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A cheap balancing system for dobsonian telescopes March 10, 2007

Posted by dorigo in astronomy, personal.

Portability is a must for dobsonian telescopes, which are meant to allow the user to use at its best the light-collection power of large mirrors. I own a 16″ scope, which weights about 80 pounds. That is, it used to weigh 90, before I found a simple way to get rid of the lead counterweight that was needed to prevent the tube from tipping over while using heavy eyepieces.

Eyepieces today can weigh from a few ounces all the way up to three or more pounds apiece. During a change of eyepieces, your telescope may not have enough friction in azimuth to maintain balance – you do not want too much friction, lest your scanning of the sky will become too bumpy. You want silky smooth movements, but a good balance regardless of where you are pointing at – the zenith, or close to the horizon. So you need a balancing device.

Counterweights have a defect: they are heavy! And bringing them along is a pain, the more so if you keep your scope, like me, quite far from the trunk of your car. So I switched to a system based on a rubber cord, thick enough to cause friction with the structure of my scope. The cord has a loop which extends when the telescope tips to lower altitudes, maintaining balance in a wide range of positions – from the zenith to 50 degrees from it. However, beyond that point the weight of the upper cell and eyepieces outdoes the pull from the elastic cord, requiring extra tension.

The solution is to use the friction between the elastic cord and the foam which lines the tubes of support of the upper cell. The “tail” of the loop of my elastic cord is wrapped a couple of times around a pole, and its free end can be comfortably pulled from a standing position, and it stays in position, providing just the extra tension that keeps the balance all the way to the horizon.

Below are a few pictures of the arrangement – not particularly esthetically pleasing, but effective – and it saves 99% in weight with respect to the former  balancing system!



1. Carl Brannen - March 11, 2007

One of the things I found fascinating was drawings of the early telescopes used to see the moons of Jupiter and the like. They were refractors of very long focal length. The problems of aligning a 3″ telescope 100 foot long with the technology of the day were considerable. And imagine how fast the thing had to be swept to follow the earth’s rotation.

2. dorigo - March 11, 2007

Indeed, Johannes Hevelius had one 150 feet long in the XVIIth century. The advent of reflectors was a slow but steady revolution in the field.
What is funny, is that nowadays for us dobsonian owners the problem is reverted: the longer the focal length F of our toys, the less we have to worry about alignment! That is because the main concern is the reduction of off-axis aberrations, which strongly reduce the usable transverse spot for placing a eyepiece as the F/D ratio (D being the objective diameter) decreases.
When one has to deal with a D=16″ or larger scope, having a F/D ratio larger than 4 (the minimum without complex correction systems) means having to climb a ladder to assemble and observe. This is rapidly impractical as D increases.That is why 16″ are the maximum size that can be reasonably carried around, assembled, and used by an individual alone.


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