Sunday, January 24, 2010
Secondary mirror and holder arrived
After adding the knobs for the pole blocks and inserting the poles, I discovered that the pole bores were tilted in opposite directions and thus I was forced to make new pole blocks after my attempt to correct the problem merely made things worse. I was able to test fit the poles and see if the pole seats are even close to being workable. It's clear that shims will be needed on both the pole blocks and pole seats, but I found some thin sheet aluminum that would make excellent shims that will be nearly invisible once installed. To prevent the problem with crooked pole bores from cropping up again, I made a jig where I took great care to ensure the fence to which the blocks are clamped while drilling the pole bores is at 90 degrees with respect to the plywood bottom of the jig. While drilling, I drilled four blocks with the pole bore on the left side, then four more with the bore on the right side as seen from the front. I then beveled the edges, then with a spade bit I made four circular recesses where the bolts for the block will be. In the center, I drilled and countersunk 9/32nd inch holes for the 1/4-inch machine screws that will eventually permanently anchor the blocks in place once I acquire the primary mirror and determine both the length of the truss poles and the angle of the pole blocks. Since I chose to use red oak instead of maple, which is very scarce locally, I stained the blocks with red oak stain then varnished them as before. I would have preferred to use maple, but since the poles are smaller than usual, and there is a greater thickness of wood left on both sides of the pole bore, I have little reason to worry about the strength of the pole blocks over time. Obsession is using plywood to make the pole blocks on their smaller Dobs with apparently no problems with durability, so I have little reason to worry when the truss poles are going to be less than four feet long. The blocks are now fully varnished and ready to attach to the mirror box once I can determine the length of the poles and the angle the blocks must be set at so the poles mate with the pole seats on the upper cage assembly.
I've also ordered over the holiday season the secondary mirror and it's holder, which arrived a week ago. I've also placed an order for a dual-rate Crayford focuser from Moonlite and next week I will purchase the finder scopes, which will be a Telrad and probably an 80mm finder telescope of some sort. I opted for the Moonlite focuser because it is adaptable to any telescope and is a high quality focuser that is built to last of high grade materials. Most likely if I choose an 80mm finder, it will be a correct image finder because I have not been able to find any straight through models that do not cost as much as a small Dobsonian. The bigger aperture will be helpful for hunting down faint objects, but I may have to add strips of steel to the tailgate to balance the telescope. So far the cost of the components to complete the upper cage have come to 500 dollars. All told, this telescope will probably cost me 3,500 dollars to build, if not more and that does not include the cost of acquiring a welding machine and other tools needed for this project. Once I have the finder scope, the next goal will be to get the money together to purchase the primary mirror, which will take at least a couple of months for the optician to produce. I expect first light for this telescope will be in the late spring or early summer at the latest. Before it will be truly finished, I will have to make a shroud for the truss-tubes, a case for the upper cage to protect it from damage in storage and transit, and fit the telescope with batteries, digital setting circles and dew heaters. With some luck, I will be able to attend a major regional or national star party and try the telescope out on some faint galaxies and nebulae.