I used that fluted plastic sign board stuff to make a baffle. Following the advice from another ATMer on the list I used a razor to slit every third flute allowing the piece to be flexible enough to mount directly to the endring. I simply painted this with flat black spray paint and used wood screws to mount it directly to the wooden endring.

...making our way down the OTA...Here's a view down the OTA.

I used simple wooden blocks with 1/4-20 carriage bolts and hand nuts to capture my truss tubes at the mirror box. The clamps are fastened to the mirror box with more carriage bolts and wing nuts. Also in these pictures you can see the grooved screen door guide wheels that serve as pivot points for the Virtual Counter Balance springs. I'm currently using string to connect the springs to the mirror box. I'll switch to wire soon, now that I've settled in on the right solutions with respect to weight. Also in these pictures you can see the central altitude fin or bearing. It rides in a triangular block with a groove that's lined with 1/16" Teflon. Epoxy does not work well with Teflon, even if you rough up the surface to be glued. Thus, the wood screws and wrap-around Teflon. The other bearing surface is Ebony Star. It's every bit as good as it's hailed on various ATM websites.

Here you can see the five 5 lb. lead bricks I had to use to balance out the scope...and the 12V muffin fan that blows on the bottom of the mirror to vent air from or into the mirror box. My preliminary results with fans and boundaries of warm air suggests this fan has little effect. The other fan, however, that blows air ACROSS the surface of the primary has RADICAL effects on improving images at the EP (as long as the mirror is not at equilibrium). There's been discussion on the ATM list recently about fans and I concur: images are aided tremendously if the surface layer of air on the mirror is scrubbed away. Just make sure to isolate the fan to avoid vibration. Rubber bands, old mouse pads...anything to dampen the vibration should work. I'm using chunks of a mouse pad.

The other two altitude bearings are mounted to the outside of the mirror box and the setup follows Mel's Tri-Dob. Aligning the three components of the altitude bearing to make them concentric has been an ongoing topic of discussion on the ATM list.

Mel suggests: "The variation in altitude rims is as great as impact as any de-centering.
So, just build to normal good 1/32 inch tolerance, and you'll be fine.  That error
calculates to a few arcminutes in most situations."

Jan van Gastel is building a large tri-dob and has implemented the use of a jig to make
sure his bearing surfaces are concentric.

I drilled lightening holes in the bracket that connects the two altitude bearings. In hindsight I would NOT drill holes in this bracket because it weakened the entire structure. I used aluminum angle to reinforce all the corners and vertices in the mirror box. A rigid mirror box is ESSENTIAL!!--see my comments below.

I'm currently redesigning and enlarging the mirror box and flex rocker (which will be motorized using Mel Bartel's Go-To Stepper Motor Drive system) and I will follow James Lerch's advice regarding driving the altitude bearing from the central bearing (or "fin") rather than from the outer altitude trunions. You can refer to James' ATM list comments HERE, which are based on motorization of his 16" binocs. You can see his creation HERE and first light images HERE. (I always have a hard time finding these pages by Google!) There was also some discussion on the Scope-Drive list (July 2005) regarding driving a tri-dob.

Kevin started it:

I was wondering if the Trilateral Dob design by Mel Bartel's could be
adapted for computer operation.
Thoughts?
Thanks in advance,
Kevin.
 

...and I replied (edited):

I've bounced several questions off Mel and the group and from the responses, it appears that the tri-dob platform can be motorized. Based on my perspective and research the altitude will have to be driven via the central altitude fin (the center alt bearing). Issues related to pointing accuracy and imprecision resulting from non-concentric alt bearings can be remedied in the software, according to Mel. I don't know how "easy" that fix will be, but given Mel's reputation, I have no doubts that problems would be solved. [comments regarding James Lerch's recommendation for driving the altitude from the center alt fin.]

I foresee problems with weight or pressure on the worm at the alt. drive contact unless there is a "trolley" or pivot bearing installed very near the worm/bearing contact point. Mel suggested the "trolley" idea...it's in the archives under "weight on the worm". Alternatively, you could widen the center alt bearing/fin by reinforcing with an additional section of ply, modify the existing contact point to a roller bearing and add teeth to the nascent ply sector. If you make the bearing "adjustable" with respect to vertical, you wouldn't need to build "tensioning" into this worm drive; you would only need rigid worm support.

Driving the azimuth appears straightforward:  Generate the teeth on the outside of the ground ring.(JB Weld, if you like.) Implement some sort of bearing or guide system on the inside of the ground ring (to maintain even contact between ground ring and flex rocker). I've done this already on mine...I mounted three bearings (another tridobism!) to the underside of the flex rocker and they ride against the inside perimeter/surface of the ground ring. I need to figure out how to make one of them adjustable. Install the stepper directly to the flex rocker and aim it tangential to the outside of the ground ring. Find a way to apply tension to the worm and you're golden.

Ultimately, I think both steppers have to be mounted to the flex rocker. I could be way off, though. Early on I kept telling myself that my flex rocker was too flexible to pull this off. But after thinking about it, the flex rocker doesn't have any real restrictions with respect to how thick or "solid" it needs to be at the two positions where stepper motors will be mounted. You could reinforce and build off of the flex rocker as much as you need 'at these two locations' and still maintain the flexure required for the OTA to sit down on the rocker and adjust to a stable three-point stance. And another thought I had was that I could actually just build a whole 'nuther flex rocker and permanently attach the stepper motors to it. Then, when my battery or pc dies, all I need to do is swap out the stepper flex rocker with my original teflon bearing flex rocker, flip or drop out the roller bearings and I'm back to manual steering. That would be pretty nifty and spiffy. As with all ATM projects, she's a work in progress. But I gotta tell ya, there are few things more enjoyable than waltzing a 16" scope out the back door, setting up and collimating in 15 minutes and be staring at the planets and stars just like that!  Sweet!
And I owe it all to you fine folks on the ATM list!
Whooohhoooo!
Dumpster Dave
 

Mel Bartels: "The only comment I'd add ... is that the altitude can be driven 
from a single long thin drive rod that stretches across and underneath
the forward altitude rim fins."

So there you have it! Now, back to construction...


 

And speaking of the ground ring, here it is in progress.

...and here's the flex rocker. Note that the rocker does not sit symmetrically on the ground ring. In the picture on the right you can see the Teflon azimuth bearing and one of the three bearings that guides the flex rocker as it spins on the ground ring. The bearing rides directly against the inner wood surface of the ground ring. I believe that this surface needs to be lined, probably with sheet metal or aluminum. And I need to find a way to make this bearing adjustable, so that it rides constantly against the ground ring surface (or holds constant tension against the ground ring inner rim). Mel's Tri-Dob uses 4 bearings that ride on the outside of the ground ring. Mine uses three ball bearings that ride against the inside of the ground ring.