Ultra light-weight 8" dobsonian

The history

5 years ago I bought a very good 8" f/6 Orion mirror (though, a bit on a thick side), secondary and custom-made helical focuser (made in Hungary). My first telescope was soon born. It was a typical, small truss-tube dobsonian. It had a 3 thick aluminium trusses connecting the upper and lower rings made from a thick (and heavy!) 25cm PVC tube. With the heavy focuser (0.35kg) and the upper PVC ring, the OTA center of gravity was quite high (more then 0.5m) - this is typical with small (less then 10") dobsonians. It had a home-made finder, adjustable weights, eyepiece holders and even a primary and secondary mirror illumination LEDs (for collimation) and mirror cooling fan. In other word, it was one meeeean scope :)

But as time went on I found more and more flaws in my design. PVC tube (though thick and quite long) liked to slightly deform under heavy stress (such as while looking at low altitudes), thus loosing collimation. With high center of gravity and inadequately strong mount the scope soon became a bit... well.. wobbly. It was time for some heavy redesigning.

After a year of planing (and finding time), simulations and calculations, my current telescope finally saw it's first starlight (after mandatory 'Murphy' cloudy period).

Goals

This time I wanted a very light and minimalistic, but solid scope. It was clear that very low center of gravity was required. After a few calculations and web surfing it became clear that this is going to be a bit tricky (low cost was very high on my priority list so light metal alloy was out of the question - it had to be wood). 16" ultra light-weight dobsonian - no problem, but 8"... can it be done (with ultra heavy focuser on top of it all!)? The road to my final design was very long and bendy and full of frustration.

Innovation

It can't be done! That was a clear conclusion after a few months of planning. You can't have both light-weight and compact/low-COG telescope, not with heavy focuser (which I liked and wanted to keep and also didn't have enough money to buy another high-quality, low profile but ultra-light focuser). I was stuck. No classical or even semi-classical design would do. Then it hit me. I could use springs for the much needed counter-balance!

Soon I made my first prototype. Much like a final design, it had a extremely low mount box, large alt disks, 8 thin trusses and a minimalistic upper ring. It also had a classical primary mirror cell (3 point) and a very strong spring. In theory it all worked. In practice, I couldn't believe how stupid I was! I forgot to calculate one very important thing simply because normally, it's never a problem. COG was, of course, not directly above a base so the telescope would top-over and fall on it's nose. :( The very large base that would make the telescope stable would also render it both heavy and hard to transport. I had to make the upper ring even lighter and move the heavy mirror even closer to the ground which was not possible with classical mirror cell. Thus, a hanging-cell was born!

Looking at zenith, the mirror was now just 4cm above the base and would pass just a few mm above it at lower altitudes! As with ultra-light design every cm of primary mirror position counts, it was now possible to build a stable telescope with a much weaker spring.

The final design album

As you can see, the rocker box is VERY low. Notice the counter-balance spring, substituting almost 5kg weight that would otherwise be necessary. Both bearings are teflon-plastic.	The altitude 'disks' are very large, with more holes in them than wood :) The mirror 'box' is just 5cm high! You can also see ALT and AZ setting circles (thanks Beri for the magnet idea!)
This is what made it all possible - the hanging cell. Not only does it significantly lower the mirror position but it is also much more convenient for collimation. At alt=60 mirror passes just 1cm above the base plate!	Back view of the hanging cell. There are no mirror holders. Mirror rests freely in the cell and can easily be removed for transport.
There are two main objectives when designing ultra-light telescope: 1) put the mirror as low as possible 2) make the upper ring VERY light With the heavy focuser it was hard. The focuser and a 40mm eyepiece weight as much as the rest of the upper ring. BTW, that strange thing beside the focuser is a telrad-like finder (homemade, but you can tell). :)	Spider design is also not very 'classic'. Both position and titl of the secondary mirror are adjusted by tilting the whole spider. Even though this design isn't as rigid (meaning that it will lose collimation if ran over by a truck) as a classical fixed design, it is much more convenient for quick collimation. Collimating secondary never takes more than a few seconds.
The end result is this! The telescope has a look usually associated with a large (16" and more) truss-tube dobsonians. By using a 8-truss design you get a VERY rigid OTA. Together with a extremely low rocker box, this makes one stable telescope.	Telescope from another angle. See the mirror hanging below the mirror box? :) Triangular mount base is optional and can be removed (telescope is usable even without this base).
This telescope is as compact as it is lightweight (shown here with a CocaCola bottle for comparison). It can be fully assembled and collimated in less then 5 minutes. The whole telescope weights about 8kg. Two people can even take it mountain-climbing with enough room left for food, tent, sleeping bag and other stuff. :)	For ALT-AZ setting circles to be of any use you need a notebook, PDA or a stronger calculator. This is my Sony Clie SJ20 handheld running Planetarium. :)