The typical elements of an equatorial mount; on display here is the Sky-Watcher EQ6-R Pro. Image Credit: www.thesecretstudio.net
The sky offers up countless wonders for us to view.
With the naked eye the motion of the stars is barely perceptible, but look again with any form of optical aid and they present a problem – they move!
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.
Fall into line
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.
The sum of its parts
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.
Click here to read our guide to AZ systems.
Equatorial mount variants
The GEM is the most common EQ mount, but it is not the only one:
The Centre-balanced Equatorial Mount (CEM) is a relatively recent introduction from iOptron that places the load-bearing RA axis between two points as opposed to one, which leads to an unusual Z-shaped design including the counterweights shaft.
An advantage of this design is that it places the centre of gravity directly over the centre of the tripod, and quite low, making for a more stable setup.
The English Mount was a very large Victorian design suited to permanent observatories.
The Portable English Mount (PEM), the scaled-down version from UK companies Astromount and AWR, holds great promise.
The load capacity is in excess of 50kg and, due to the popularity and interest in the mount, a mini-PEM with a capacity of up to 25kg is in development.
Watch this space.
Camera tracking mounts
These mounts are extremely lightweight and small; essentially, they are miniaturised EQ mounts.
With a load range encompassing anything from DSLRs up to small telescopes they have revolutionised travel astronomy, being ideal for solar eclipse and wide-field, deep-sky photography anywhere in the world.
As they are compact and run on batteries, they are great grab-and-go systems.
Paul Money is BBC Sky at Night Magazine’s reviews editor