Variations on the BallScope Telescope Mount

Mel Bartels, January 2013

The ballscope is a wonderful mount; it commands a loyal following. With a single push the scope can move in both axes and rotate the eyepiece to a comfortable viewing angle. And there is no overhead 'Dobson' hole at the zenith, the bane of altazimuth scopes.

Ballscopes can track. Norm James (San Diego, California) in the late 1960's devised a motorized polar axis attached by suction cup on his 'spheremount'. See http://www.cloudynights.com/ubbthreads/attachments/3729721-JAMES%2C NORM - spheremount - 1A.jpg.
Circa 1990 Alphonse Pouplier (Belgium) invented a two drive wheel and two motor 'crossed parallel' mounting that tracked, slewed to an object and tracked satellites (August 1993, Sky and Telescope magazine).
Pierre LeMay (Quebec) in the mid-1990's devised a polar oriented two roller drive system. Pierre's 20 inch f3.9 scope is pictured here http://stellafane.org/convention/2013/images/scopes/scope14a.jpg.
About 2010 Jerry Oltion independently invented a single motorized polar axis drive system that he calls the TrackBall. See http://www.sff.net/people/j.oltion/trackball.htm

The size and weight of the ball becomes prohibitive in scopes larger than 20 inches [0.5m] aperture.Here are my ideas to modify the ballscope for larger apertures. First, the standard ball scope: a ball for pointing with the attached upper telescope tube:


As you can see, the ball is about twice the aperture. For a 20 inch [0.5m] scope, the ball size would be approximately 40 inches [1m] but with careful thought as Pierre LeMay has demonstrated, it is possible to use a ball as small as 30 inch diameter. Otherwise such a telescope would be difficult to transport.

What would the ballscope look like if we start removing unused sections of the ball? If we give up pushing the scope all the way through the zenith to the far horizon and instead allow a modest movement through the zenith to cover 'Dobson's' hole, then the trimmed ballscope might look like:

Here's Frank Szczepanski's hemi-ballscope 8 inch f/5 which uses a half sphere from a barbecue lid and a plain old 5 gallon plastic bucket for a base.


In this design the scope must be pushed around to reach the far horizon. However, large Dobs are often built with mirror mounts that use slings or two bottom edge supports at 90 degrees which preclude moving from horizon to far horizon by pushing through the zenith.

The ball is still too large but its weight has been cut in half. Let's keep going by acknowledging that in very large scopes, the eyepiece points out the side of the scope roughly horizontally within an angle of +- 20 degrees or so. Now the doubly trimmed ball scope might look like:


The ball has been trimmed severely to a manageable size and weight. In fact, other than the extra effort to make the smooth spherical surface, it is about the same dimensions from side to side as a Dobsonian styled altazimuth mount. But unlike the Dob we can push through the zenith forbidden zone and rotate the focuser to a comfortable viewing angle.

Trimming the ball to a saucer shape and adding altitude bearings for motion up and down results in the following design:


In this design the Dobsonian rocker box has been replaced with a curved saucer or Frisbee shape with an optional hole in the middle to allow the tube to swing through. The saucer acts to close Dobson's hole and allow the focuser to be adjusted to a comfortable angle. The altitude rims will need stops to keep the scope from sliding off. This has been demonstrated successfully by amateurs, e.g. see Rob Adam's Rob-a-Dob.

It is also possible to position the ball somewhat up the tube assembly. Then the telescope looks like:

Here's Frank Szczepanski's 4.25 inch slipped ballscope whose lower tube is from a round ice bucket and a sauce pan and chunk of lead that extends through the opening in the brown wooden box.


At the cost of a rocker box and the customary azimuth motion of a Dobsonian telescope, we can push through the zenith hole a few degrees and rotate the focuser to a good viewing angle at will. The rocker box contacts the ball at three points so the front panel needs reinforcing as shown and the side panels need trusses to not split apart (not shown).

These variations on the standard ballscope design show how the aperture limitation on ballscopes can be overcome.

Taking a completely different tack, the ballscope can be modified in an interesting and elegant fashion.

Let's start again with a side profile of the standard Dobsonian showing the telescope pointed horizontally and vertically:


For large scopes with lighter primary mirrors, the altitude rims can be surprisingly large. It's possible to compress the altitude rims into a smaller elliptical shape, like a sleigh:


Besides the obvious squeezing of the altitude bearings, something else very interesting happens: the tube assembly slides backwards as the telescope points down to the horizon. This means that elliptical bearings can handle telescopes when the center of gravity is located uncomfortably further up the telescope tube. Explained differently, the center of gravity slides vertically downward as the scope transitions from pointing at the zenith to pointing at the horizon. The consequence is that the altitude bearings can be made smaller. This is a design that I've successfully prototyped and discussed privately for several years now. The drawback is that the elliptical altitude bearings need a varying drive rate if the scope is to track and slew across the sky to point at an object.

If we marry the elliptical concept with the ballscope, then we get a new design, the 'Egg Scope':


This ovoid design is shaped in a complex elegant manner, inspired by Nature. Though difficult to track and slew, I find this design compelling. I wonder if it will ever be built.

The Sketchups can be found here:
ball scopes
elliptical bearings
eggscope

Pierre Desvaux from France has designed an elegant egg shape with beautiful symmetry. See http://dobsonfactory.blogspot.fr/2013/01/ze-egg.html  

Both the trimmed ballscope and egg shape are two interesting approaches for overcoming the aperture limitation with the standard ballscope design.

Tom Conlin's SudiBall designs http://ix.cs.uoregon.edu/~tomc/Hobbies/Astronomy/ATM/SudiBall.pdf

Thanks to Jerry Oltion, Tom Conlin, David Davis and others for their thoughts and inspiration.

eod