Telescope Reviews: My thread for building a 12.5" RC.

Hello:

I'm quite interested in your progress since I'm making a 12.5" cassegrain in a carbon fiber truss tube as well. In my case, it's a classical cassegrain (around f/15 or so), and the scope is currently in a solid aluminum tube that I made a few years ago. The plan for the new tube is to have three aluminum rings connected by two trusses of 6 poles each. Since I don't have the capability to make nice machined rings like you see on RCOS scopes, I'm bending 3/4" square aluminum tubing into the rings. Here's how you do it: just have someone get you really mad until you turn green and roughly double in size. At this point, bending the aluminum tube is very easy. If this doesn't work for you, follow the instuctions here:
http://home.wanadoo.nl/jhm.vangastel/Astronomy/50cmscope/bending.htm

Regarding mounting the primary mirror, the mirror should be thick enough that a 3 point cell will work fine. My experience with PLOP shows that as the central obstruction gets larger, the radius of the mirror support points gets smaller. In the case of Josh's 10" RC set, the 3 points should be on the edge of the central hole. Silicone glueing your mirror with three blobs to a chunk of aluminum might work. You could also use a mirror cell like that used by Novak, Parks, or University Optics. Since the scope will be for astrophotography, warping the mirror a bit isn't too terrible. But you really want the collimation to be stable and your don't want the mirror flopping around. I found that my cassegrain is VERY sensitive to collimation, and a laser collimator won't help you a whole lot.

While you're thinking about the primary mirror cell, also think about the primary mirror baffle. Ideally, it shouldn't touch the primary mirror, and it should be able to be aligned independently of the primary mirror. Since you have a lathe, you may be able to machine things accurately enough that you don't need to build in a way to align the baffle. Typically, the primary mirror cell also supports the baffle, but this isn't necessary. In my case, the hole in the primary mirror is too small to allow the baffle to pass through. My solution was to suspend the baffle tube in the main tube using fine wires. Thus, the alignment of the baffle can be adjusted, although it's a royal pain to do it, and the baffle doesn't touch the mirror.

Soda can aluminum is an excellent source of baffle material. Just chop the ends off of a few cans and put them end to end. They even come with built in ring baffles inside.

Here's a web site you should see, if you haven't found it already:
http://www.trivalleystargazers.org/gert/rc10_technik/rc_10.html

One interesting thing about a scope like this is that, although it's a truss, you never need to take it apart. This allows the truss connectors to be simpler and more robust. My plan was to cut a 1/8 wide slot into a wood dowel to accept a 1/8" x 3/4"x2" piece of aluminum strip. The wood dowel would then be glued into the carbon fiber poles like a plug, and the piece of aluminum that sticks out can be bolted to the end rings of the "tube". One reason I've come to this design is that I don't have a lathe to make nice sockets! I haven't seen carbon fiber tubes split from the use of a plug, but I have seen it delaminate when something is glued to one side and the glue bond is shocked.

Regarding the spider, Josh is right in that it's a large cantilevered mass, and using a spider where the vanes don't meet in the middle will reduce torsional vibration. The guys who have really big dobs have secondaries as heavy as yours, i.e. this is a previously solved problem. I imagine that a heavy duty spider will work just fine.

Cheers
Mike Connelley
Original owner of Josh's 10" mirror set