North Devon Rocket Building a 12.75 inch Dobsonion Telescope
I have got several blank primary telescope mirrors in various stages of polishing, last year I got given a 12.75 inch aluminised mirror, I decided last month to make a telescope to fit it into, and the Dobsonion is the easiest way to go, so over the next few weeks, this will be my project.
First I tested the mirror for faults and the state of the curvature of the mirror surface, checked with a light the surface of the mirror and its focal point, also did several other tests, all where good, its focal point is F6, my 10 inch SkyWatcher is F5, so the 12.75 is quite a fast mirror.
I bought some 6mm ply from a local firm, which I have glued together to get 12mm thickness , next I drew out plans for the telescope, so that I would get the measurements right, allowing for the size of the primary mirror, next I cut two 15 inch squares from the ply, then measured to where the adjustment bolts will be, and firstly drilled a 20mm hole to 5mm deep in one piece of ply, and drilled a 10mm hole through both pieces of ply, with some springs I purchased from a local firm, this will make up the adjustable mirror cell, there will be wooden blocks glued a screwed to the piece of ply holding the mirror, this will be support for the mirror, when the telescope is in use.
The next step is making the lower body of the telescope, which will hold the mirror cell, this will be cut from the same 12mm ply, and will be box shape, 15 inches on the inside, for a snug fit of the mirror cell, and when telescope is finished, will be screwed in place, and will be 500mm long, the depth will help get rid of any stray light, also I will bolt the 10 inch discs to both sides of the bottom body, these will have Formica strips glued on to the edges, for smooth up and down operation of the telescope when finished and in use, next is to build the top section of the telescope body, basically similar size to the bottom section, but is 300mm shorter, this will hold the 'Spider plus flat mirror' and the focuser, these will be joined together by four wooden trusses when the body sections are measured for the focal length.
The base is the next part to build, there is two parts, the floor section, I made out of an old kitchen work top, this is quite thick around 30mm, and has Formica glued to it already, so it perfect, a friend of mine had an odd piece, which he was going to get rid of, so I screwed four rubber door stops on each corner, act as feet, drilled a hole in the centre, to put a 15mm bolt through, and nuts, this will act as an anchor for the top base to swivel on, the top base is made from 12mm ply, with PTFE blocks screwed on the bottom, which will act as sliding bearings on the formica surface, this is also drilled to slide over the 15mm bolt, a metal plate is screwed on top to hold a ball bearing, with the bolt adjusted up or down, this will balance the top and bottom bases, and swivels easier.
Since starting this project a few months ago, I have had some modifications done, I have modified the mirror cell, I have cut a lot of the ply away to make it lighter, and have added three metal brackets, to hold the mirror in place, I have made a longer box section for the focuser, ( I tried to make the spider for the secondary mirror out of ply, but was to heavy, and not practical), so I got on to a friend of mine at Beacon Hill Telescopes, and he made me a spider and mirror holder, I glued the secondary to it screwed into the box section, with the main and secondary mirrors in place, I rested the telescope onto a metal tool box, out side my garage, put a 25mm lens into the focuser, aimed the telescope at some distant trees, and moved the focusers box section on the four trusses, to focus the telescope, once done, screwed the box section to the trusses, I then balanced the telescope to find out where exactly the bearings should go (wooden discs), and found that are now further forward, so I have taken the discs of the bottom box section, and will be moving them forward to the balance point, the melamine, that I glued to the bearings, to make the up and down movement of the telescope easier, could not take the weight of the telescope, so I have sent the wooden discs of to Beacon Hill Telescope, and my friend will be fitting PTFE rings on the outer edge of the discs, and strips for the base mount, which will also be made taller.
well after taking me five months of and on, and several modifications, such as re-positioning the bearings, making a new focal mount box, my telescope is nearly finished, all I got to do now it sand and paint it, in the mean time I am waiting for clear skies, so that I can check it out, and to align the spotting scope, and do minor adjustments where necessary, I cant wait to see Jupiter through it.
