Hack42.Improve Dobsonian Motions with Milk Jug Washers

Hack 42. Improve Dobsonian Motions with Milk Jug Washers

Eliminate the jerky azimuth motions that start to plague your scope after a while.

The azimuth (right and left) motions of inexpensive Dobs may be adequate when they are new, but often the azimuth motions become rough and heavy as the scope breaks in. This occurs because inexpensive Dobs use hard plastic or mechanical Teflon pads riding on a smooth laminate surface, and the friction and stiction (static friction) of this combination are difficult to control.

The real fix is to replace the bearings with virgin Teflon pads riding on bumpy Ebony Star laminate, but that requires spending money and devoting several hours' work to the project. But there is a fast, free solution, albeit a temporary one. You can supplement the existing azimuth bearings by adding a center bearing made up of several thicknesses of plastic washers cut from flat-sided milk jugs. (Although milk-jug washers are traditional, some astronomers use old AOL CDs or even LP records; they're called milk-jug washers regardless.)

The improvement in azimuth motions can be significant and, depending on the material you use for the washers, may last for dozens of observing sessions. When the washers begin to lose effect, it takes only a few minutes to remove the old washers and replace them with fresh ones.

To make the washers, cut full circles from the flat portion of the milk jug, or simply use recycled CDs. (In fact, a CD also makes an excellent template for cutting milk jug washers.) Don't worry too much about the size. We used washers about the diameter of a CD, but even 2" washers work fine. (The sole purpose of the washers is to bear some of the weight that would otherwise be borne by the azimuth pads; central washers contribute almost no friction.) Punch a hole in the center of the washer that's large enough for the Dob center bolt to pass through without binding. If you use CDs and the center hole is too small, use a file or reamer to enlarge it until the Dob center bolt slides freely within the hole.

You'll need to experiment to determine how many washers to use. The number you need varies with the thickness of the material and your particular scope. (Our scope needed five washers cut from milk jugs or two CDs.) If the washer assembly is too thin, it makes no contact and accomplishes nothing. If it is too thick, it takes all the weight off your existing azimuth bearing pads and can actually make the motions worse. The goal is to let the center bearing washer take just part of the weight.

To install the washers, remove the nut from the center bolt and lift the rocker box free of the baseboard. Place a few of the washers over the center bolt, as shown in Figure 3-29, replace the rocker box, and test the azimuth motion. (You needn't replace the nut on the center bolt each time; it's there simply to keep the rocker box connected to the baseboard, but it applies no tension.)

When you reach the proper number of washers, the improvement in azimuth motion should be obvious immediately. You should be able to start the scope moving with the gentlest of finger pressure, and keep it moving smoothly with almost no discernible pressure. When you stop pressing, the scope should stop moving instantly, without any overshoot.

It's best to try the washers without any lubrication first. If they work to your satisfaction, great. If you'd like the motions to be lighter still, try lubricating the washers slightly. We've tried everything from machine oil to Vaseline to WD-40 to paste wax to spray-on furniture polish to graphite lock lubricant, with both milk jug washers and CDs.

Most lubricants seem to reduce friction well enough, but at the expense of increasing stiction. Stiction is static friction, which gives the washers a kind of undesirable inertia. Although they move freely once they are moving, it takes significant pressure to get them moving in the first place. Stiction makes it very difficult to track objects, particularly when you are using high magnifications. We got the best results with graphite lock lubricant, but, as always, your mileage may vary.

Figure 3-29. Installing milk jug washers (actually, CDs in this case)

If milk jug washers work for you, keep a spare set or two in your observing kit. They stop working as they become worn, and once they start degrading they go downhill fast. Still, it takes literally a couple of minutes to replace them, and they may be all you need.