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Diffraction from spider vanes, confusion.

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#1 Joe1950

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Posted 23 September 2019 - 01:49 PM

I have a 6", f/6 mirror from the 1970s that I rebuilt into a DOB. Using a sonotube and Poplar wood, rather than a metal tube and press-board, it is very lightweight for taking out. 

 

I bought a 3 vane spider for it and it is well made. But the vanes are made of 1/8" screw stock, painted black. So I did some rough calculations.

 

6" mirror (full size full thickness, real Pyrex).

1.52" secondary.   25.3% central obstruction.

Each vane (3" length over mirror - 0.76" secondary radius  =  2.24" of each exposed (in the light path) spider vane, X 3 vanes =  6.72" of exposed (in the light path) vanes.

 

So using the 1/8" thickness of the vanes (machine screw stock) the linear amount of mirror obstructed by the vanes is 0.84" (6.72" x 0.125"), or 14% more obstruction.

 

 

If I'm figuring this correctly, that works out to an additional 14% obstruction, in addition to the 25% for the secondary or a total of 39% total linear obstruction?

 

If I'm mis-calculating this, please let me know.

 

 

 

What I ended up doing, was to replace the screw stock with 1/2" flat brass, which is 0.016" thick. The additional obstruction from this is, with painting added, about 2.25% additional obstruction, for a total of just less than 28% total obstruction.

 

 

So that is a big difference.

 

 

I ask if this line of thinking is accurate since I do see some commercially available scopes with very thick (plastic) spider vanes. Some are made for imaging, but some are for visual also. I would guess, some come close to 45% obstruction.

 

With spider vanes, does the diffracted light go into the Airy rings, or does it add to the overall background of the FOV.

 

I have the feeling, I may be missing something.

 

Thanks, joe

 



#2 Jon Isaacs

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Posted 23 September 2019 - 02:04 PM

Joe:

 

I think you need to keep in mind the units, you are calculating areas. In engineering, keeping the units straight is half the battle.  I think your area calculation is right,  0,84 square inches.  

 

The area of the secondary mirror is 1,82 square inches.  So the total obstructed area is 0.84 + 1.82 square inches = 2.66 square inches.  

 

The area of the primary mirror is 28.3 square inches.  

 

That means by area, the obstruction is   2.66/28.3 = 0.094 = 9.4%

 

If you want to convert back to linear obstruction, then you need to take the square root of 0.094 = 0.307 = 30.7%

 

Jon


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#3 JamesMStephens

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Posted 23 September 2019 - 02:14 PM

The diffraction from the spider vanes doesn't go to the rings, and a direct comparison of the area obscured by the vanes to the area of the secondary mirror isn't really valid.  I don't recall ever having seen a calculation of this sort for the vanes themselves (it would be a more complicated calculation than looking at the energy directed to the rings vs Airy disks for a circular obstruction--it seems someone must have done it and it might be something another member knows about)  Your three vane spider will produce six diffraction spikes vs the four you see with a four vane spider (Technically the four vane spider produces eight diffraction spikes, but they overlap!)  

 

Jim


Edited by JamesMStephens, 23 September 2019 - 03:05 PM.

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#4 Joe1950

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Posted 23 September 2019 - 02:23 PM

Jon, James, thanks very much. I had a feeling I was going about it all wrong because I was mixing area and linear. And, yes, after thinking about it, the diffraction from the vanes are doing something different than a central obstruction.

 

Very good! Thank you!

 

I always wanted to be an engineer but was too short to see out of the Locomotive window.


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#5 MitchAlsup

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Posted 23 September 2019 - 07:05 PM

In any event, My 20" F/4 DOB uses 0.007,5" thick roof flashing (galvanized mild steel) as its spider vane material.

It is plenty strong,

It blocks as little light as possible.

The image looks more like a binocular than a vibrating telescope.

 

You see the amount of light diffracted into the spikes around stars is proportional to the area of the aperture that is blocked.

 

You vanes at 0.125 are 16× thicker than mine (and I am trying to imply) way too thick.

 

If a 20" can use such thin material, so can a smaller scope.


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#6 Joe1950

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Posted 23 September 2019 - 07:57 PM

Hey Mitch! The .125” vanes, which I measured with my calipers and are actually 0.134, or #6 screw stock, came with the spider and secondary holder. Nicely made, but I thought too thick also. 

 

So I bought some flat brass stock at the home store that came in strips of 1/2” x 6” x 0.016”. So I replaced the screw stock with the flat brass at 0.016” thick, 3 vane spider. 

 

Also I got some brass #6 machine screws and nuts and was able to solder the flat stock to the brass screws to make up the vane ends. 

 

So it worked out pretty good. Where the mirror is, and at the center of the tube where the altitude bearings are, and at the front, where the focuser and spider are, I doubled the thickness of the sonotube by gluing an extra layer inside. It really helps the strength at those points.

 

I think some use thin wire for their vanes, but that’s too much work for me.

 

Thanks Mitch! That 20” scope must be really nice!


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#7 Joe1950

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Posted 25 September 2019 - 03:45 PM

A fantastic essay on maximizing the effects of light management and minimizing the effects of diffraction was written by the talented Ed Turco. He tuned a 6” reflector to get refractor like contrast and detail. Very much worth a read!


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