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 Post subject: Sauron's Other Eye
PostPosted: Mon Nov 17, 2008 8:36 pm 
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Super Nova
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Time for the official introduction!

I've used it twice now, but its still not 100%. I've got it down now for some stiffening modifications and some work on the mirror cell to make collimation easier. Tomorrow is "third light", and I think that will be the charm!

Here's Elaine's introduction:
Image


This is the top end, showing the wire spider. That's working surprisingly well.


Image


And here's the bottom end, showing the center mounted conical mirror. It takes 35 pounds of counterweights to keep the center of gravity low enough for this short rocker box. Whether that's worth it will be found out as I use it.


Image


Still needs some tweaks, but overall I'm pretty happy with it.

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PostPosted: Mon Nov 17, 2008 8:58 pm 
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Wow, very nice, Dave!!! :cool:

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PostPosted: Tue Nov 18, 2008 5:18 am 
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molecule
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Wow indeed Dave! Very nice! Love the design, including the paint scheme.

I'm sure more questions will come to mind eventually, but for now, how do you adjust collimation at the secondary. I could see enough from my computer screen to tell how it's suspended, but not fully how you adjust it.

Gary


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PostPosted: Tue Nov 18, 2008 6:46 am 
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Velocepedist
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Dave, just gorgeous! The color, the unique box design, the conical mirror, the wire spider...it's the whole package. I absolutely love the box design, with the circular bearing that almost resembles the joint of an elbow or knee. Holy cow!

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PostPosted: Tue Nov 18, 2008 9:53 am 
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Super Nova
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Gary wrote:
Wow indeed Dave! Very nice! Love the design, including the paint scheme.

I'm sure more questions will come to mind eventually, but for now, how do you adjust collimation at the secondary. I could see enough from my computer screen to tell how it's suspended, but not fully how you adjust it.

Gary


Here's a schematic:

Image

The basic support is a pvc pipe with a piece of plywood that mounts a 5/16" bolt. The bolt passes through a rubber grommet that supports the weight of the secondary (that's the the black rectangle in the picture) and absorbs vibrations. It's a pretty tight fit, which is good. The bolt is nested inside a smaller length of pvc, which is drilled to accept three small bolts penetrating radially. One of them is shown in the graphic. These adustment bolts press against a regular hex nut that's mounted on the threads of the main support bolt.

The advantage of this system is that the swivel point for the secondary is closer to the centerline of the mirror face than most secondary adjustment points. This way when you adust the angle of the mirror, there is less lateral movement to compensate for. Also, the central nut and bolt arrangement allows for easy rotation if necessary, and a straightforward method of centering the secondary longitudinally.

There are a number of design flaws in the structure of this scope, but the secondary arrangement is one of the things that is working out well. I like it enough to want to use it when I redo my 10", and if I ever build a bigger scope, I think I'll stick with it.

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PostPosted: Tue Nov 18, 2008 1:05 pm 
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molecule
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Very nice Dave! The advantages of the design are obvious (now that you describe them;-) Please keep us updated on any future tweaks.

Thanks.

Gary


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PostPosted: Tue Nov 18, 2008 8:08 pm 
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Now you've got me thinking about tweaking my design on my new dob project.

Man, that is sweet! :cool:

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-Erik Wilcox
Homebuilt 16" Truss Dob
8" Antares f/5 Newt
SV 80mm ED Nighthawk NG on M1 ALT/AZ
Nikon Prostaff 65mm spotter on Trekpod
Konusvue 20x80 binos/Peterson PipeMount


"Newt Gingrich is what stupid people think smart people sound like."
-The Great Paul Krugman

Evolution is both fact and theory. Creationism is neither. -Anonymous


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 Post subject:
PostPosted: Tue Nov 18, 2008 8:58 pm 
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Velocepedist
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Dave, not only does collimation of the secondary in your design cause less impact on optical axis shift, but it has a better center of gravity in regards to weight distribution. I imagine it will actually hold collimation better than a standard design. Intruiging. I like.

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PostPosted: Wed Nov 19, 2008 6:58 pm 
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Super Nova
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I really like the mechanicals of the support so far. I'm not quite as confident about the wire arrangement yet. I guess I won't know until it's stretched as much as it will stretch and holds its tension for a reasonable period.

I must admit, though, it's very quick and easy to tighten when the tension does decrease.

I think I still need to work some on the mirror cell. There's definite collimation shift that I don't think is coming from the secondary, although I haven't ruled out the truss structure yet either.

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 Post subject:
PostPosted: Wed Nov 19, 2008 9:25 pm 
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quasar
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She's a beauty, Dave.

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