Ball Scope Project--Source for 22-24" sphere
Posted 11 June 2013 - 02:52 PM
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.)?
Posted 11 June 2013 - 03:12 PM
Posted 11 June 2013 - 03:58 PM
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.
Posted 11 June 2013 - 03:58 PM
Posted 12 June 2013 - 12:47 AM
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.
Posted 12 June 2013 - 09:51 AM
Posted 12 June 2013 - 03:32 PM
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
Posted 14 June 2013 - 11:29 AM
Posted 14 June 2013 - 12:05 PM
Posted 14 June 2013 - 12:27 PM
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.
Posted 14 June 2013 - 12:30 PM
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.
Posted 15 June 2013 - 03:38 PM
Posted 17 June 2013 - 04:02 AM
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.
Posted 17 June 2013 - 06:43 AM
Posted 17 June 2013 - 12:50 PM
Posted 26 June 2013 - 02:22 PM
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).