This article explains how German Equatorial Mounts work. It consists of 2 parts: The first part provides a general overview about these mounts. GEMs require polar alignment; it will be discussed what this is and how it is done. Part 2 talks about more specific subjects like Declination, Right Ascension, setting circles, mount balance and load capacity.
Most people are familiar with camera tripods that offer vertical movement and head rotation. Telescope mounts based on this principle are called Altitude-Azimuth mounts, or Alt-Az mounts for short. German Equatorial Mounts (GEMs) are more complex, but they are preferred by astrophotographers and observers who like to view particular objects for long periods. Furthermore, GEMs are ideal for finding faint celestial objects just by coordinates.
German Equatorial Mounts are designed to compensate for the earth’s rotation. They can keep a telescope steady at any object in the sky. For this, GEMs have special rotating axes and additionally a gearbox. Astronomers can either turn a slow motion gear knob manually to keep the object in view, or they can attach a small motor to move the telescope for them. If the mount is properly aligned and the motor is running (or the slow motion knob is turned), an object will stay in the field of view (FOV) as long as the observer desires.
The earth rotates, and so do observers who are standing on it. This gives the appearance as if stars circle in the sky. If observed with bare eyes, stars move very slowly (one revolution in 24h), but the telescope magnification increases this speed proportionally. A telescope with a power of 100, let stars “move” 100 times faster. If an observer uses for example an eyepiece with apparent FOV of 50 deg, and a telescope with magnification of 100, it takes only about 2 minutes for a star to wander from one side of the eyepiece to the opposite site. What might be slightly annoying for stargazing is completely unacceptable for astrophotography. GEMs offer a solution
Exact compensation for the earth’s rotation is only possible if both, the polar axis of the mount and the polar axis of the earth are in perfect alignment. Other than Alt-Azimuth mounts, which offer only 2 axes of movement, German Equatorial Mounts have no less than 4 axes. These mounts use Altitude and Azimuth settings solely for aligning the polar axis of the mount. Azimuth adjustment turns the mount so that its polar axis points north. Altitude variations set the angle to point exactly at the north or south celestial pole (NCP or SCP). Astronomers in the northern hemisphere use the Pole Star as reference. Polaris is conveniently located within 0.8 degree of the actual NCP. Observers in the southern hemisphere refer to the constellation Octans for aligning the mount to the SCP. All GEMs are equipped with a tight tube like opening for aiming at the NCP or SCP. At most GEMs this opening is actually a bore in its polar axis.
Astrophotography demands very precise polar alignment because imaging faint galaxies or nebulae requires very long exposure times. Polar scopes improve alignment accuracy dramatically. These are small telescopes to be inserted in the aiming tube of the mount. These scopes have a small magnification but more importantly, they come with etchings on a lens. Etchings show constellations and a marker for Polaris. Using these markers provides a much more accurate polar alignment. More information about precision polar alignment methods will be provided in future AstronomySource articles.
Once the German Equatorial Mount has been properly aligned, latitude and azimuth screws are secured; they should not be touched during the remaining observing session. At first glance it seems rather strange that these axes are secured and left alone; are these not exactly the degrees of movements that are used with Alt-Az mounts to point at the objects we want to see? Yes – however GEMs offers two more degrees of movement to set Declination and Right Ascension, or DEC and R.A. for short.
Continue reading Part 2 of this German Equatorial Mount article on Declination, Right Ascension, Setting Circles, Mount Balancing, and Load Capacity.