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Home Made 10" Dobsonian Telescope

Article by Paul Harris

Hampshire Atronomical Group

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.

Searching the Internet for 'Dobsonian' revealed many amateur telescope maker (ATM) sites giving useful ideas, plans and pictures of the finished telescopes. Dobsonians range from the very simple to exquisitely designed and engineered instruments. Like any project of this type a lot of time was spent in background research and the design of the telescope changed daily in my head. I also purchased a copy of 'Build Your Own Telescope' by Robert Berry. This is an excellent book and though I didn't follow any of the plans it did provide some good guidance throughout. There is also something comforting about having a reference book to hand rather than just a list of Internet bookmarks!

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.

The basic design was built around a set of octagonal rings which would be the baffles in the finished telescope (to reduce stray ambient light from reaching the primary mirror). A Newtonian design program called 'newt' or 'newtwin' is available for free and this program will calculate all of the critical dimensions given a few parameters such as primary mirror size, focal ratio, internal diameter of the tube etc. Newt calculated the number of baffles required and their internal diameters.

Once the baffles had been cut they were pinned and glued to one of the eight lengths of plywood that would be the sides of the tube. I then similarly attached another six lengths of plywood and things began to take shape! The last length was left whilst I required access to the tube for painting etc. All of the inside surfaces of the tube and any parts located inside the tube were painted matt black to reduce light reflection and these surfaces were also deliberately left unfinished and 'rough' prior to painting.

At the 'business end' of the tube some 1/2" batons were cut and screwed around the edge to provide a mounting for the mirror cell. The mirror cell comprises of an 11" diameter plywood disk (18mm thick) to which three posts were attached which would hold the mirror in place via silicone adhesive pads.

Holes were then drilled through this mirror plate for similar silicone pads to key into, for three sets of collimation bolts and to ventilate the mirror via a fan mounted on the back plate of the telescope (to accelerate the cooling of the mirror & tube). The use of silicone obviates the need to construct floatation plates; its use is generally accepted for mirrors up to 12" in diameter. A larger octagonal 'back plate' was cut and three sets of collimation bolts, in push/pull pairs were used to mount the mirror plate to the back plate.

I also managed to cadge an old broken computer which was cannibalised for the case fan, mounted in the back plate and for the power supply. The mirror cell completed it was time to mount the primary mirror. Three blobs of silicone were squeezed over the appropriate holes drilled into the mirror plate and the mirror gently lowered onto the blobs, supported by three nails while the silicone set. Three further blobs were squeezed through holes cut into the supporting posts.

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!

The assembly of the spider proved to be a tricky business and a few attempts at soldering the vanes failed as I could not hold everything in place well enough. After some time and much swearing I knew I was beaten as I reached for the Araldite. Never mind, a good spray with matt black paint covers a multitude of sins. The spider completed, it was mounted in the tube (twice as it happens, after a basic mistake of not accounting for the thickness of the secondary mirror). The mirror cell was then bolted in place and the focuser and red-dot finder attached. The last side of the tube had already been attached via screws to allow future access to the tube innards.

Once the tube was complete the tube holder box and the telescope base were constructed requiring only rudimentary carpentry skills. Attached to the sides of the tube holder are wooden rings with kitchen worktop edging strips glued on. The base rests on a larger disk of worktop via three furniture gliders with a coach bolt through the centre. Four more gliders, cut to size were screwed to the side plates of the base where semi-circular cut-outs accommodate the circular runners of the tube holder box. The tube holder box has a removable top so that the tube can be removed if required.

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.

Once these rough adjustments had been made I could then use the laser collimator (borrowed from HAG) to complete the adjustments. This involved bringing the primary and secondary optical axes into line by first adjusting the primary until the laser pattern was centred on the primary mirror and then adjusting the secondary until the reflected beam was centred in the focuser.

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 construction took place from late December 2004 to the end of March 2005. Richard Berry's book reckons two weeks of evenings which is probably about right since I managed about half a day at the weekend and perhaps one or two evenings each week. The whole experience was enjoyable and I've inevitably learned valuable lessons from my mistakes.

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)

• Primary mirror diameter : 254
• f ratio : 5.5
• Diagonal minor axis : 52
• Focal length : 1397

 

Oct 2007 Update

The telescope has undergone a number of modifications since I initially completed it in 2005. A finder was added (not seen in these pictures) and the mirror cell re-designed (using the marvellously named Plop freeware program to calculate the triangles used to make a nine-point cell). The spider has also been re-designed as I was particularly unhappy with my initial attempt.

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