From: "Tom Krajci" 
To: 
Cc: 
Reply-To: "Tom Krajci" 


>From: Rik Hill 

>I did experiments with different "spiders" in the 1970s and reported
>on these in the Warren Astronomical Society newsletter. Among the
>designs tried were wide spider supports, narrow ones and 8 wire
supports.
>The latter was the worst. As I eventually learned (and refused to learn
>by simply reading) was that the diffraction is caused by EDGES not
>by obstruction....

I could have sworn, that Suiter, in this book: http://www.willbell.com/tm/tm5.htm
mentions that amount of aperture blocked (area) also plays a role in
diffraction patterns.  (In other words, a thin wire/vane/whatever and a
thick wire/vane/whatever...both have two edges of equal length...but the
area of obstruction changes.)

If I'm wrong on this citation, please let me know.

Also, anecdotally I've heard that wires are better than vanes because they
have smaller mass...and as they cool by radiation to the night sky...the
smaller mass of a wire support produces a smaller boundary layer that
perturbs incoming light less, compared to a vane.

>Recently, in the work I do with the Catalina Sky Survey, we ran
>an experiment where we were looking for the support that would
>provide the LEAST diffraction for a 4-vane spider support for our
>camera....

>...the best was a vane (regardless of
>the number of vanes) that had small circles along the entire edge....

>...I would be glad to post a before/after image
>on my website if you think it would be useful.

Yes, please!

Tom Krajci
Tashkent, Uzbekistan


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