[Wavytone]

Steve,

MDF is cheap but it's both heavy and nowhere strong enough for the rings, and if you put screws into the side grain, it will split and disintegrate. One way to deal with that might be to soak it in a watery glue that can soak in, and wait for it to dry, or better, choose marine plywood.

But... if you're going to make two rings strong enough for this to work well, you may as well make a truss.

Cut off the top end, fairly short and keep that. Cut off the bottom end, also quite short. The piece of tube in the middle (ie most of it) will be discarded (reducing weight too).

Devise some short poles or a truss to connect the top end to the bottom, there are a number of ways to do this. The simplest I have seen so far is a 2-pole truss which Mental (Alex) used for his 13" dob. In this:

- the two poles are aluminium tubes, fairly large at 2" outer diameter, which makes them quite stiff enough for this.

- the lower tube with the primary mirror has 2 wooden rings around it, at top and bottom. The top ring has 2 holes on either side and the two tubes slide through these. The bottom ring has two wooden plugs, and the poles are a neat firm fit over the plugs, which keep the tube and the poles aligned precisely. The plugs were the blanks from using a hole-saw to cut the holes for the tubes in the top ring (d'uh, stroke of genius).

The upper ring has small saw-cuts on either side of the holes, with a pair of coach bolts through the ring; tightening a pair of wing-nuts so you can clamp the poles by tightening the wing nuts. Its a simple, easy to make, secure and foolproof way to clamp the poles.

- the top end has just 1 ring and slides over the poles and clamps the same way. If you feel its too wobbly add a second ring as per the bottom tube assembly.

- the altitude bearings are plywood disks with laminex around the perimeter, and slide on the poles, so you can adjust the balance of the scope.

The result is quite elegant and very portable, his 13" would fit in my Honda CR-Z - and that is a VERY small car. The only thing to note is that you'll need to cover the primary mirror - or remove it and store in a box - to protect it from dust and grime.

Lastly, your present OTA is metal - no doubt heavy and its going to be a messy noisy PITA to cut this. if you are tempted to make the two piece above with the 2 poles, I'd suggest instead there is an even better alternative: leave your heavy metal tube alone and instead buy some 1.5 or 2mm 3-ply - this stuff rolls into a 14" tube quite neatly and if you have cut some rings, use these to hold it in place at exactly the right size to fit the rings (!) - then you can forget about the heavy metal completely.