Back

Members

S*T*A*R Events

Observing

Astro Photos

Telescope Making

Light Pollution

Library

Spectrogram Newsletter

Discussion Board

Club

Links

On February 7, 1998, there was a meeting at Don Odegard's house for STAR members interested in building an equatorial platform. For those unfamiliar with the term, an equatorial platform is a low platform that a dobsonian or other scope can be set on (the platform can replace the ground board of a dobsonian mounted telescope). A motor tilts the platform slightly so the scope will track objects as the earth turns. A well-made well-aligned equatorial platform is capable of tracking accurately enough for astrophotography.

The purpose of the meeting was for all of us to learn about how an equatorial platform works, examine several sets of plans for building them, and come up with an idea of how to build one of our own. In future meetings, we'll nail down the design and actually build one. Assuming this prototype works, we'll be building several of them.

Don had done his homework, and had handouts of articles on designing and building several different equatorial platforms. Kay Sears brought a small model he had made to help us all understand how one works. Several people wanted to take it out and try it despite the small aperture and low power of the toilet paper tube used to represent the telescope on the model.

Most people are familiar with the german equatorial mount (see figure 1). The mount's polar axis points towards Polaris, so it is parallel to the earth's axis or rotation. As the earth rotates, the polar axis is rotated in the opposite direction to keep the telescope pointing in the same direction.

The principle behind the equatorial platform is the same, but there is no real polar axis. Imagine a large cone, whose axis of rotation is parallel to the earth's when it is laying on its side on the ground (see figure 2). Now imagine a large saw parallel to the ground comes along and cuts off all the parts of the cone more than a few inches from the ground. What's left is essentially an equatorial platform. It rotates around the axis that the cone would have if it were still in one piece. Because the axis is mostly missing, it is called a ``virtual'' polar axis.

Unlike most equatorial mounts, the equatorial platform can't swing 360 degrees (if it did, the telescope sitting on top of it would fall off). Instead, it rotates at most about 7.5 degrees to either side of horizontal. This translates to at most one hour of tracking before the platform has to be reset. In practice, this is more than enough for observing or astrophotography.

We examined several designs, by Tom Osypowski (of the company ``Equatorial Platforms'' at http://www.rahul.net/resource/regular/products/eq_platforms/), Alan Gee, Chuck Shaw, and others. The major differences in the equatorial platform designs seem to be in the bearings and in the drive mechanism. Since the platform needs to be supported by at least three points, there needs to be a south bearing and two north bearings. Figure 3 shows the Osypowski platform, for comparison.

Generally, the south bearing (near the point of the cone) is a shaft (an alternative south bearing is a ball and socket which can be made from a trailer hitch), but the north bearing would be inconvenient if it were a shaft. Note that if both the north and south bearings are shafts, we've made a modified english mount. Instead, the north bearing is usually a plate, or a section of a cylindrical or conical ring, as shown in figure 4 (these sections are called ``sectors''). Each of these bearings can be constructed using rollers against metal, or with teflon on formica (they have been drawn as roller bearings).

The shaw platform uses cylindrical sectors for both the north and south bearings. We all felt it would be easier to use a shaft for the south bearing, because two sectors would result in four support points for the platform, and if they are not perfectly aligned, the platform would wobble.

As for drive mechanisms, the candidates are rollers (which sort of implies you have roller bearings) or a tangent arm. The drive is at the north end of the platform, since the larger radius makes for more accurate tracking. The tangent arm consists of a threaded rod, with a nut in the middle of it. The nut is coupled to the platform. The rod is held stationary, and as it is turned, the nut moves, moving the platform with it. The problem with the tangent arm is that the angular speed of the platform will not be constant if the angular speed of the rod is, so one has to either live with tracking errors or vary the speed of the motor according to the position of the platform.

For our design we opted for a tangent arm, because we felt it would be easier to make. A roller drive system would require gearing down the motor, whereas the screw on the tangent arm automatically reduces the motion. Next, we discussed what kind of motor to use. The options here are a stepper motor or a DC motor. The DC motor has the advantage of simpler drive electronics, but the stepper can be more precisely controlled and varied. Ultimately, we put off the decision until we do some research into how expensive a stepper controller circuit will be to make.

Figure 5 shows the design we have so far. It looks surprisingly like the model Kay brought in (sans toilet paper tube)!