Equatorial mount
Introduction
An Equatorial mount is a type of mount used for telescopes that compensates for the Earth's rotation by having one rotational axis parallel to the Earth's axis of rotation. This type of mount is used for most research telescopes and for many amateur instruments. It is a key component in astrophotography where long exposure times are necessary to capture faint celestial objects.
Design and Function
The basic concept of an equatorial mount is to allow an object to be followed in the sky by rotating the mount about a single axis, the polar axis. This axis is aligned parallel to the Earth's axis of rotation. The mount has two perpendicular axes of rotation, a right ascension axis (polar axis) and a declination axis.
The right ascension axis is aligned with the Earth's rotation axis. A motor drive system turns this axis to follow the stars as they move across the sky. The declination axis is perpendicular to the right ascension axis and covers movement north and south in the sky.
Types of Equatorial Mounts
There are several types of equatorial mounts, each with their own advantages and disadvantages. The most common types are the German equatorial mount, the Fork mount, and the English or Yoke mount.
German Equatorial Mount
The German equatorial mount (GEM) is the most common type of equatorial mount for amateur astronomers. It has a large counterweight at one end of the right ascension axis to balance the weight of the optical tube assembly (OTA). The GEM is known for its stability and precision tracking but can be complex to set up and use.
Fork Mount
The Fork mount is a type of equatorial mount that is often used with Schmidt-Cassegrain telescopes. It consists of a set of two prongs (the "fork") which hold the telescope between them. The fork can be attached to a wedge, which adjusts the angle of the fork to match the observer's latitude, allowing the telescope to track the stars as they move across the sky.
English or Yoke Mount
The English mount, also known as a Yoke mount, is a type of equatorial mount that is similar to the German equatorial mount but has a single arm that supports the telescope. The English mount is simpler and easier to use than the German mount, but it is less stable and less suitable for long-exposure astrophotography.
Setting Up an Equatorial Mount
Setting up an equatorial mount involves aligning the mount's polar axis with the celestial pole. This process, known as polar alignment, is crucial for accurate tracking of celestial objects. The mount must be levelled, and the latitude adjustment must be set to the observer's latitude. The mount is then rotated until the polar axis points towards the celestial pole.
Advantages and Disadvantages
Equatorial mounts have several advantages over other types of mounts. They allow for precise tracking of celestial objects, making them ideal for astrophotography and long-term observations. They also keep the orientation of the object constant in the eyepiece, which is not the case with alt-azimuth mounts.
However, equatorial mounts also have some disadvantages. They can be complex to set up and use, especially for beginners. They are also generally heavier and more expensive than alt-azimuth mounts.