I just completed Kirk's 8" F/6.3 Ballscope yesterday and Kirk is coming over to pick it up. I am pretty excited and nervous about it. This is my first telescope that I have built for someone else (and getting paid for it). I applied some modifications to my original ballscope design that makes this one the best one yet.
Since this was a longer focal length, one of the things I wanted to do was to allow Kirk the ability to tear the scope down for transport. My 8" ballscope is short enough that I can leave the struts in all the time. If I left Kirk's that way, he would have to lay the scope down for transport. Since my mirror cell is based on R.F. Royce's design, the mirror stays in the cell by friction, but when tilted at 90 degrees (under normal use is fine) and bumped around too much (like during transport), it will fall out when at 90 degrees. So being able to remove the struts is a must.
With previous designs, I used threaded rods to push up against the two rings that sandwich the lip of the ball and down against the bottom part of the mirror cell. This is fine when the struts stay put. But when unscrewing the struts, there is leverage there that can cause the mirror cell to twist inside the ball. The threaded rods are not rigid enough to prevent that from occurring. So I placed some 1x2 struts inside the ball that attaches to the mirror cell and the top two rings. No more twisting now!
Another problem I have had in the past was centering the mirror cell in the ball and making sure it is parallel with the ball opening. I solved this by using a threaded spacer (1/4 20 thread) so that the bolt that sticks up from the center of the ball attaches to another bolt that attaches through the center of the mirror cell. This lines everything up making the mirror cell parallel and centered. As in my previous ballscope designs, collimation is done from the top.
On the secondary ring, I use captive knobs to prevent anything from dropping on the mirror during assembly. They are held captive by use of nylon lock nuts and washers. The washers are 1.25" washers so that the strut tubes don't damage the wood.
The red-dot finder bracket is attached using angle aluminum and is attached from the top of the ring. This is important as it supports the corner of the aluminum to prevent bending. I used Kydex for the light baffle and attached Flocking paper from Protostar here in Columbus. The light baffle is attached using velcro for easy attachment and removal. The focuser is a Kineoptics HC-1 1.25" helical focuser. This focuser is only 2 ounces which is great considering the weight considerations. The spider is from Meridian Telescopes in Michigan.
Speaking of weight, I placed about 15 lbs of weight at the bottom of the ball. It is sandwiched between the bottom of the mirror cell and the ball. I used a combination of barbell weights and good old sand bags from ankle weights.
The 16" Polyethelene ball was purchased from Formed Plastics, Inc. out of New York. I went with Polyethelene because it has less friction than Polycarbonate and also strong. Maybe not as strong as Polycarbonate, but strong enough for a purpose of an 8" telescope.
I had the pleasure of using the telescope last night under the stars. Kirk really did a nice job on his mirror. I was surprised to see how nice it moved. Much better than my 8" ballscope (it uses Polycarbonate which has way more stiction). The scope does vibrate, but it dampens in less than 3 seconds. Scopes that use a single secondary ring are more prone to direct light pollution than scopes with a solid tube or two ring designs. But when away from those lights, they perform very well. Plus, they are very light and I like the aesthetics of it anyway.
That pretty much wraps this project up. I am using the proceeds from this project to fund my next project: a 16" ultralight built just like my 12.5". The scope will be the replacement for my 18" Obession I sold. I hope to have it completed by winter.
Let me know if you have questions.