DIY 4.5" Dobsonian


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DIY 4.5" Dobsonian
I built this telescope as a present for my sister. She is a rank amatuer and is fascinated by the moon. I considered buying her the Orion XT4.5 Dobson, but was turned off buy reviews that I read, particularly by the recent S&T article. Since this was intended primarily as a lunar/planet scope, I did not want any spherical abberations. Orion sells an imported 4.5" f8 spherical primary for $40. This one was $80. Also, University Optics sells good, small mirrors as does Edmund Scientific.

The first step was to manufacture a focuser. One the advice of an old timer at our local club, I made one using two diameters of PVC tubing. The narrower one fits inside the wider one. The narrow tube has a strip of sandpaper down its length. The focus knobs are a pair of epoxy filled soda bottle caps (bigs ones) stuck on the ends of a steel bolt. The bolt is wrapped by a latex tube and is held agaist the sandpaper by office elastic bands. The drawtube is pressed against a furniture glider inside the larger tube. It sounds crude, but it works pretty well. The 1.25" eyepieces are held in place by friction created by a black, foam lining.

I had a hard time finding a tube. While I know it exists, no one locally could supply my with 6" Sonotube. I resorted to 6" plastic drain pipe, similar to PVC but more rigid. The interior is painted with flat, black latex. For glare control, the tube extends 5" beyond the focuser and 2" beyond the cell. The focuser end is lined with black velveteen and is baffled with black, foam weather stripping. Another baffle is just above the objective. The exterior is painted with Krylon Hammer Finish Bronze and an acrylic clear coat (prone to scratches).

The altitude bearings are tuna cans filled with auto body filler and painted gloss black. They attach by way of a piece of 1x4 shaped to fit on the tube. All the visible hardware on this scope is brass. All hardware in contact with the plastic tube are cushioned by rubber washers. The secondary is oversized [1"] for easy collimation. It is glued to a bevel cut closet pole. A 2-vane spider made of a piece of threaded rod supports the eliptical. The entire assembly is flat black. The mirror cell is two pieces of 1" oak and is vented. (I should have used pine. Oak is very hard to cut and shape, especially if you use only hand tools like me.) Three brass wingnuts collimate the primary. The finder is a Daisey air rifle unity finder ($14 at Kmart), the same one Stellarvue sells. I built a wooden dovetail that stands away from the tube about 1.5"

The rocker box is 3/4" plywood with two sheets as the ground plate. The lazy susan is spaced with furniture gliders. Three short, thick table legs screw into the ground board to bring the eyepiece to eye level when seated. The box is held together with L brackets and brass machine screws. (I had to be able to disassemble it for shipping). All wooden parts were stained with Minwax Cherry and coated with polyurethane. The final result is attractive enough.

I only had a chance to test it once before shipping it and that was around low level light pollution. I hope I do not sound boastful, but for its size, it is the best Dob I have ever used. The ring nebula was clearly visible. Stars were tack sharp. Contrast was superb, as good as a refractor and much better than my 8" Dobson. The motions were smooth, better than a commercial version. The focuser was easy to use, though it had to be refocused with each eyepiece since there is no focus lock. With a 10mm occular an a barlow, Mars resolved into a tiny, orange disk with some visible surface markings (this was one month before closest approach).

While I built this to prove the adage, "If you want something done right, do it yourself" it took a lot longer than I planned (mostly due to numerous small engineerg problems) and finished over budget. (I had to buy a lot of wood working tools). The result is a superior product, but at a cost of much time and money.

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By: Anonymous (xxx.xxx.204.163)
Link to this vote: http://www.excelsis.com/1.0/displayvote.php?voteid=41260

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