Tuesday, September 22, 2009
After careful consideration, I chose a method to connect the truss-poles to the upper cage that will not result in the danger of parts dropping onto the mirror or being lost. I made four blocks from Brazilian Lyptus I had left over from the construction of my dresser. After the glue set, I made a jig that would hold them at the angle I calculated to be the best for the pole bores, which was 9 degrees.
After trial and error I got the jig perfectly positioned on the drill press and clamped it into place. A 1-inch Forstner bit was then used to bore all the way through the block, which was clamped to the jig. After the pole bores were drilled, I needed to drill the holes for the bolts that will hold them onto the lower ring of the secondary mirror cage. Using locating pins I marked the exact location of the holes on the blocks, then drilled a 1/4-inch hole with a Brad point bit.
Next, I made a simple jig to hold the pole seats stationary while I beveled the edges just like the pole blocks for the mirror box. Unlike the pole blocks, I routed one side on the back of the blocks to make sure the light baffle doesn't get in the way of them resting flush on the ring.
The next task I did making the pole seats was making two nine degree saw cuts in the front of the pole seat blocks after drilling a hole for a bolt and knob. A third cut will be made to split the seats into two parts tied together by a bolt and a knob. These pole seats will work like upside down buckets when the telescope is set up and taken down. When the knobs are tightened, the poles will be gripped tightly without crushing them. Any forces that try to pull the cage out of position will wedge them more tightly into place and prevent major damage to the telescope from occurring if the upper cage should separate from the truss poles. The final step would be to sand and varnish them with several coats of exterior Polyurethane to protect them from the harmful effects of moisture.
Thursday, September 10, 2009
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.
Tuesday, September 8, 2009
Now that the altitude bearings are nearly finished, I decided to make the mirror box in order to determine where to attach them to it. After cutting out the panels from the rest of the 1/2-inch plywood I bought and checking them for squareness, I laid out and cut out slots for biscuits to join them together. After applying yellow carpenter's glue to the joints and the biscuits, it was a simple matter to tap the panels together with a rubber mallet, ensured the box was square with a steel framing square, then tightening the clamps. After a final checkout with the framing square, I left the box on the table saw so the glue can set. After a few hours, I removed the clamps and cut out corner braces with more 1/2-inch plywood. After applying glue I used scrap plywood to hold the braces in place while the glue dries. Then it was time to check the box for proper fit of the tailgate, which turned out to be smaller than I expected. There's a 1/16th inch gap between the side rails, a minor problem that can be fixed using washers as shims. Had the box been too small, it would have forced me to build it all over again, so if your tailgate turns out to be a little undersized, don't fret.
Wednesday, September 2, 2009
Now that I completed the upper cage as far as I can until I can buy the rest of the parts, I am now making the altitude bearings and pole seats. To make the altitude bearings, I set up my router with a circle cutting jig I made to route out a plywood arc 23 and a half inches in diameter and 3 and a half inches wide. The altitude bearings will be hollow arcs to avoid interference with two of the poles and pole blocks, and I can use a pie shaped piece of plywood on each bearing to stiffen the unsupported end if the need arises. Since this is not a very large Dob, I doubt the need for that will materialize, but I can add them if it does.
I then routed out the outside curve by running the spiral cutting bit to the inside of the pencil line drawn with a compass made from a piece of scrap plywood. Then I routed the inner curve with the bit running along the outside of the pencil line but stopping just past the line the compass pivot point was placed in the exact mid point thereof. Then I cut out the arc on the table saw, which also yielded smaller semi-circles that can be used for bearing on a smaller telescope.
I will glue them together with another sheet of 1/4-inch plywood to get the 1 and 1/4-inch thickness I was planning for them, then I will trim off the rest with a trim bit. Then I will locate the holes for the bolts that will attach them to the sides of the mirror box, then drill and countersink them. I'll make a template that will also be used to position the bearings and the holes for their bolts on the mirror box correctly. Everything will be made in a manner that will allow disassembly for either repairs or upgrades if necessary. The outside edge will be covered with Ebony Star Formica and the rest of the bearings will be coated with four coats of exterior grade polyurethane.
Tuesday, September 1, 2009
Now that I've applied the final coat of varnish and paint, all parts have been assembled and the cage is complete. All that is needed now are the secondary mirror holder, focuser and finder scopes. The attachments for the truss poles have not yet been made but the cage is ready for their installation. Presently the upper cage weighs about 7 pounds, and when these components are added the weight will be 11 or 12 pounds.
The rings, focuser board and finder scope base have been coated with four coats of exterior grade polyurethane, while the outside of the light shroud was painted with glossy black spray paint. The inside was flocked with flat black oil-based enamel mixed with sawdust and the plywood shroud was attached to the rings with 3/4-inch long, stainless steel wood screws. The shroud stiffens the rings and allows the placement of another finder scope at a convenient location.
Aside from final touch ups, the cage will be set aside until the pole attachments are finished and the rest of the components are in hand. I will begin the production of the pole seats and altitude bearings next.