Saturday, May 23, 2009

Errata corrected

In examining my mirror cell, I found a couple of minor problems I overlooked. The aluminum triangles were turned the wrong way, so I had to drill and tap another hole in them and do the same to the plastic ring to keep them from turning. I drilled out the original slots with a 3/4-inch Forstner bit and made a new slot for the screws.

The other problem was that the eye bolt mirror clips would not tighten down securely. I couldn't find eye bolts that would fit tightly around the threaded rod, so I used the smallest size stainless steel eye bolts I could find. Evidently, they weren't small enough so I took some left over pieces of 1/2 by 1-inch aluminum bar and made mirror clips from them. With a rubber bumper pad added and stainless steel flat washers, lock washers and hex nuts these clips stay put and the mirror will never contact metal. I sanded and painted the fan board with flat black paint. I was originally going to varnish it, but decided against it since I will be putting a LED on it that will be lit when the fan is operating. The fan itself was bought from a local electronic supply store and it was attached to the fan board with stainless steel pan head screws, nuts and washers. I put it in the inside of the board to keep it from bumping the bottom of the rocker box or the azimuth encoder for the digital setting circles that will be on the pivot bolt.

I made a dummy mirror and installed it in the cell to see if the mirror will fit correctly in it when I receive it. It does not contact metal even as I move it around in the cell, though it does stop only 1/8th of an inch from the split bolts. I may replace the eccentric side pins with bigger ones but I anticipate once I wrap the sling around the split bolts the risk of glass crunching collisions with them will be eliminated.

Complex as this cell is, it's easy to take apart and repair. The mirror cell is now complete, and now that a cooling fan has been added to it, I will begin making the upper end assembly. The total weight of the mirror cell is now between 12 and 13-pounds, and the mirror it will contain will weigh about 25 pounds.

Sunday, May 17, 2009

Mirror cell completed

After attaching the triangles to the bars, then the bars to the collimation bolts, I needed a way to keep the bars and triangles from rotating, so I bought some black plastic an 1/8th of an inch thick. This material is fairly stiff but plenty flexible enough to permit adjusting the collimation once the telescope is complete. After determining how large the outside diameter had to be, I set up my router with a circle cutting jig and spiral bit and cut it out from the plastic sheet. A 4-inch hole saw made cutting the inner circle a snap. To attach the ring to the triangles, I took the plywood template used to cut the triangles out, and drilled a hole for a marker. I then marked each triangle along one inner corner and drilled a small hole, then used an electrician's tapping tool to thread the holes. Finding out where to make the holes on the plastic ring was tougher than I expected, so I just made slots through which 3/8-inch stainless steel screws were driven.

It turns out that they hold the ring tightly enough to keep them from turning when I turn the collimation bolts. I could have used double sided tape just as easily, and that would have been far easier. The fan board was cut out of some left over Baltic Birch plywood I had lying around. I used a fly cutter to cut out the opening for the cooling fan, then drilled four holes for the mounting screws. I made a notch for the bottom collimation bolt, then positioned and clamped the board into place. I then made starter holes with a cordless drill, then removed the board to drill the holes into the back side of the middle and bottom rungs. Then I tapped the holes with the same tapping tool, and secured the board in place with four 3/4-inch stainless steel screws. At a later time, I will sand and varnish the board, then attach a 3 1/2-inch cooling fan, switch and RCA jack. The mirror clips consist of 3/16-inch diameter stainless steel eye-bolts threaded into pieces of Nylon rod to cushion and protect the mirror.

I just drilled a 1-inch deep hole with a 3/4 Forstner bit into a block of wood. Then I drilled a hole where the center spur of the larger bit one all the way to the other side . Then I drove the Nylon rod into the block, and cut it off flush with the wood's surface. Then with the same bit I used to drill all the way through the wood, I drilled a 3/4-inch deep hole into the pieces of Nylon rod and tapped the holes with a tap that matches the eye bolts. Then I screwed the eye bolts into the tapped pieces of Nylon rod, and then bolted them to the tailgate with stainless steel washers and hex nuts.

The 3/4-inch diameter of the Nylon rod will ensure the glass mirror will never contact metal if the telescope tips over or I hit a severe bump while transporting it. The reason why the mirror must never touch metal is that there is a great danger of the glass chipping or even breaking if the mirror hits metal as a result of the telescope being bumped or tipped over, and these bumpers along with the wooden dowels are there to catch the mirror and prevent a costly accident.