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Rebuilt my imaging reflector from the ground up.

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#1 Dan Watt

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Posted 16 January 2013 - 07:29 PM

Decided that my 8" f3.9 Orion imaging reflector could use a few tweaks. I wasn't happy with its weight, the stiffness of the tube and the stiffness of the secondary support. I've made CF tubes before so I decided to rebuilt the OTA from the ground up. This scope is only used for imaging so that gives me quite a bit more freedom in the design.

First I optimized the design using Newt, plugging in the numbers for the stock primary and secondary mirror. A slightly wider tube was desired as well since the stock tube didn't offer much in the way of cooling and off-axis light was vignetted. I also wanted to make the tube shorter so it wouldn't be affected as much by wind gusts.

I picked up a 9.5" concrete forming tube from Home Depot (after carefully measuring them all before I selected the one I would purchase to be certain I got the most-round sample I could find) and covered it in a thin layer of mylar sheeting followed by some Velocity 85 mold release wax.

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Then I slid the first layer of CF sleeve over it

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A layer of fiberglass was sandwiched in there to increase tube wall thickness without straining the budget

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And another two layers of CF 2x2 twill fabric

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#2 Dan Watt

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Posted 16 January 2013 - 07:38 PM

Next I cut the tube ends with a hacksaw using a piece of tape as a guide. Hacksaw cuts through CF pretty quickly.

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Using a holesaw I carefully measured and cut the focuser hole. I cut the hole along the seam... since the focuser (and camera) will be pointed towards the ground when the scope is mounted this positions the seam in the least conspicuous spot.

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Plenty of sanding to get things smooth. Its been unusually cold here and I don't have heat in my garage so the epoxy was a bit thick and hard to get it to flow out evenly. No matter, it sands like butter with the right tools and abrasives.

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After sanding up to 600 grit I sprayed two coats of adhesion promotor and then four coats of PPG automotive urethane clear coat.

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Since I won't need to be rotating the tube I could get away with hard-mounting the dovetails using radius blocks. This saves quite a bit of weight too when you compare the radius blocks to big heavy parallax rings.

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Built a custom spider out of 1/16" aluminum. The stock spider, while featuring some pretty thin vanes isn't very stiff... you can easily bend the vanes while performing secondary mirror adjustments. In this picture you can see the focuser is test fitted... I also upgraded to the Moonlite with stepper motor.

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#3 Dan Watt

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Posted 16 January 2013 - 07:41 PM

Finally, the scope in action.

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I also made a new mirror cell out of CF and a few parts from the old cell. The original cell was a clunky cast piece that weighed as much as the mirror, if not more so.

And a cool sunset picture:

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I still have a few things on the to-do list... first is to spend some time carefully adjusting the secondary spider vanes so everything is perfectly square. I also need to get some tube flocking material.

I used it Saturday night... Scope was around 60F when I got it out of my car and did my initial align and focuser... temp dropped to below 20f that night. I checked focus and I just barely had to give it a tweak. Nothing like the old steel tube that would need babysitting throughout the night. It is significantly lighter... I have no bathroom scale but look at the photos and note the position of the counterweights... with the steel tube and rings the weights were at the bottom of the counterweight bar AND I needed an extra three pound weight.

Here is first light with the scope, IC443.

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#4 Tim Gilliland

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Posted 16 January 2013 - 08:07 PM

Very, VERY impressive! Now I need to build a tube for my 10". And spider, and mirror cell.

#5 Pinbout

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Posted 16 January 2013 - 08:15 PM

that is awesome.

looks like the radius block worked great.

how was it to release the tube?

#6 Dan Watt

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Posted 16 January 2013 - 08:28 PM

Thanks!

Releasing the tube was quite easy. I realized the darn thing only costs $6 or so and I don't plan on reusing it anytime soon so some quick work with a razor blade and I was able to tear it right out. It didn't adhere to the CF at all because of the mylar sheeting + mold release. I've used other plastic sheeting in the past for CF and fiberglass work and I gotta say, Mylar is by far the best I've tried. Practically no effort to separate it.

#7 MDB

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Posted 16 January 2013 - 09:14 PM


Nicely done......the scope and the image!

Mike

#8 azure1961p

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Posted 16 January 2013 - 09:29 PM

Beautiful job!

Pete

#9 David Pavlich

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Posted 16 January 2013 - 10:22 PM

That is pure terrific! You should be proud.

David

#10 Raginar

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Posted 16 January 2013 - 10:35 PM

Wow. Question, how is your focuser? Obviously great picture, but do you get any flexing? I don't have much experience working with CF.

#11 Dan Watt

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Posted 16 January 2013 - 10:41 PM

Focuser is amazing, I have had several other focusers including the Orion dual-speed and the JMI EV3n and I must say the Moonlite is miles ahead of all of them. The stepper motor system makes things really sweet, I can achieve perfect focus without having to get up from the laptop.

There is no discernible flexing, the CF tube is much much stiffer than the old steel tube. In fact, I can stand on top of it and it hardly bends and I weight nearly 270 lbs.

#12 Raginar

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Posted 16 January 2013 - 10:48 PM

Hey Dan,

Never mind, i re-read your HOWTO and it makes sense. Can you explain how you re-did the mirror area as well?

I have an AT8IN that would be fun to do this way.

#13 Dan Watt

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Posted 16 January 2013 - 10:54 PM

Not familiar with using a balloon for this, could you elaborate? My method, as far as I can tell is as simple as possible. Take a cardboard tube, put a layer of plastic on it and then wrap CF around the tube while applying resin. No need for a vacuum bag or anything. A layer of peel ply might be helpful but we aren't going for every last bit of weight reduction, to me the stiffness and thermal properties of CF were the most desirable. Being significantly lighter is a nice plus.

#14 nirvanix

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Posted 16 January 2013 - 11:11 PM

You could/should be in the telescope making biz. :bow:

#15 jtrevino

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Posted 16 January 2013 - 11:43 PM

WOW!!!! that thing looks AWESOME!!!! Great job!!!!

#16 Dan Watt

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Posted 17 January 2013 - 12:00 AM

Hey Dan,

Never mind, i re-read your HOWTO and it makes sense. Can you explain how you re-did the mirror area as well?

I have an AT8IN that would be fun to do this way.


I'll have to take some pictures, I'm afraid I never got a picture of the cell. I should take it apart this weekend to do some finishing touches so I'll try to remember to grab some then.

#17 Raginar

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Posted 17 January 2013 - 12:30 AM

Cool, thanks for making this HOWTO. It's really awesome :)

#18 Mirzam

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Posted 17 January 2013 - 02:42 PM

Where did you get the CF material, and are there any special considerations, such as type, thickness, weave etc? Are all the sleeves made of the same material? What type of resin was used?

Thanks for the report! Really makes me want to try it myself.

JimC

#19 Dan Watt

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Posted 17 January 2013 - 03:51 PM

I used 3k 2x2 weave and 6k 10inch sleeve CF from Soller Composites along with West Systems 105 epoxy with 205 hardener. When laying down the 2x2 weave it helps to do it at different 45 degree angles for each layer to get maximum strength.






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