Building an Equatorial Platform from Scrapwood – Part 1 – What’s an EQ Platform?

In astronomy, there are essentially two types of frames on which to put your telescope. The first is the altitude-azimuth (Alt-Az) mount, where the telescope points up/down/left/right, according to terrestrial coordinates. The pointing direction is referenced to the direction along the horizon, as well as elevation measured from the horizon to zenith. The other type of mount is called equatorial, and the pointing direction is referenced to the celestial pole and equator. From our Earth-bound vantage point, and in human time scales, the positions of the stars are essentially constant. What changes from month to month is the region that is up in the sky (as opposed to being below the horizon). In a single night, the rotation of the earth can result in significant changes in the position of objects, sometimes in a matter of seconds at high magnification. Below is a gif showing the movement of the sky in a few hours.

Incidentally, this is what causes star trails, which are nice images in their own right, accomplished simply by increasing the exposure time, and stacking large number of frames. It’s generally not possible to create star trails from a single super long exposure digital shot, since the ambient light will soon wash everything out. Furthermore, camera sensors heat up with long exposures, and I don’t think a 3 hour exposure is necessarily the best way to get the images, especially not if the battery runs out halfway!

You may notice that a single star – Polaris, has been highlighted in the image above. This is our Northern Pole Star – it sits near enough to the celestial pole that it appears to move very little throughout an entire year. There isn’t really a Southern counterpart. Polaris, contrary to popular belief, is NOT the brightest star in the sky. Not even close. It’s only the 50th brightest star, at an apparent magnitude of just over 2.0. However, it is bright enough to see at high latitudes, and a much more reasonable Pole Star than Sigma Octantis in the south (a measly magnitude 5+ star).

Because of this movement, objects in the eyepiece can move quickly out of view. One way to counteract this is to simply motorize the mount – move it by an amount to counteract the rotation of the earth. And that’s what most Goto mounts do. For Alt-Az mounts, this means continually moving the scope left and up or down, to track the object. While that keeps the object in the eyepiece, it is continuously rotating in the field of view – the so-called field rotation problem. You could tilt your head, or rotate your camera if you are imaging, and indeed those are valid solutions. But that means you now have to install three motors, and calculate each one carefully to counteract a simple rotation.

That’s where EQ mounts really shine, because one of its axes (the Right Ascension, or RA axis) is designed to be perfectly aligned to the polar axis. In other words, a single, appropriately timed motor will be able to counteract the effects of Earth’s rotation. Pretty sweet. Except that EQ mounts are huge, heavy monsters, with counterweights designed to offset the lever arm of the telescope. Many scopes are simply too large to fit on an EQ mount of reasonable size. Dobsonians, those relatively low-cost, large aperture instruments, are particularly unsuited for mounting on an EQ.

A typical EQ platform. Image from http://www.equatorialplatforms.com/about.our.platforms.shtml

However, clever astronomers learned quickly that it is possible to put the entire Dob onto a platform, aptly named EQ platform, that has an axis of rotation pointing at the celestial pole. The best part is that these are reasonably small, don’t have to be accurately aligned to Polaris, can easily track for around 1 hour before requiring a reset, are typically driven by a single motor at a constant rate (no computers needed!), and can be built for cheap, under the right conditions. An EQ platform is generally considered good enough for latitudes within 5 degrees of its design latitude. So, I decided to build one for my ultraportable dob!