CONSTRUCTION

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CUTTING TIMBER

BELLS & WHISTLES

THE BEARINGS

THE PLAN

You can just about get the idea from this picture, but click on it to go to a larger, clearer version with the dimensions, if you like.

N.B. The dimensions on the drawing are applicable only for my telescope tube.

Remember - measure twice - cut once !

CUTTING TIMBER

Well, laminated chipboard, to be precise. Two 18 inch diameter disks were first cut with a router and circle guide from 3/4 inch thick plastic-surface laminated chipboard . This is very flat and dimensionally stable - provided it doesn't get damp (but then I don't use the 'scope in the rain!). A centre hole was drilled through both and a 'T-nut' fitted in the bottom one to accept a bolt which holds the top assembly to the base and acts as the pivot point.

The two side panels were cut from 3/4 inch plywood (stronger than chipboard) and the 8 inch; diam. semi-circles cut as one circle by pinning the two boards, head to head, onto a scrap board underneath and cutting the circle with the router. Separating the two,after cutting, left two very neat arcs. You don't need to go to this trouble, V-shaped cuts in the boards would work just as well because the tube assembly only touches at two points. Similarly the base doesn't have to be round - facetted or even square would do. The front panel was cut from 1/2 inch; ply for weight saving (actually, I didn't have enough 3/4 inch; stock left). It's important to note that this piece will act as a stop to prevent the 'scope tube from swinging down past the horizontal, or, if it's too tall, stopping it from ever reaching that position.

The three pieces of the mount were then glued and screwed together and them glued and screwed to the base. No problem, the screw-heads on the underside will not interfere with the bearing. Now for the bells and whistles - two gussets were glued either side to firm up the side shimmy of the mount. Two shelves were fixed on the sides - one with holes in to hold spare eye-pieces (works well) and one with a ledge round it to hold pens, torch etc. Finally, a strong carrying handle was made.

The complete assembly was sealed, sanded and painted a fetching metallic grey - all except of course, the laminate bearing surfaces !.

The tube cradle was made from 1/2 inch; ply and the top fixed with a piano-hinge. A clasp (and two bolts - an afterthought) were fitted so the top could be firmly closed, afterall, it does hold the tube, and a carrying strap was then fitted. There is a row of holes drilled in the bottom of the box, along the centre-line. Any pair of these will mate with the corresponding dowels fitted into the telescope tube to stop it slipping backwards out of the cradle. Balance can be adjusted by the choice of holes. The two bearing disks were router-cut from 3/4 inch; chipboard. Some say the disks should be the same diameter as the mirror - well, I made mine 7 1/2 inch; for luck. They are screwed onto the box making sure thay are dead-central and square with the telescope - but not until the bearing surface had been added. I've added a separate page on bearings because it's a topic all in itself. After that, come back here and read about the final:-

JUMP  TO DOBSONIAN BEARINGS FROM HERE

BELLS AND WHISTLES

After a short period of use, a few improvements were called for.

DOBLOK The tube tended to move as I changed eye-pieces  because different ones upset the balance. Initially I tried an adjustable  counter-weight at the back, but that proved too fiddly to adjust in the  dark. So I built a friction lock for the altitude control consisting of a  P.V.C. shim bearing on the disk and pressure adjusted by a threaded rod with  a knob on the end.

The Doblok - versions 1 and 2

Recently I changed from the simple friction lock to a tangent arm system allowing the tube assembly to be finely adjusted in altitude. The hand-wheel in the centre tightens the arm for fine adjustment but when   loosened, allows the tube to be moved by hand for coarse positioning and tube removal.

DOBTAB Where to put the papers, sky charts  etc.? and how to stop them blowing away ? I built a small table extension   which clips onto the main stand with two metal bars. Inset into the table is a very shallow magnet - a large steel washer holds papers fast against  this.

The Dobtab

FAN COOLING Even with a modest mirror like  this, distinct improvements in image quality are still occurring after  1 hour's viewing in the garden and after 1 hour standing before . Time to  try a fan. I had prepared a hole in the rear plate of the mirror cell at  the construction phase, so no mods were needed other than to make a  thin plastic plate, mount the 2 inch 12 v computer processor fan to it and  screw it to the plate. It makes a real difference ! and as there is no  discernable vibration, it can be left on.

LEGS Despite all my careful planning, the eyepiece was  a little too close to the ground for comfortable viewing at moderate  elevations, so I added four  sturdy legs screwed to the underside of the base disk. There are four  screw level adjusters to stop rocking on uneven ground. As a bonus, it keeps  the dodgy chipboard of the damp grass.

That about completes the Dobsonian project - I enjoyed building it, now for just a few more clear nights ! My Equatorial Platform project is on these pages.

I can be reached by e-mail at molyned at ntlworld dot com