Telescope mount

A telescope mount is a mechanical structure designed to support a telescope.

Its primary purpose is to hold the mass of the instrument. It also allows for highly accurate pointing. Scientists must point telescopes at specific locations in space. Mounts are engineered to manage the weight of the telescope. They also help track the motion of stars as the Earth rotates. Different mounting systems are used depending on the specific goals of the observer.

Fixed mounts are designed to stay in one permanent position.

Some fixed mounts are Zenith telescopes. These instruments only point straight up. Other examples include the Green Bank fixed radio 'horn' used by the National Radio Astronomy Observatory. This specific horn observes Cassiopeia A. There are also fixed-altitude mounts. These mounts keep the primary optics at a set altitude angle. They can rotate horizontally, which is known as azimuth. However, they can only observe objects for a short time. This happens when an object passes through a specific altitude and azimuth.

Transit mounts are another specialized type of fixed structure. They are fixed in azimuth but can rotate in altitude. Most transit mounts are oriented on a north-south axis. This allows the telescope to view the entire sky. The telescope can only see objects when they cross a specific line. This line is called the meridian. Transit telescopes use this method for precision astronomical measurement. These mounts are also useful for saving on costs. They are helpful when the mass of the instrument makes multi-axis movement difficult. Large radio telescopes often use this type of mounting.

Altazimuth mounts, also called alt-az mounts, allow for two separate motions. The telescope can move in altitude, or up and down. It can also move in azimuth, or side to side.

This design is mechanically simple. Early telescope designs used this mount frequently. Until the second half of the 20th century, it was a less sophisticated option. It could not easily track the motion of the night sky. Because of this, it was mostly used for inexpensive hobbyist telescopes. Digital tracking systems changed this situation. Now, almost all modern large research telescopes use altazimuth mounts. Digital tracking has also made them popular for amateur astronomers.

Despite their popularity, altazimuth mounts have specific mechanical limitations. The field-of-view rotates at a varying speed as the telescope tracks. The telescope body itself does not rotate. This requires a counter-rotation system for astrophotography or imaging. There is also a blind spot known as a "zenith hole."

This occurs near the zenith, which is the point directly overhead. In this spot, the tracking rate in azimuth becomes too high. This makes it difficult to follow equatorial motion accurately.

Alt-alt mounts, or altitude-altitude mounts, offer a different solution. These designs are similar to Cardan suspension gimbals.

They provide an advantage over altazimuth mounts because they lack a zenith blind spot. They also minimize field rotation for objects near the celestial equator. However, these mounts are complex. They have the mass and engineering challenges of equatorial mounts. Because of this, they are used for specialty applications like satellite tracking. Some include a third azimuth axis to allow for smoother tracking.

Equatorial mounts use a different mechanical approach to track the sky. These mounts have a north-south polar axis. This axis is tilted to be parallel to Earth's polar axis.

This tilt allows the telescope to swing in an east-west arc. A second axis is perpendicular to the first. This allows for north-south movement. By driving the polar axis in a direction opposite to Earth's rotation, the telescope follows the stars. This process is called slewing. There are several types, such as German equatorial mounts (GEM) or equatorial fork mounts. Some versions include equatorial platforms like the Poncet Platform.

Equatorial mounts require significant engineering due to their complexity. Tilting the polar axis adds difficulty to the design. Some mounts, like fork or yoke mounts, must support one or both ends of the axis. German equatorial mounts also require large counterweights. These weights balance the heavy mass of the telescope. Because they are so large, they require big domes for coverage. This mechanical size makes them less viable for very large telescopes. Most modern large research telescopes have replaced them with altazimuth mounts. Some modern systems use a hexapod. This uses six extendable struts to support the mirror. This allows movement in all six spatial degrees of freedom.