A Question of Balance


Barlow plus Laser Collimator, and counter-balance weight straps visible

There may be occasions when the telescope becomes top-heavy. For example when a large 1.25" eyepiece (such as my Hyperion 13mm), is used in conjunction with a Barlow to increase magnification, and both the standard Finder Scope and an additional red-dot finder scope are mounted in place, and perhaps a dark cap observing shield (prevents overspill into the telescope from nearby streetlamps), or when you use a webcam or SLR camera with the Barlow, again this can be quite a combined weight that will make the front of the scope tip downwards.

While the CorrecTension system on the Orion XT Skyquest dobsonian scopes can be tightened and acts as a "brake" against the scope tipping, the added friction means the scope cannot be so easily adjusted in Altitude which in turn causes difficulty aligning on stars.

Really the better balanced the scope is the less you will have to use the CorrecTension system. Ideally you want the scope to balance perfectly when the Correctension is fully slackened off, although perfect balance is difficult because its centre of gravity changes as the scope tilts from horizontal to vertical.

 

 

 

 

The answer is of course to provide some means of adding weight at the bottom end to compensate, and the following photographs show a simple, cheap and effective method I used to make some counter-balance weights. The weights can slide up or down the length of the scope tube to provide the desired counter-balancing effect.

Making the Weights

As you can see it is quite simply a stretchable elasticated fabric strap, which has velcro fasteners to allow adjustment of the tightness of the strap around the telescopes circumference. I found a pack of two of these straps at my local DIY store (B & Q in UK). I purchased the largest size strap they had to stretch all the way around the 12 inch diameter tube of my (10inch mirror) XT10i. The circumference is Pi x Diameter = 3.14 x 12 = 37.68inches.

The straps pass through a self-fashioned plastic loop to hold the weight in place. The weight was covered over with black electricians insulating tape, and a couple of yellow/green "earthing" coloured insulating tape, to make it easily visible in the dark if it is accidentally dropped on the ground.

The actual weights are simply two very large brass door hinges per weight you want to make. These are then slotted together over each other to provide a greater combined weight. Other items could probably be used also, e.g. lead or other suitably sized heavy objects.


Black insulating tape was then used to completely cover the hinges, so holding them together, and paying particular attention to providing a good thickness at the sharper edges so as to prevent the metal coming through the tape which could scratch the paintwork of the telescope tube.

After the initial layer of tape was applied, I then fashioned a loop of stiff (but bendable) plastic cut from a clear polythene plastic food tray (the kind used to deliver take-away meals) to make the strap loop. This kind of plastic is thin enough to bend with breaking, and hold its shape reasonably well after bending. More tape is used to attach the plastic loop to the weight.

Adjusting the Weights

Its not easy to get a perfect balance for all situations, but the weights can easily be moved for extreme situations, e.g. move both weights to one end.

Generally as a compromise I position one weight on the topside of the scope tube right at the bottom (primary mirror) end, and the other about halfway between the altitude bearing mount point and the focuser, but on the underside of the scope tube. This seems to help even out the weight as the scope is tipped up toward vertical.

Most of the time only very light use of the CorrecTension adjustment is necessary, while tightening a bit more is required when the scope is either near horizontal or vertical.