Monday, August 10, 2009
Secondary mirror cage and spider
After making the split blocks for the mirror box, I turned my attention to making the secondary mirror cage, which will hold the secondary mirror and it's holder, spider, focuser and at least one finder scope. I was planning to make the rings from Baltic Birch, but found it to be unavailable locally and ridiculously expensive to have shipped to me in the small amounts I needed. The shipping would have cost more than the plywood itself. This plywood isn't as stiff or strong as Baltic Birch, but since I'm not building a very large Dobsonian it will serve. I'm thinking about using 3/4-inch plywood of the same type for the mirror box and the cutouts from the rings as gussets for the inside corners. My Discovery Dobsonian's base is made from softwood core, hardwood veneer plywood and is going strong. The 1/2-inch plywood when double layered will make a nice, stiff rocker box.
So I opted to use a Birch veneer plywood that has numerous thin plies inside, but is made of softwood instead. It has voids that not desirable, but can be filled in. I routed the rings half an inch wider than the plans from the book called for, and may use seven instead of four struts to make the cage stiffer. Or find some thin hardwood plywood and make the inner shell for the cage from that, which would stiffen the rings substantially. I routed out the rings, rounded the edges and drilled the holes and seats for the four struts I have already made. I made the focuser board from a piece of Baltic Birch plywood and left over maple from the pole blocks. I will makes the pole seats from more maple when I acquire it.
After weighing my options, I decided to make my own spider, since I have the means to make it. For the vanes I uses 1 1/4-inch wide, 12-gauge mild steel flat stock. The central hub will be a piece of 1/2-inch square mild steel tubing, and the lugs are 10X24 coupling nuts used for threaded rod. They too are made of mild steel. Welding them to the vanes was the way to go, so I notched the vanes so the coupling nuts would fit in the slots, then welded them in place. The clean metal was fairly easy to weld for a change. A tap cleaned out any slag or spatter than got into the threads, and now all that remains to do before welding them to the hub is to cut each vane to the correct length.
After that I trimmed the vanes to the correct length, then welded them to the hub in the jig pictured, then welded a 3/8-inch washer welded to both ends of the hub. I sanded the hub bright and clean, then welded one side of each vane at a time, allowing the metal to cool to the touch before welding again. That stopped porosity and cracks from forming in the welds, which do result from the weld bead being overheated. The washers I welded onto the hub will help support the nuts and washers on the stud that will secure the holder to the spider in place. When tightened, the nuts and washers on the stud will lock the secondary mirror in position. After welding, the spider will be primed then painted in flat black paint. I could have made the spider from stainless steel, but it would still have to be painted, therefore I took the easy way out this time and opted for mild steel. Stainless steel fasteners on the other hand will be used to hold the cage together after I remove the temporary steel screws used to hold the cage together so I can verify everything fits correctly before finishing and final assembly. After completion of the welding, I wire brushed and filed away left over spatter and slag so I can apply primer then paint to the spider to prevent the onset of rust and blacken it to stop scattered light from ruining the views.