Support for your Dobsonian

Bungee cords not only help to transport your Dobsonian telesope. You can use them to balance it too!

Balancing a Dobsonian is a tricky issue, especially when the instrument has to be portable and therefore light: the fast focal ratio requires complex eyepieces and your favourite eyepiece can often weigh well over 500g. But what can you do if it is not easy to bring it into balance?

The weight of a heavy eyepiece becomes critical, especially when observing close to the horizon, because there the lever effect is greatest. Counterweights do not have the same lever arm and generate a significant moment near the zenith. A spring mechanism can help here.

Bungee cords are particularly suitable for this purpose, as the aim is to have the greatest possible spring travel. The spring system should be subject to strain as the telescope tube nears the horizon. For this purpose, a simple mechanism with a deflection pulley on the rocker box can be used on the Dobsonian. The construction is secured in such a way that travel is generated in a spring running between the end of the altitude trunnion on the opposite side of the mirror box to the observer, and the edge of the rocker box directly below. Additionally, the tension can be diverted over a pulley, so that additional horizontal spring travel between the pulley and the other end of the rocker box is created. The long spring travel means that the counterforce of the spring mechanism changes only slightly over the adjusting range. If the tube is near the zenith, then the altitude wheel end is close to the deflection pulley, and the spring system is slack and has no effect. However, the more the tube is panned towards the horizon, the stronger the tension in the spring system becomes.

Author: Sven Wienstein / Licence: Oculum Verlag GmbH