How about this one?

I've been watching Lee and Chuck have all the fun so I thought I'd add mine to the list. This is an 8" F6. The tube is 10" in diameter. The rocker box side is about 12" wide.

I finished this in May after 5 months of work. I wish I could say Lee learned everything he knows from me, but it's really the opposite. He was very helpfull in some off-line exchanges that made thing go much better.

The only major problem is that the base flexes more than I would like. I have plans to rebuild it using baltic birch in the near future. Quick question, would 3/4" be enough to support a 30 pound OTA without flexing or should I laminate a couple of pieces together for the sides?

thanks,
jerry

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Hi Jerry,

Hubba hubba ;^). That's nice!

To arrest all flexure in this design, you'll need to do a few things (I learned it the very hard and very expensive way):

1) Widen the sides of the arch on the rocker uprights to 3.125" or greater.

2) Laminate two sheets of baltic birch together for the rocker uprights. The width for each upright, if you are going to cut out the center of the panel as you have (and for a ~30 lb. OTA), should be at least 1". I'd go with 1.25" myself. 1.5" would likely be overkill, but it won't add that much weight, will look nice and will put you well in the safe zone. The dark cherry one in the other thread has 1-3/8" rocker sides and it is plenty.

3) Triangulate the rocker just inside the upper eyepiece racks as shown in detail the other thread.

4) Optional (but to achieve ultimate stiffness): Cut and install a semi-circular aluminum brace from 1/4" thick aluminum, 1" wide and long enough to span across the back of the rocker bottom plate from rocker upright to rocker upright. Install it with countersunk screws just inside the Ebony Star ring of laminate and Teflon pads underneath the rocker plate (hidden from view). This will all arrest flexure in the bottom plate that results from those attractive and decorative half-moon cutouts. Or, just leave the bottom plate one solid 3/4" piece. I prefer to reinforce, though it is only necessary on a tall 8" f/7 and 10" scopes f/5 and longer.

5) Optional: You notice I used a really thick ground board because the load is cantilevered. You can cut some more reinforcing aluminum in the shape of your extension feet and screw it to the bottom of each one to add stiffness. I do/did this on all scopes larger than an 8" f/6 until I went with the the double thickness laminated ground board.

Aluminum cuts pretty easily with a bimetal blade in a jig saw--18 TPI to 24 TPI. Keep the speed on the blade down so you don't heat it up and clog the teeth. You can add some oil to help. To give it a nice finished look, sand it with 100 to 150 grit sandpaper using an orbital sander.

If you're going to rebuild the rocker from scratch, consider making the rocker sides/uprights the same width as viewed from the side as the tube bearing box.

Aaah, the challenges of balancing aesthetics and structure on a higher center of gravity tube scope... kept me up many a night and cost me many a dollar.

Good luck!

"Lee"

PS - This early scope of mine, below, had to be redone as described for the same reason. Any telescope I make now is so stiff you have a hard time seeing a visible response to any tapping or movement of the tube when viewing through an eyepiece at high power. Lesson learned.

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Jerry, the sides of the rocker box on my 13" are made from single thickness 3/4" plywood, and it has virtually no flexure. The OTA weighs considerably more than 30 pounds. I think the secret is to have adequate triangulation/cross-bracing - on mine there's another panel about 16" high running across the front of the rocker box joining the two sides. You can see a picture of it here: http://www.astromart.com/messages.asp?message_id=140099&page=

I think that 3/4" will be fine.

Regards,
Doug Scobel

Jerry,

Taking another look at your scope, I think I might remake only the sides, without the center arches. You might be able to get 1" sides on the bottom plate without extending over the edge. And even if you used 3/4", it would be much stiffer after triangulating near the inside top. It would also balance nice visually with your bearing box/"rocker".

Just something to consider--a quick and effective correction without having to redo everything.

Take care,

Lee

Jerry, another place where flexure may be creeping in is in the piece(s) joining the two sides of the rocker box. If there is a large opening in it, or if you have separate pieces, top and bottom, or if it is too short, then you may get some flexure side to side. Simply adding corner braces won't help much - you need triangulation. Can you post a picture from the front?

The key in making the structure stiff is not necessarily thickness, but rather triangulation. And the bigger the triangles, the better.

Doug