From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 


>From: "Bill Kocken" 
>
>I don't think you'll have any unusual problem.  Your diffraction spikes
>should look like the squashed X that your spider looks like.  I tend to
>think that each straight spider vane produces 2 spikes, one that follows the
>vane and another on the opposite side of the secondary obstruction.

Hi
 The diffraction spikes are created perpendicular to the spider
vane. As you've stated, a 4 vane spider produces 8 spikes. If the vanes on
opposite sides of the secondary follow parallel paths, they will have the
spikes overlapped to seem as 4 spikes. If the vanes do not follow parallel
paths, they will seem as multiple spikes.

>  A 3
>vane spider thus produces 6 spikes.  A 4 vane spider actually produces 8
>spikes, but you only because the opposite sides produce coincident spikes.
>Your design should do this.
>
>Curved spiders and offset spiders produce diffraction, but not straight
>spikes.

 The curved spider produces a halo of diffraction rather than
spikes. This type doesn't have the advantage of making things more ridged
by tightening the vanes and requires heavier vanes for a given seconary
size. This means more total diffraction. There is no free lunch. Curved
vanes are more practical for smaller scopes.

 The diffraction spikes can be used to aid in focusing for thing
like photography. One can make a small cross that can be flipped into the
aperture, on the side. One then focuses on a star such that the diffraction
cross from this side obstruction matches the secondary vanes. If using a 4
vane, rotate the aid cross by 45 degrees. This gives two diffraction images
with 8 spikes. You just need to match the centers.
 There are other similar obstructions that can be used this way.
try different obstructions, like dowel rods and such. Dwight

>
>Bill Kocken
>
>
>>From: "Russell Jocoy" 
>>Reply-To: "Russell Jocoy" 
>>To: atm{at}shore.net
>>Subject: ATM Spider design
>>Date: Tue, 15 Jul 2003 21:21:01 -0400
>>
>>
>>("atm{at}shore.net")
>>
>>    Hello all again,
>>
>>             I have made my own spider out of extruded aluminum
1"x 1/16"
>>thick. My question to the list
>>is if my spider veins are not at  90 degree angles does this cause an image
>>problem. My anchoring
>>position is more like a squashed X.    I have seen curved spiders, but I
>>don't see how any tension
>>can be applied. I have heard that the diffraction spikes are eliminated by
>>the curved supports.
>>         Am I clogging areas by squashing my supports.. Here are the
>>dimensions
>>   Diameter of cage.... (9.5")
>>  The "X" is squashed to 4.5" between supports on
each side leaving  about
>>11" (on the diameter
>>measurement) on the opposite gap..something like this........
>>
>>                                                              *
>>     *
>>                                                                 *
>>  *
>>                                                                     *    *
>>                                                                        *
>>                                                                     *
>>*
>>                                                                 *
>>    *
>>                                                              *
>>       *
>>
>>                          Russ Jocoy
>>("atm{at}shore.net")
>>                                                                 *
>>  *
>>
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