The Practical Telescope #6
by Dennis Allen
Our club receives orphan telescopes all the time. We fix them up and give them away. We only ask, when you get a better telescope, bring back the old scope. That way, we can fix it up for the next person.
One of these orphans is a 4" Newtonian. In testing this scope, found a couple problems. First, it wouldn't focus. Turns out, the tube is simply not long enough for the mirror. So much for quality control at the factory. Taking an old 2x Barlow, I removed the lens and made an extension tube. Tested the scope out, images weren't half bad. The other problem was the mount. The design is awful, way too top heavy. Tighten up the mount enough for the scope not to droop and the mount becomes too stiff to move (in the image below, the old mount is under the tripod).
The tripod and tube cage looked in good shape, so I decided to convert the scope into a Dobsonian. The tube cage already had one mounting plate, so I cut and drilled a matching piece of steel. Mounted a pair 3" PVC end caps to the plates for altitude bearings. I replaced the old mount on the tripod with a 3" long 3/8" azimuth bolt, held by a couple nuts and fender washers. For the rocker box, I found scrap 3/4" plywood and cut a couple 9"x17" side pieces and a couple of 9"x9" pieces for the bottom and front. To each side I drilled out a 6" half circle and tacked couple of Teflon furniture feet. Drilled out a 3/8" hole at the bottom of the rocker box. Countersunk the hole, so the bottom wouldn't catch the nut holding the 3/8" azimuth bolt.
As you can see, the mount is a very simple design. The rocker box is a bit tall, but allows the scope to look straight up. Oh, usually we cover the bottom of the rocker box with Formica. This scope is so light, however, I just applied candle wax.
I tried this scope the other night. Once I got the tripod bubbled/leveled, worked nice! Better than I expected. What little shake settles down quickly. Has a red-dot finder, but wouldn't hurt to add a 5x24 or 6x30 finder. Otherwise, I think this will make a nice beginner telescope.
The Practical Telescope #6a
I was so happy how that telescope turned out, decided to convert another orphan telescope. For this 4" Newtonian, I had to build a tube cage. I cut out three pieces of plywood. The bottom piece is the width of the tube. The side pieces, couple inches longer. Screwed the side pieces to the bottom. At the top of the side pieces, drilled holes for couple long 1/4" bolts with wing nuts. To the sides I also added couple 3" PVC end caps for altitude bearings.
Based on the size of the tube cage, I built a rocker box same as before. Found an old camera tripod with a 1-1/2" pipe adapter. To this adapter I ran a short piece of 1-1/2" pipe, to a pipe flange bolted to a square piece of plywood. The center of this plywood was drilled for hold the 3/8" azimuth bolt. Also added three Teflon pads to hold the rocker box.
This scope is a bit more shaky than the last one, no doubt due to the camera tripod. At the last open house, we tried out both mounts. Both worked nice! Visitors ended up taking the first one home. For 4" Newtonians, this is a the way to go. Oh, I ordered a 5x24 finder for this scope.
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