Intro: Dobsonian Mount Replacement for Mirror Telescopes
What is a "Dobsonian" ? What is it for? What is the main purpose of it, why are there people who love it? Well, if you are an amateur astronomer, you probably have one or more scopes, and perhaps one of them is a reflector. (Reflector: a telescope with mirror as objective, while the 'refractor' is a telescope with a lens as objective. These are differences in the astronomy, because of the attributes and possibilities of them.) When you buy a mirror telescope - or a reflector -, you most likely get also a tripod like object in the pack. And often this is the weakest part of the system - the tripod usually is equatorial, but is a weak, underplanned industrial garbage, that is alvays moving in all directions, is not stable at all, in the smallest wind the telescope on it will tremble, tilting, and so you are not able to observe anything in the eyepiece, because the celestial ojects are just running stripes or sparkling lines in the sight field.
This is the most discomfort for a telescope owner; having a good optics and you can't use it because of a weak mount - of course, you may buy a stronger tripod, but a really strong and proper tripod is a very high-cost item, and is very large. If you want to observe as an amateur astronomer, you should know dobsonians; the best azimuthal telescope support, that was invented by John Dobson (died in this year) - a monk, amateur astronomer, telescope maker, mirror grinder and the man who started the renaissance of the easy azimuthal telescopes.
Here you can read about it, however:
The 'Dobsonian' is an easy way to have a very stable, accurate and portable, but a very low budget telescope. The only inconvenience may be the azimuthal system, but this is not so wrong when you don't use the scope for photography.
Now we will build a mount, not a complete mirror telescope, but this task will take only 1-2 hours.
Step 1: Let's Start It!
Well, there is not so many pics about it - and you dont need them to build a dobsonian mount for a small telescope with short focus. Perhaps you can use this type of mount for longer focal tubes, too, but the stability is optimal for the shorther tubes. The normal dobsonian mount is a gun carriage type system with two arms, but this special mounting method is similar like GoTo mechanics or half-arm mounts. Let's start!
You need a decopir saw, a drill, a screwdriver, a flat-iron, or a cutter - oh, sorry, of course a pencil and paper, a measuring tape and a rubber for the planning, but no any other stuff.
The materials will be: a laminated board and some iron-glued stripe for the naked edges (sorry, I haven't found its proper name in English:-( ), some screws for wooden works, and some for the metal works, nuts, a palm-size teflon piece, and a telescope with a useless tripod, BUT(!) in this project you must have the tube rings for the proper mounting.
So, you may build this instructable, if you have a short focal lenght telescope with a weak, unbalanced, cheap EQ-mounted tripod with tube rings, and you want a stable, highly portable and very useable, but AZ-type mounting system instead.
Step 2: The Panels
There are 4 main panels, and 2 auxiliary panel for the half-arm dobsonian. The sizes depend on your telescope, I have a 114/500 one, and this is the ideal size for this mount. You can go until 130/650, but not higher. These telescope sizes are wide popular, most of amateur astronomers have similar in this category.
Panels. I planned them, and went to te local DIY-shop, and they have cut them on correct size in five minutes. But you may cut the lines yourself as in the followings:
Cut 2 pcs. square panels in same size: the edges are 3 times of the outer(!) diameter of your tube (cca. 40 cm at me). These are the bottom panels on each other, with a central axis, with 3 small teflon pieces between. This is the horizontal movement axis Then cut a square panel, which edge size is the full lenght of your tube (cca. 45-50 cm at me). If you are a sophisticated person, you may form rounded shoulders or triangular cuts at the upper section of it; I cut a triangular from it, but I'm sure, you can imagine more aesthetic shapes than I:-). The purpose of this some cut is the weight loose of the system. And the prettier design, of course:-)
Well. The fourth panel is a smaller front support of the rectangular direction, it is height is cca. the half of the third panel's, and you can form rounds here, too - you can see mine, and cut similar, or prettier:-) Or leave it as a rectangle...
And the last, small panel - the tube ring support. Its edge size is 10-12 cm, and is a square, with a central axis and again 3 teflon pieces - this is the vertical movement axis.
Step 3: Assembling
Now plug a flat-iron and iron the edge stripes for the professional looking, and cut the sharp edges of it. When ready, drill the axis holes: draw traverses on the firs two panels, and try to drill very accurate a hole with 8 mm, for a 8 mm screw. Then measure a point on the third panel, (which is a standing one), with 15 cm from the upper edge, on the vertical center line of it, and drill a 8 mm hole here. Now take the small square panel (the tube ring support), and make a central hole with 8 mm, and after it, in one of its traverse line drill two holes for the tube ring symmetrycally at the corners of the panel, from 6-6 cm far from the centre. These two holes must be the same size of the tube ring's holes at the bottom, because here come the ring bolts, when you attach the tube rings here, instead of the old, weak, unbalanced tripod mount. With this method you haven't modified anything of your original mounting system, and you can restore the original states anytime, if needed.
Now measure the standing panels to the base and to each other, as seen on my pics, and as you imagine; the optical axis of the tube in zenith-position must be about in the center of the base. It is not necessary to be absolutely correct, but recommended to stand the tube safely in every position. Drill the holes with an appropriate diameter for your wooden work screws, for the standing parts from the blind side of the upper base panel, and drive the screws; first the higher, side-panel, and after the smaller, front panel. And at the end drill and screw together this two panels, too, from the outer side of the standing panel. This is the only part of this dobsonian, where the screws are visible; i put small plastic aps onto them, so they are like small buttons now. Now take the teflon plates, cut a 2-2 cm squares from it, drill them out to 2-3 mm, forming place for the small screwheads under the surface, because the laminated surfaces have to slide fluently on the teflon surcaces, the screw mustn't scrape it. Screw them to its place, between the sliding areas, 3-3 pieces in 120-120 degrees, to the horizontal and vertical axis parts. (If you can't find teflon, use any glossy and solid plastic plates instead, it's almost perfect. The teflon is not glueable, don't try with it, but, if you can get teflon stripe in roll - similar to duct tape -, that is a good idea to cut pieces, and cover the other plastic plates with it, because the teflon stripe has a glued side.) Now take the smaller square at the tube ring piece, place the tube rings onto their place, and put the axis screws into their place, and strenghten them to firmly, but let them moveable. If you want, you can attach a knob to the vertical movement axis to loosen-strenghten the axis, but, if you have trim well the strenght of the screw, you won't need it. I have put one, anyway... At the end you can 4 small bottle caps to the lower base of your dobsonian as stands. These stands may be wooden blocks, too, or any solid material, but never use rubber, because we are working for the avoid the tremosrs, and rubber is the material which does it:-)
Step 4: Using This, or Other Dobsonians
You can carry this mount separatelly, you can carry it with or without the tube rings. The tube installing is ready in some seconds while you put it into the rings, and no any external tool needed to find anything on the sky, when you observe even if in zenith! Oh, sorry, you need one auxiliary object, however - a table, or some elevation for the mount - but be careful, don't use tilting table. Where I used to observe the sky, I found a concrete block, which is ideal. But this is perhaps the only convenience of this mount with a small telescope like this, and in my opinion this is worthy for it. Who builds this dobsonian, will admire at the stability and useful attributes of it, and even a novice is able to build it quickly. While I've builded it, I use my other, larger scopes rarely, because I fall in love with this system. A larger backpack, or an empty corner in the car, and I have a professional stable telescope anywhere, I go. (N.B.: the mirror of this telescope was a cheap, made-in-china- garbage, but I've asked an expert to re-grind an coat again, and after I have a semi-professional optics, so it was a double reason to use it with a good, stable mount.)
Good seeing and cloudless, starry nights to you!