Sunday, January 31, 2010

Pole block problem resolved

After making and then staining and varnishing new pole blocks, they were placed on the mirror box to see if the poles all align in the same way. Unlike the previous set, they do and thus my previous problems were resolved. That still leaves the issue of getting them to fit in the pole seats, or possibly making new ones altogether but for the present I will leave them as they are until I get the primary mirror and determine the length and angle the truss poles and blocks must have before the blocks are bolted to the mirror box.

I've also ordered a Telrad and a 50mm straight through, non-illuminated finder scope after finding that straight through 80mm finder scopes are scarce. The smaller aperture will not be a problem because ultimately digital setting circles will be my main means of locating objects anyway. The focuser has already been shipped from the manufacturer and will arrive here soon, with the finder scopes soon afterwards. When they arrive, they will be attached to the upper cage assembly and an accurate weight can be derived for determining how the telescope will balance and what, if any corrective measures I will have to take.

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.

Saturday, January 2, 2010

Keepers and knobs added

After planing a piece of Brazilian Cherry left over from the construction of my dresser, I made two "keepers" to prevent the mirror box from sliding from side to side when the telescope is pointed near the horizon. After planing, I ripped a piece one and a half inches wide, then cut two pieces six inches long. I then bobbed the top corner and drilled two holes then countersunk them after reducing their width to 1 and 1/4-inches. Two holes 5/16ths of an inch were drilled all the way through for the 1/4-inch machine screws to pass through. On the rocker box, I made two pairs of blind 3/8-inch diameter holes to accept threaded inserts, which were installed with a bolt and nut after dabbing epoxy on them to keep them from coming back out. The keepers were varnished with three coats polyurethane and before finishing I notched out the sides of the tops facing the altitude bearings to prevent them and the keepers from scuffing each other.

The knobs for the pole blocks were ordered from Woodcrafters, a very good company for any woodworker to deal with. Among their numerous products are knobs with an internal threaded insert which are used to make jigs and other fixtures used to build furniture. I ordered eight of these knobs, one for each pole block. When tightened on the carriage bolt, they press the middle section of the pole block inwards, gripping the truss pole tightly without crushing them. However, the ends of the poles do not for some reason don't all lie in the same plane, the left hand poles are farther out than the right side poles by about three quarters of an inch at the ends. Since it is consistent on all four sides, I'm going to shave a small amount of wood off the left hand blocks to bring them in line with the right hand poles. The pole seats on the upper cage also need to be altered so the poles will slip into the seats. Apparently an angle of one or two degrees is all that's needed to rectify this problem.

I've also begun to order the components necessary to complete the telescope. I've already ordered the secondary mirror and a holder for it, and soon I will purchase a focuser and a finder telescope. I am planning to also install a Telrad. While I made the assumption the upper cage will be quite heavy for a 15-inch Dobsonian, I can add strips of steel to the tailgate if the telescopes turns out to be top-heavy.