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Ed Grissom Curved Spider

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#1 Daniel Mounsey

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Posted 01 February 2005 - 09:32 PM

For those of you who never got to see what Ed Grissom's curved spiders look like, here you go. This is the way it should be done. As I mentioned before, the complete dispersal of diffraction requires that the curve be 180 deg. This is accomplished using three 60 deg. arcs totalling 180 degrees.

People say a four vane spider is just as good. Well, they are dead wrong! There is no scintillation bleeding off planetary limbs and there is no glowing around planets as seen in poor designs. My observing team and I have seen this during very critical planetary observations in side by sides at Mt. Wilson and there's no comparison. The remaining image looks like a steel ball bearing from a pinball machine. Overall there is less diffraction with the removal of one vane and the thickness has been cut in half, equivalant to about 6 sheets of paper.

The width is 1.5" all the way down to maintain rigidity and yes it holds collimation rock solid. Another thing that floors me about Grissom's work is that after everything is installed, The vanes can be tilted like the flap on a plane wing by adjusting two screws. This allows you to get the diffraction down to the point where it's barely even there.

Note, if you look carefully at this first picture, the upper right vane looks a tad thicker because I left it improperly tilted for comparison. The second photo coming up gives you and idea of the vane after the tilt has been made.

Like I mentioned before, this allows you to fine tune the diffraction down to a minimal. This is a completely rebuilt and modified 12.5" F-5 Zambuto Starmaster which will be featured in my latest planetary shoot-out against the best. Numerous changes were made, but you'll see them in the review. Hope you guys enjoy it. I could have never accomplished this without the help of others.

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#2 Daniel Mounsey

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Posted 01 February 2005 - 09:33 PM

pic#2

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#3 Daniel Mounsey

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Posted 01 February 2005 - 09:36 PM

Pic#3 for overall view.

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#4 celestial_search

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Posted 01 February 2005 - 10:31 PM

Thanks for the info and pics. Interesting.

#5 erik

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Posted 01 February 2005 - 11:43 PM

very nice! i'm sold on curved spiders as well, although i've never tried ed grissom's spiders. i have heard about them though. for planetary viewing, i wouldn't even consider a reflector with a straight spider...

#6 Brian Carter

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Posted 02 February 2005 - 01:05 AM

i just recently upgraded to a curved spider, 1800destiny i believe. I finally got a look at Jupiter for the first time since changing. I never noticed objetionable diffraction around saturn, but Jupiter is bright enough to make it really ugly. But the curve eliminated them, made it looked great.

#7 Guest_**DONOTDELETE**_*

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Posted 02 February 2005 - 01:22 AM

Where or how are they available?

#8 Guest_**DONOTDELETE**_*

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Posted 02 February 2005 - 01:23 AM

Daniel, that is the best looking spider I've ever seen. Grissom is a master.

#9 erik

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Posted 02 February 2005 - 01:57 AM

as thin as those vanes look, i'm a bit surprised that there's no vibration problems. with the large heavy secondary mirror in my 16" dob, i had to go with 1/8" thick vanes...

#10 Matthew E

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Posted 02 February 2005 - 11:10 AM

Pardon my ignorance, but why does having curved spiders help jupiter? or any other object for that matter?

Thanks guys.

#11 Daniel Mounsey

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Posted 02 February 2005 - 01:01 PM

Hi Matthew,

What happens with a four vane spider is that light bleeds off the planetary limbs, making the edges of the planets look less defined and the brighter the planet, the worse it gets. This is not as good when studying the structure of Saturn's "A" ring for example. By eliminating the spikes, there's less light concentration for scintillation to take place. Using a curve does not eliminate diffraction, but instead, disperses it evenly against the entire field instead of it just being concentrated into the diffraction spikes. If the curve is too thick though, it creates a white cloud that looks invisible until seen against a bright object, and the halo can destroy the overall contrast of the image.

Erik,

1/8" is thick but remember too that the diffraction or thickness of the vane is relative to the area being used as well. I'm glad that people like the Destiny. The only qualms I have with the Distiny are two. One, the vanes are still thick and two, it uses three 180 degree arcs. Only one 180 degree arc is needed to completely disperse the diffraction and having more only multiplies the diffraction. In a three vane, the arcs only need to be 60 degrees. This uses less material and cuts down diffraction even more. You won't notice this until you have a frame of referance or compare it to another curved design.

Bryan Greer at Protostar sent me one of his first curved Prototypes for testing and it's actually the correct design because it uses only one 180 degree arc and the vanes are relatively thin. Even though it may not look as aesthetic on the outside to some, it does produce aesthetic images.

As far as vibrations, there are none in Ed Grissom's. He's not selling his spiders, but he observes with us. That's why I was able to have him to do this one. I'll post another pic from the side so you can see why it's rigid. The metal he uses has a serious tendancy to stay straight, which also adds to the rigidity.

#12 Daniel Mounsey

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Posted 02 February 2005 - 01:14 PM

Here's a side pic. It doesn't taper in like a typical four vane.

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#13 Daniel Mounsey

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Posted 02 February 2005 - 01:28 PM

Here's another used on a 12.5" F-6 super planetary Newtonian which belongs to the Pons master. This vane material is a bit thicker because it's older and still produces exquisite planetary images. I've seen numerous divisons in Saturn's ring system with it and we've conducted numerous side by sides with the big apos.

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#14 kiwisailor

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Posted 02 February 2005 - 01:48 PM

As far as vibrations, there are none in Ed Grissom's. He's not selling his spiders, but he observes with us. That's why I was able to have him to do this one

You tease :)
Good looking setup though Daniel.

Steve

#15 Starman1

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Posted 02 February 2005 - 02:49 PM

Daniel,
Planetary scopes with shiny silver screws and nuts inside the tube? And no flocking? For shame!
:lol:

#16 Daniel Mounsey

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Posted 02 February 2005 - 06:22 PM

You crack me up Don. I knew someone would say something about that. That's my next project. Right now, I'm just getting this thing back together.

#17 erik

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Posted 02 February 2005 - 09:14 PM


Erik,

1/8" is thick but remember too that the diffraction or thickness of the vane is relative to the area being used as well. I'm glad that people like the Destiny. The only qualms I have with the Distiny are two. One, the vanes are still thick and two, it uses three 180 degree arcs. Only one 180 degree arc is needed to completely disperse the diffraction and having more only multiplies the diffraction. In a three vane, the arcs only need to be 60 degrees. This uses less material and cuts down diffraction even more. You won't notice this until you have a frame of referance or compare it to another curved design.



yeah, i've heard that the total area should add up to 180 degrees. i wonder if the difference would really be visible though. i've never seen any hint of diffraction in either of my destiny spiders, no matter what i'm viewing. i really like the ed grissom design though, very sleek looking. it's too bad that he doesn't sell them...

#18 sixela

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Posted 02 February 2005 - 11:11 PM

yeah, i've heard that the total area should add up to 180 degrees.

The sum of the angles the vanes are turning (if they're all sections of a circle, just to be pedantic).

i wonder if the difference would really be visible though. i've never seen any hint of diffraction in either of my destiny spiders, no matter what i'm viewing.


You don't see diffraction spread out evenly very easily, but that doesn't mean it's not there -- diffracted light that is evenly distributed simply reduces contrast.

It's all a fine line to tread: there's actually *less* diffracted light with a straight spider, by virtue of the fact that it's rather hard to make something that's shorter than a straight line. But the detail it blurs, it blurs in preferential directions.

There's little point in making each vane of a three vane spider turn 180°, though, given that it makes the vanes longer and that you don't need each individual vane to spread its diffracted light evenly. Three vanes turning 60° are shorter, and spread the diffracted light in three sets of two opposed 60° sectors (and 3*2*60°=360°).

I have to :bow: looking at that spider - the inside end of each vane is perfectly parallel to the outside end of the next vane...

#19 Daniel Mounsey

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Posted 03 February 2005 - 12:10 AM

Sixela,
I agree. What you stated is quite true. Thanks for the compliments as well.

#20 gazerjim

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Posted 03 February 2005 - 01:08 AM

I like it. Might it also be even better to line up the intersection of each vane and the tube with a mirror tab?
Thanks for posting.

Jim

#21 Daniel Mounsey

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Posted 03 February 2005 - 10:46 AM

Hi Jim,
It really wouldn't make mich difference since the tabs are turned during observation. I didn't bother to turn them for the photos, but point well taken. I would say that if the mirror tabs are left, it would be nice to line the vanes up with them.

#22 NHRob

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Posted 03 February 2005 - 09:04 PM

How do I get in touch with Ed Grissom to get one of these?

Rob

#23 Barry Fernelius

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Posted 04 February 2005 - 11:06 AM

NHRob,

What an avatar! After seeing yours this morning, I spewed hot tea all over my monitor!

This particular... configuration... makes astronomical viewing a bit more difficult!

#24 Daniel Mounsey

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Posted 04 February 2005 - 12:11 PM

Hi Barry,
Please fill us in.

#25 NHRob

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Posted 04 February 2005 - 12:53 PM

Hi Barry,

Thanks. I was wondering if it would offend anyone.
It's a great new Pilates move.
Rob
:john:


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