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| Wednesday 23rd of July 2008 - 6:47 pm BST |
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Home Made 10" Dobsonian Telescope
The Dobsonian telescope is basically an alt-azimuth mounted Newtonian reflector built from basic components requiring little or no machined parts. It must be said that when John Dobson first invented this type of telescope there was a real financial incentive to building one given the price of similar telescopes from the limited suppliers at the time, if you are embarking on this type of project today however you must have reasons for building a Dobsonian beyond saving money as equivalent telescopes from the vast array of suppliers will not cost much more than making your own. My own reasons involved learning about the construction and principles of a Newtonian and the sheer joy of having a precision instrument, built by my own hands, that never fails to amaze friends and family when they see Saturn, Jupiter or the Moon for the first time through the telescope. Before deciding on the design of the telescope I acquired a 10" f5.5 mirror (made by Robin Gorman). I had already decided that grinding my own optics was a little ambitious for my first project but otherwise the plan had rapidly grown to the point I was going to build a motorised, equatorial mounted telescope and I had become somewhat confused by the bewildering array of possibilities. Robin gave some sound advice at this point - 'keep it simple' and I reverted back to a classical alt-azimuth Dobsonian.
Whilst mulling over the finer points of the design I purchased the necessary parts I could not (or didn't want to) make myself. These were the secondary mirror, a focuser, eyepieces (3 powers and a 2x Barlow), and a simple red dot finder. A Dobsonian in its simplest form is a cheap and cheerful tube holding the primary mirror in one place, the secondary mirror in another and the focuser attached to the side. The tube sits in a small box (with circular runners) at its centre of balance. The whole assembly then rests on another swivelling box on Teflon pads allowing the tube to be tilted up and down. The most popular budget tube used is a length of wound cardboard used for forming concrete pillars. So called 'Sonotube' cylinders seem to be widely available in the US but I only found one supplier in the UK and I thought the cost of the required length plus postage was rather expensive. I decided therefore to build the tube from plywood in the form of an octagon.
After 48 hours curing the mirror cell was complete. Whilst the silicone cured it was time to construct the spider. This was built from disks of brass (turned by a work colleague on his metal lathe), a microbore straight connector, four steel vanes cut from the cladding of an old fluorescent tube and a piece of cherry wood turned on my father's lathe and then cut to 45 degrees. In addition to this secondary holder the wood lathe was also used to turn the mirror plate and the runners on the tube holder. Incidentally, apart from lending his skill and advice to me in the construction of parts my dad 'Fred Harris' also painted signs for an HAG open day many years ago after someone from the group spotted his signs for the Catherington Show!
Nearly there, I only needed to collimate the telescope now. I approached this with some trepidation, would the focal plane actually be in the focuser or had I got some measurement or other wrong? Reading up on collimation proved a frustrating business as there seems to be a wide range of techniques and opinions on what is critical or not. One piece of advise I can give which I did not see anywhere is just get things roughly right by eye to begin with. The spider was centred (actually offset slightly as recommended) and the focuser shimmed slightly to square it in relation to the secondary. The secondary was roughly positioned by looking down the focuser. I then stood away from the tube and with one eye on the reflection of the secondary moved my head till the secondary obscured it's own reflection, this view then shows the spider to be offset from the centre of the tube. Adjusting the collimation bolts of the primary corrects this.
As it was a cloudy night the only thing I could try focussing on was a building at the bottom of the garden. I found that the focus point was beyond the fully racked out position! The following day allayed my fears when I succesfully focused on the local church spire, the building at the bottom of the garden was just not far enough away. Another cloudy night, but the following evening was clear (if not good seeing due to the fullish moon). I placed the telescope on its base swung it round to the moon (silky smooth movement, no stiction to speak of) and wow! Some great views of the moon along the terminator, I'll need a moon filter though, a 10" mirror gathers a lot of light. The nights have not been great since with the Moon or haze deteriorating the view but so far I've seen the Moon, Jupiter, Saturn, M3 globular cluster, M44 globular cluster ('Beehive'), M13 globular cluster, M97 ('Owl Nebula' - just!) and some great views of the stars in general.
The next project will be to build the equatorial mount from some scaffolding tubes I've been given and drive the scope. I'll also be adding a finder scope (6x/8x?) to aid finding beyond what I can do with the red-dot finder. Some further collimation may also be necessary - I think I could get the stars sharper but then the seeing's not been good yet. All that can wait for now though while I enjoy the sky through my completed telescope! Specifications (millimetres)
Oct 2007 Update
A Crayford focuser has been added and a Sun viewing port cut into the front cover (with a Baader film insert). Another major upgrade has been the addition of motors to track the sky. One of the altitude supports has also been replaced by a steel axle resting on a bearing (to remove some sideways motion when using the motor).
The telescope is still work in progress (it will probably never be finished - there's always something else that can be done. The next job is to improve the azimuth bearing to remove some backlash. Paul Harris
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