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Ball Scope Project--Source for 22-24" sphere

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#1 astrobug

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Posted 11 June 2013 - 02:52 PM

I'm going to be rebuilding my 13" f/4.5 into a ball scope. Steve Swayze is refiguring the mirror right now, and I'm hoping to get it back from the coater in a month or two.

The old mechanics were fairly crude and left the mirror too exposed (as evidenced by my daughter's handprints). My plan is to build it into a ball scope for now, and then make a trackball base down the road. I'm now looking for a reasonably priced 22-24" plastic sphere. I've read that polycarbonate is preferable to acrylic for a ball scope, but it still may require reinforcing (in which case, I will add a layer or two of fiberglass to the inside). So far, this is the cheapest source I've come across:

Formed Plastics 24" spheres

They want $190 for a 24" polycarbonate sphere, plus another $65 for shipping, which seems a bit much for this project. I could go a little smaller and/or switch to acrylic to save some money, but this will mean more counterweight and/or reinforcing.

Does anyone know of any cheaper sources for a 24" polycarbonate sphere? Any other ideas about what might work (aluminum, etc.)?

Thanks.
-Brett

#2 mark cowan

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Posted 11 June 2013 - 03:12 PM

Have you looked at the large animal toys (like zoo supplies) - these are durable spheres. I don't know where to find them but I've seen them online so I know somebody linked them at some point. Also there are buoys and other floatation spheres that might work, again, no links. :shrug:

Best,
Mark

#3 Mike I. Jones

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Posted 11 June 2013 - 03:38 PM

Google "large plastic spheres", "large acrylic spheres", "large polycarbonate spheres", etc.

This looks interesting, though comparable in price.

Mike

#4 mconnelley

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Posted 11 June 2013 - 03:58 PM

Hello:

For my 20" ballscope, I needed a 30" sphere. I went with an aluminum hemisphere, such that the 'equator' is tilted to the optical axis by 30 degrees to allow the scope to point to the horizon without coming off of the ground pads. I used a spun aluminum hemisphere. The key is to find a shop that does metal spinning and already has an appropriate tool. One lesson I learned is not to underestimate the need for the ball to be stiff. Even though I reinforced the hemisphere, the scope still 'bounces' quite a bit, which is especially noticeable at high power in windy conditions. I believe that this is due to flexure in the hemisphere itself. Regardless of what kind of ball you go with, make sure you know the wall thickness.

Cheers
Mike Connelley

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#5 bremms

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Posted 11 June 2013 - 03:58 PM

acrylic is lousy compared to polycarbonate. Lousy to machine and not very strong. Any situation requiring any mechanical strength acrylic is not a great material. It will work, but poly carbonate is much better.

#6 astrobug

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Posted 12 June 2013 - 12:31 AM

Mark-
Thanks for the tips. I found these: Boomer Balls . If they can stand up to a polar bear, they should be sturdy enough for a telescope. Cons for my project are weight and size (20" will be too small, 30" is probably too big).

#7 astrobug

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Posted 12 June 2013 - 12:47 AM

Mike-
That's quite a scope...I like it. IIRC, there was going to be an 18" Portaball with a similar configuration (tilted hemisphere). I think I'll use a fuller sphere on mine, though, as part of the appeal of this design for me is the ability to change the eyepiece orientation, depending on where the telescope is pointed. Spun aluminum might be a better choice if I can get it thick enough.

#8 kfrederick

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Posted 12 June 2013 - 06:30 AM

http://engineeronline.ws/spheres.htm

#9 kfrederick

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Posted 12 June 2013 - 06:33 AM

http://www.hooksandl...ss-spheres.html

#10 kfrederick

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Posted 12 June 2013 - 06:35 AM

http://www.alibaba.c...tal_sphere.html

#11 kfrederick

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Posted 12 June 2013 - 06:39 AM

http://www.cn-sphere.com/

#12 kfrederick

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Posted 12 June 2013 - 09:51 AM

The fiberglass ones come in two halves .and 1/8 thick should be easy to beef it up on the inside

#13 coinboy1

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Posted 12 June 2013 - 03:32 PM

Here is an interesting idea that I thought of while checking out my friends new composter. Its a 27
or 32" diamater sphere and has an opening... You can pick it up for about $150-$200 online. Google: compost sphere The 50 gallon is 27" dia, 70 gallon is 32" diameter

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#14 don clement

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Posted 14 June 2013 - 11:29 AM

In May 1930 R.W. Porter had pencil on vellum concept plans using a 50 ft. diameter sphere mounting for proposed 200" telescope. Looked amazingly like a scaled up version of the ball scope except in Porter's version the 200" tube assembly fit inside the 50 ft. diameter sphere.

Don Clement

#15 kfrederick

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Posted 14 June 2013 - 12:05 PM

Machine a 50ft ball that would hold the 200 inch and track might not be easy.

#16 don clement

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Posted 14 June 2013 - 12:27 PM

Actually a sphere is one of the few geometric forms that can be automatically generated, precision ball bearings for example or the sphere generated before parabolizing.
Porter's ball mounting was only a concept in 1930 however Porter's split-ring mount was used in the final design for the 200". In addition Porter had plans for tracking his 200" ball scope on the same drawing in one of his cutaway views.

Don Clement

#17 orlyandico

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Posted 14 June 2013 - 12:30 PM

I tried making a ball scope before using a lighting fixture. What I learned:

1) acrylic is too soft, I ended up reinforcing the interior with fibreglass tape and lots of epoxy

2) if the ball is too small, you will need lots of weight inside the ball to balance the UTA. When the insides of the ball gets heavy, the ball surface "dents" where it touches the teflon contact pads

3) no matter what you do, balance is only achieved with a small range of top weights (and the UTA must be as light as possible)

4) this guy - http://www.sff.net/p...kball_mount.htm - has a very nice refinement of the ball scope: he adds tracking!

he's got some discussion on spheres to use here - http://www.sff.net/p...ball_optics.htm - and mentions a 16" mussel float that's only $17. That would have been big enough for my 10" mirror, but it's obviously too small for yours.

#18 don clement

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Posted 15 June 2013 - 03:38 PM

R.W. Porter's May 1930 Ball mount concept for the proposed 200" with 50 ft. diameter ball note the OTA is totally enclosed by the ball:
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#19 careysub

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Posted 17 June 2013 - 04:02 AM

Their hemispheres might be the right thing - a 24" (suitable for a 16" mirror?) is $133, a 36" hemisphere is $300, but they are thin (as little as 1/16" at the apex) and need serious reinforcing.

One idea for reinforcing I am throwing out for consideration is to use sheets of Divinycell, foamed PVC, that is heat formable and with good compressive strength (87 psi). The idea is to cut pieces of the foam sheet to tile the inner surface, then heat each one and press into place with epoxy resin adhesive, doing just one tile at a time.

1/4" thick Divinycell costs about $2.40 per square foot, a small incremental cost.

When the tiling is complete then finishing the inner surface with fiberglass and resin. Perhaps a weather balloon inflated could press the fiberglass in place while the resin sets.

#20 kfrederick

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Posted 17 June 2013 - 06:43 AM

If the contact points could have a triangle and the one point be three like on a mirror cell.Then the ball could be thinner . A fiberglass layup could beef up the inside . Here is a pic of a tracking mount for a ball

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#21 don clement

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Posted 17 June 2013 - 12:50 PM

R.W. Porter's drive concept in May 1930 for the ball mount of the proposed 200"

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#22 astrobug

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Posted 26 June 2013 - 02:22 PM

Sorry to take so long in getting back to the thread I started. Thanks for all of the great ideas. The spherical composter idea is perhaps the most intriguing, but the reviews from people using it as a composter say it doesn't roll too smoothly, so I'm doubtful that it will work as a telescope mount. Anyhow, I think I need to make some phone calls to local places that do metal/plastic/fiberglass and see what I can come up with.

#23 careysub

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Posted 26 June 2013 - 07:43 PM

...The spherical composter idea is perhaps the most intriguing, but the reviews from people using it as a composter say it doesn't roll too smoothly...


This could be because it is partly filled with a heavy wet plant mass that makes it grossly weighted off-center (piled up in the bottom) no matter what you do, not because it is not round (but that might also be the case).

#24 AlphaGJohn

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Posted 27 June 2013 - 09:41 AM

Amazing drawings, Don. Thanks for posting those !

John

#25 neotesla

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Posted 27 June 2013 - 12:03 PM

20" but probably would need to be reinforced...

http://www.1000bulbs...s-with-opening/

found this link in this article...

http://www.sff.net/p...n/Astroscan.htm






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