February 2001 Newsletter

Stockton Astronomical Society

Valley Skies

Valley Skies - February 2001 Issue

The Telescope Nut
by Jeff Baldwin

Driving Dobsonians

Dobsonian Telescopes are so easy to set up and use; they are basic and low-tech. They don't track the stars, though. There are a few ways to make this happen.

One is to mount the telescope on an equatorial platform. Another way is to place stepper motors on both axes and control their motions by either a calculator or a computer. The third way is to move to the North Pole and let the telescope's azimuth axis rotate once per day (less convenient). Let's start with the equatorial platforms.

Imagine a conic shaped object that has its axis of revolution pointed toward the Celestial North Pole. It could be rotated once per sidereal day and it would keep in step with the Earth's motion. If we were to remove most of this conic solid leaving only a small part that acts as a bearing surface, we could let the top of it act as a table. The Dobsonian telescope could be set on this table, and it would still rotate once per sidereal day, maintaining alignment with the North Celestial Pole.

This would be great until the platform runs out of travel and the conic object falls off the end travel of the bearing. This type of equatorial platform usually has a little over an hour of use before needing to be reset. The weight of the cone onto the bearings is vertical making this a fairly good type of platform for heavy scopes.

Another type would be a cylindrical solid. It also has bearings in contact with the solid, but they are not aligned vertically with the gravity force.

Let's remove most of this cylinder, leaving a table onto which the telescope can be set. Again, it has a time limit before needing to be reset.

If the telescope's center of mass leans over too far it will either fall over or place an abnormally large amount of push or pull onto the drive system, so the telescope and the equatorial platform must be fairly well matched and designed. Also, with today's large, thin mirrors, the mirror may be held in its strap with a lean in it due to this constant change in attitude, and this asymmetric placement of the mirror may deform it enough to alter the performance of the telescope. The alternative is the stepper motor drive system.

A high-speed computer can instruct stepper motors to adjust their position at short intervals on both the altitude and the azimuth bearings to step the telescope across the sky as you view. A few problems occur with this system.

First, if the steps are not tiny enough the discrete stepping may be seen in the telescope's view, or an oscillation may occur, amplifying the steps and shaking the telescope. Mel Bartels of Eugene, Oregon has overcome this problem brilliantly by changing the voltage to each pole of the stepper motor trigonometrically rather than with square wave as normally occurs with stepper motors. This allows the stepper motors to 'sweep' rather than 'step'. Smooth motion is the result, and incredible accuracy and targeting are possible.

Another problem is in field rotation. Since the telescope is driven in altitude and azimuth, the field of view rotates in the eyepiece. This is no problem for visual use, but if you are attempting to do long exposure photography the field rotation will cause arcs to exist in star images. This can be overcome by field de-rotation equipment and software. The big advantage to using steppers instead of equatorial platforms is that the steppers don't have to be periodically reset like the platforms, and the weight of the mirror in its strap is always symmetric, as designed by the ATMer.

Here is a rough layout of Mel's system showing only one of the two circuits.

In the old days, telescopes that were large (greater than 10" aperture) had to be on an equatorial mount and had to be massive. It was crazy to think otherwise. Today we think nothing of having a 24" scope that we move along by hand as the Earth rotates. Furthermore, telescopes are made of lightweight material (wood) rather than a ton of steel. We don't really care about how the field rotates as we observe, so the state of the art is now large thin mirrors in lightweight Dobsonian mounts driven by equatorial platforms or stepper motors.

We have it good these days. It makes you wonder what people will be using in 20 years, and what they will be saying about the way we do it today.

Clear Skies!...Jeff Baldwin
For more information on Telescope Making jump to the ATM page.