes are exactly rotated so that they present the minimal thickness to on-axis light, they will present ever greater effective thickness as field angle increases, this being greatest for light entering the system perpendicular to the vane orientation.

For example, a 0.5mm vane 50mm wide will project as having a thickness of 0.94mm, or nearly double the on-axis thickness, 0.5 degrees off axis.

Do diffraction spikes have, as a result of larger projected width, greater intensity toward the field edges? No. This is because, for vanes which do not have completely ridiculous thickness, diffraction scales as the linear *length*, not *area*.

Scalloping vane edges is a waste of effort. For one thing, they have no effect whatsoever on light which passes by in the plane of the vane, part of which is nominally on axis. And for off-axis light, which will pass the scalloped edge at some small angle, the effect is really minimal due to the very small angle, with the scalloped edge effectively appearing as still practically straight. In any event, a scalloped edge on a straight vane has zero effect on on-axis light, which is the most important part of the FOV.

One might as well employ relatively thick, rigid, straight-edged vanes and be done. Diffraction spike intensity will be no worse.

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How big is the telescope?
Can you try a plate glass window?

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For example, a 0.5mm vane 50mm wide will project as having a thickness of 0.94mm, or nearly double the on-axis thickness, 0.5 degrees off axis.

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Glenn:

It seems to me though that diffraction effects are only a concern at high magnifications which would typically be centered in the field of view, not 1/2 degree-off axis.



Jon

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