Thursday, September 10, 2009
Baffles and gussets
Now that the mirror box had been made, it was necessary to reinforce it with corner braces and gussets. The mirror box is arguably the most vital component because all of the structural loads converge in it, and that requires it to be very rigid and strong. I places two corner braces in each corner and a gusset made from the leftovers from building the upper cage. Additional scrap was used to make the corner braces, which were cut on on the table saw with the blade set at 45 degrees. Brads and sticks held the corner braces and gussets in place while the glue set.
The next step was to make a light baffle for the front end of the box to reinforce it. The baffle will also help keep the box square and free from flexure as the telescope is moved. I took two thin sheets of birch veneer plywood and glued them together, placing a stack of floor tiles to press them together as the glue set. Then with the router and a circle cutting jig I cut out a round opening a little smaller than the inside width of the mirror box. Brads kept it from sliding on the slippery glue when the clamps were tightened.
I then cut out another circle of plywood from a surplus sheet of birch veneer plywood a little smaller than the width of the box. Using the cut out from the baffle, made a cover for the mirror box by gluing the cut out and the larger piece together, again placing a heavy stack of tiles on top to ensure the glue bonds the plywood together. The cover will serve two important functions. One will be to protect the mirror from any falling objects that can strike and damage it. The other is to prevent sunlight from getting into the telescope and turning it into a powerful solar furnace, which would likely damage the telescope severely, even set it ablaze. After sanding and varnishing, a knob and a couple of door catches will be added to facilitate easy removal and replacement.
After that, a router fitted with a trim and a 1/8th-inch round over bit was used to trim off excess plywood around the top of the box and break the sharp edges. I then used wood filler to fill in damage I did to the veneer next to the light baffle. Unfortunately, I couldn't find Baltic Birch plywood locally so I opted to use a 1/2-inch thick hardwood veneer, softwood core multi-ply material instead. It has seven interior plies a little more than 1/16th-inch thick plus the face plies which are very thin. That means it's easy to sand right though them, and I did in a few small spots. After filling them in and sanding, they're not as noticeable as before. I decided to leave the box as it is, after considering covering it with veneer. So I applied the initial coat of varnish to seal the box and help protect the face plies from getting dirty. Before I completely varnish the box, I'll locate the holes for the altitude bearings and drill small pilot holes for them. This way if I botch something, it will a lot easier to fix a small hole than a large one.