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Marking Out and Setting Up Azimuth/Altitude Inscriptions On Dobsonian Type Telescopes

Marking Out and Setting Up Azimuth/Altitude Inscriptions On Dobsonian Type Telescopes

Also a Quick and Easy Way Of Setting Your Telescope To North

Hey guys, Nic here. As promised, finally got my act together, and have put this little piece to paper (as it were).

No stuffing around straight into it the process is pretty painless and not time consuming nor does it need a great deal of technical skill, but to help you out I will go through it stage by stage.

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For ease I will use the measurements that I worked out for my Telescope (an 8” Dobsonian) but you will be able to easily transpose these dimensions to any size scope.

On the base, mark out a circle with a compass firstly plugging the central hole with a suitably tapered wooden dowel (I whittled mine out of a solid piece of scrap wood), ensuring you have plenty of clearance between the edge of paper and the edge of the base.

Download Download Setting Circles program. (just click on the link) and as you will see, and  from memory I think, you only get a 90 or 180 degree print out over 4~8 pages so you will need to print out twice to get the required amount to get the full 360 degrees. Tape them all together then mark out the required diameter and cut to suit. For my 8” I used the following dimensions; Outside diameter 350mm, Inside diameter 200mm.

Reasonably important, that you have marked out those dimensions on the base as it makes gluing and placement of the of the cut-out azimuth scale easier. At the same time mark out circles 25mm larger and 25mm smaller as well as these are the guide lines for the placement of clear plastic cover film over the scale to protect it from the elements. After gluing let it dry then apply the cover film. Cover film dimensions O.D. 400mm I.D. 150mm.

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I’ve used a pretty green only because that was all that was lying around but still works OK, from here we now turn our attention to the telescope carriage. Firstly measure in from the edge of the base to a point that you will be able to imagine a rectangular viewing box that you will be reading the azimuth scale through from the carriage base. Then mark out those dimensions and cut out with an electric blade saw (firstly drilling a hole large enough for the saw blade to go through)

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Now turn carriage base over and mark out the ½ way point and drill two holes countersink them and attach a piece of wire using screws, smaller the gauge the better. This is your scale reading mark or pointer.

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Sorry for the picture quality hope you’re getting the idea though, now we move onto the altitude scale and reassembly.

Again measure from the edge of radius, where the Telescope attaches to the carriage base, and cut a rectangle for viewing the altitude scale, on mine of I put two different versions one mine and the other from the downloaded program mentioned earlier. To find and mark reasonably accurately the center of this rectangle, with the Telescope barrel in place, but without the adjusting handles in place, hold a stringline against the side face with the string hanging over the center of the hole and mark above and below the central hole, draw a line joining these marks. This is now your centreline from which to draw and cut out your viewing port, ensure you project this line far enough up and down so it is not cut out when you cut your view port. After cutting out the port, on the inside again as on the carriage drill and countersink two holes and attach wire as before.

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You can also attach one piece of wire for the side mounted altitude scale this is easy to adjust for accurate reading.

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This is how I originally marked out altitude face using the good old πD² for the circumference then dividing it by 36 to give you the graduations in10 degrees admittedly this is not pretty and a bit time consuming but it does work.

Now on to reassembly, with the Teflon bearing that is supplied with the scope, between the base and the mounting carriage, I have found that the surface area isn’t large enough to work effectively so I simply measured the thickness of it with replaced with two pieces of plastic cut out from an ice-cream container, bit from the lid another from the base they’re just the right thickness (original on right) on mine the thickness was 3mm.

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I have also put adjustable legs on the base, as if the Telescope is not levelled you will never get an accurate reading and will be forever chasing those objects that you are searching. For mine I used adjustable legs from an old washing machine welded the lock nuts together and while still red hot pushed them into the original plastic feet and then held them there till cool then quenched them in water to firmly lock them in place, then screwed them back onto the base.

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When putting it all back together I use a graphite powder dry lubricant. This is specifically designed for use in Plastic, Teflon etc and works extremely well and makes for very easy manoeuvring and still allows positive lock when you’ve got what you want in your sights. Apply this to all mating and moveable surfaces then get ready to set scope to north and start observing.

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Now on to setting the scope to north firstly get a descent Silva compass, minimum type 3 best type 4/54 or better and a GPS that will tell you grid convergence angle (or even a survey map that will give you the information in the legend). For your information the grid convergence angle for the Townsville area is 140mils or 8degrees for those civilians out there so set your compass to 6260Mils/352degrees ensure the pointer on azimuth is set to 0/360 degrees  put the flat of the preset compass on the edge of the carriage base and move the telescope base until you have the needle pointing at the 0 on the dial .

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Then using the adjustable legs, whatever design you’ve settled on, get the telescope levelled in two planes using an ordinary cheap level but if you can obtain a bubble level it will simplify the process.

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To accurately get the scope set, use a program like Stellarium (again follow link and use the program that suits your system), pick a known star, planet, moon etc and fine tune the position of the base by setting the dials (both azimuth and altitude) to the position given and nudge the base till the object you’ve picked is central in the eyepiece.Then adjust the compass till it is set to the correct heading and don't touch it, makes for easy setting up at a later date if you have to move the scope.

Well I think that’s it guys, any questions post them and I should be able to help this little setup is cheap, didn’t actually cost me a cent, and after the scope is zeroed in there is nothing you can’t home in on, within the bounds of your scope anyway, but I’m see galaxies, nebula, clusters etc out to mag10 then some on a good night. Well for some this should be a good project for the monsoon months up here in the north (if it ever gets here) and with a bit more care and patience you can make a much better example than mine but I was in a hurry and was results I was after, not looks. Happy viewing Nic.