A beginner's guide to equatorial mounts

What equatorial mounts are and why they are used in astronomy.

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Published: October 25, 2018 at 11:00 am

An equatorial mount is an astronomical device designed to follow the apparent movement of the stars across the night sky, which is in reality caused by Earth's rotation. The equatorial mount enables you to stay focussed on a specific target without it drifting from your field of view.

With the naked eye the motion of the stars is barely perceptible, but look again with any form of optical aid and the fact that they appear to move presents a real problem.

For brief observing sessions you can keep adjusting a telescope’s orientation to compensate for this, but it’s a different matter if you want to study a single target for an extended period of time, or take a long-exposure image.

This is where an equatorial (EQ) mount comes into its own.Most commercial models are based on the German Equatorial Mount (GEM).One axis is aimed at, and ideally lined up accurately with, Earth’s axis of rotation.

For the northern hemisphere, we have a convenient star almost on top of that point, the north celestial pole.This star is Polaris (Alpha Ursae Minoris), which is why it is called the pole star.

Southern hemisphere astronomers have much fainter stars close to their celestial pole and so offset from nearby Sigma Octantis.

For more info, read our guides on why you need to align your equatorial mount, and how to set up an equatorial mount. Or, find out more about the subject in our basic guide to telescope mounts.

If you're on the lookout for your first mount or an upgrade, browse all our mount reviews or read our guide to the best telescope mounts available.

Aligned on the north celestial pole, an equatorial mount makes it easy to track stars as they move from east to west through the night.
Aligned on the north celestial pole, an equatorial mount makes it easy to track stars as they move from east to west through the night.

Why do you need to align an equatorial mount?

By aligning one axis with the pole star, the mount’s rotation will match the rotation of Earth itself.For most visual purposes a rough alignment is all that is required to keep the target in the view of the eyepiece.

However, with prolonged viewing, there will be a steady drift of the target and occasional ‘nudges’ will be needed to bring it back to the centre.

For more accurate tracking, an accessory called a polarscope, which is usually fitted on the axis of the mount pointing northwards, will be of great help when it comes to achieving alignment.

Although early commercial equatorial mounts were undriven and required users to use slow-motion hand controls to keep targets in the field of view of the eyepiece, today many beginner systems have drives on both axes and are supplied with a computerised Go-To hand controller.

We are well and truly spoiled in this modern age.Go-To gives you the option to use up to three stars to help align the mount, assuming it has been roughly aligned already.Once aligned then you can select and slew to a whole range of targets.

You can take a tour, locate and view deep-sky objects and the fainter members of the Solar System, such as comets, asteroids and even Pluto if your telescope has the aperture.

Computerised mounts also allow for the even finer accurate alignment needed for deep-sky astrophotography and have brought this specialty within reach of the many.

Setting up an equatorial mount is a relatively easy, one-person job once you get the hang of it. Credit: Steve Marsh

The different sections of an equatorial mount

The principal part of a mount is the mount head, which has two fully rotatable axes: one in declination (dec.), the north-south axis, and the other in right ascension (RA).It is the RA axis that needs to be aligned with the pole star and it forms the main bulk of the mount head.

On the top is the saddle, which is where you attach your telescope.It is usually one of two variants: the ‘standard’ Vixen-style dovetail bar or the wider Losmandy-style.Some mounts now have dual saddles, giving you both options.

The mount body also contains the electronics and has ports for connecting the Go-To hand controller, power and an autoguiding port for astrophotography.

With the GEM, counterweights are attached to the dec. axis, with the weight you need dependent upon the weight of the telescope plus any additional equipment or accessories, such as imaging cameras and guidescopes.

Finally, for stability the mount is fixed to a sturdy tripod with adjustable legs.

Equatorial mounts have become very versatile, with a wide range of carrying capacity from very portable small systems for cameras up to large systems for deep views and imaging of the cosmos.

Today there are even variants that can act as both equatorial and altaz (AZ) mounts for even greater flexibility. For more on this, read our guide to altaz mounts.

Paul Money is BBC Sky at Night Magazine’s reviews editor.

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