I just finished a 6″ f/8 Dobsonian telescope as a gift for my great-nephews, one of whom I discovered is VERY interested in astronomy and happens to live in a place with pretty dark skies – about the middle of Maryland’s Eastern Shore. The mirror is excellent, and the mechanical parts all work very well — in my opinion. Let’s see what the recipients think.
I finally succeeded in putting in the mirror yesterday afternoon in a very stiff and cold wind right outside my house, estimating where the mirror should go by aiming at a distant chimney. This takes patience because it’s trial-and-error, no matter how much you calculate beforehand!
Later that evening, after the nearly-full Blue Moon came over the trees at DC’s Chevy Chase Community Center where we have our telescope-making classes, fellow ATMer and all-around interesting person Jim Kaiser helped me collimate it by pointing the scope at the illuminated curtains in the windows of the CCCC. We then verified that the Moon actually did come to a focus with the eyepiece nearly all the way screwed in. If you want to focus on closer things than the Moon or galaxies, you need to screw the eyepiece out towards you, the observer in the cheap but effective helical focuser that was lying around the shop.
This scope incorporates a couple of innovations by me, and a bit of artistic whimsy.
First small innovation: I made the secondary diagonal mirror holder so that no tools are needed at all: you just rotate the part holding the elliptical mirror and turn a little thumbscrew to collimate it quickly and easily, while you watch. Here is a sketch of how I made it.
Second innovation can be seen near my right hand (to your left) atop the cradle: two 1/4″-20 machine screws with simple homemade knobs on top, going through threaded inserts (T-nuts would work too), which push against a piece of lumber in the shape of prism with an isosceles right triangle at each end. I call this the tube brake, which can be applied or released quite easily, whenever needed. Small springs (almost impossible to see in this photo) pull this brake up against the corner of the tube, while the machine screws press it down. If you want to change the position of the eyepiece because a taller or shorter person has arrived, no problem. A few CCW turns of the wooden knobs releases the brake, you rotate the tube to the desired position, and then you lock it down again with a few clockwise turns. If you add or remove a heavy eyepiece or a finder or whatever, same procedure, except this time you can slide the scope up and down inside the cradle.
The artistic whimsy is partly seen in a photo Jim took of me after we got it collimated but before we rushed back inside: lots of colors, thanks to several tons of paint cans salvaged by fellow ATMer Bill Rohrer from being thrown away by a third party who lost his warehouse lease, and also because smurf blue is the favorite color of one of the boys. The altitude bearing is made out of the Corian countertop that my wife and I got rid of a few months ago when we had our kitchen remodeled. (30 years ago we did it ourselves, mostly. This time we hired professionals. They are SOO much faster and better at this than us!)
So that my young relatives can keep this thing looking good, they also get four or five quart or pint cans of paint – the ones I used on the scope. Free, of course. The more we
get rid of put to use, the better. They can repaint anything that gets scratched, you see?
You can also see some wood-cutting fun above and below. This retired geometry teacher had a lot of fun figuring out how to lay out and cut out stars with 5, 6, and 7 points, as well as a crescent moon and a representation of Saturn seen with its rings edge-on. I guess you could show Saturn’s rings a 30 to 45 degrees to the viewer, if you instead carved it out of solid wood or did wood burning, but I just had a hand-held jigsaw and a Dremel knockoff. And plus, this is supposed to be a scope that is USED rather than just admired for its artsy parts.
I designed what I wanted onto two sheets of paper and then taped them to the plywood. This worked, but it wasn’t the most wonderful plywood, so on many of the pull strokes, the wood splintered a bit. So that side got to face inside.. Painting all those little nooks and crannies was tough!
(The purpose of the cut-outs was simply to make the telescope lighter. It’s got a very heavy and sturdy base. Each square inch of plywood removed saves about 7 grams. Also, more holes means more hand-holds!)