Equatorial Mount
An equatorial mount is a type of mechanical telescope mount in which one axis of rotation—called the polar axis or right ascension axis— is aligned with the celestial pole, that is, in the same direction as the Earth’s axis of rotation. This configuration compensates for the apparent movement of the stars in the sky by rotating only a single axis, which is essential for long-duration observations and astrophotography.
Why align with the celestial pole?
The Earth rotates on its axis, causing the stars to appear to move from east to west across the sky. To track a star, this movement must be precisely compensated for. A standard azimuth mount must move two axes simultaneously to track a star. An equatorial mount, on the other hand, needs to rotate only one axis—the polar axis—at a constant speed of one revolution every 24 hours, which corresponds exactly to the Earth’s rotation.
Alignment
To function properly, an equatorial mount must be carefully pointed toward the celestial pole— Polaris in the Northern Hemisphere. This process, known as alignment, is crucial in astrophotography: an imprecise alignment causes the stars in the field to drift during long exposures, resulting in blurry images or star trails.
Importance in Astronomy
The equatorial mount is essential for long-exposure astrophotography. It allows a motor to automatically compensate for the Earth’s rotation, keeping the observed object centered in the field of view for hours. Professional observatories use this type of mount almost exclusively for their instruments, sometimes in combination with highly sophisticated automatic guiding systems.
Practical example
During an astrophotography session of the Orion Nebula, a properly aligned equatorial mount can keep theobject at the same point on the sensor for several hours, allowing for 3- to 5-minute exposures without the stars shifting by even a single pixel.
Frequently Asked Questions
What is the difference between an equatorial mount and an azimuthal mount?
An azimuth mount rotates on a vertical and a horizontal axis. It is simple and intuitive but requires moving two axes to track a star. An equatorial mount has an axis aligned with the celestial pole, which simplifies tracking to a single axis but requires precise alignment.
Do you need an equatorial mount for visual observation?
No. For visual observation without astrophotography, an azimuth mount is often sufficient and easier to use. An equatorial mount is essential for long-exposure photography or for precise automatic tracking.
What is a fork-mount equatorial mount?
It is a variant in which the telescope is supported by two fork-shaped arms; the entire assembly can be tilted to align theaxis of rotation with the pole. It is often used for Cassegrain telescopes and allows for a more compact design than a traditional German equatorial mount.