In the hobby market, there are two types of telescope mount designs that dominate: altitude/azimuth and equatorial. We’ll look at both and compare their benefits and drawbacks to help you decide which is best for you. Depending on what you want to do, you may wish to own both.
We will also examine the coordinate systems that can be used with each. By understanding coordinate systems and how to use them, you can find your targets in the sky.
The mount is the part that has the attachment for the optical tube. The mount is typically supported in one of two ways: on a tripod or on a pier.
A pier is a post or pipe-like structure that is usually set into the ground or the floor. It may be bolted down or set in concrete. A pier is usually a permanent structure that is used in home observatories or large observatories. Piers are usually extremely stable.
If you want your telescope mount to be portable, it is usually mounted on a tripod. If properly designed and if the weight and size of the scope are well matched to the tripod, a tripod can also be very stable. If the tripod or mount is overloaded, then the optical tube will tend to shake and may not track well.
If you are reading an advertisement for a telescope package, the name may end in AZ, which would indicate that the package includes an Alt/AZ mount. You might see a similar optical tube, but with the name of that package ending in EQ, which would indicate that it is on an equatorial mount. Always read the details to make sure you know what you are getting.
The Obvious Difference Between the Mounts
The altitude azimuth mount operates as if the world were flat. This is how we operate on a day-to-day basis, as that is how the world appears to us in our everyday lives. You identify the things around you as to where they are around the circle of the horizon. Compass points are azimuth points, and they are designated by the number of degrees from magnetic North, which is marked as zero. Then we look to see how high things are above that horizon. This is called the altitude and is measured in degrees. We set the horizon as zero degrees and straight up as 90 degrees.
The equatorial mount recognizes that the world is round and that it spins on a tilted axis. It was specifically created for astronomy to make it easier to track celestial objects as they move through the sky.
From this point on, I will be explaining why there are two types of mounts and the two coordinate systems you can use when operating them. One coordinate system, AltAz, will likely be very familiar. The equatorial coordinate system, right ascension, and declination, may not be familiar. I will provide some insights on how the mounts are used, but this is not a “how-to” on how to use the mounts.
The Altitude/Azimuth Mount
First, let’s get the terms straight. You will see this type of mount identified as altitude azimuth, altitude/azimuth, AltAz, Alt/Az, Alt-Az, or simply AZ. They are all describing the same type of mount, so don’t let the terms or abbreviations confuse you.
You’ve probably seen or used an AltAz mount if you’ve ever seen or used a camera tripod. The key characteristic of this type of mount is that it allows you to move the camera, binoculars, or telescope in the vertical plane, up and down, or altitude. It also lets you move in the horizontal plane, left and right, or azimuth.
If you place a telescope on an AltAz mount, adults and most children will immediately understand how to use it in terms of moving it up and down or left and right. It works the way we normally see the world, so we say it is very intuitive.
If you wanted to describe the direction the mount was pointing or the direction you should point it to find your target, you would use degrees around the circle of the horizon, 0-360 degrees.
You may have encountered this if you have ever used a compass. North would be zero degrees. East would be 90 degrees, South 180 degrees, and West would be 270 degrees.
Likewise, if we look straight up, we think of this as 90 degrees of altitude, also known as the zenith. As we come down toward the horizon, the numbers decrease till we get to zero when pointing at the horizon, parallel to the ground.
When working with first-time telescope users, I usually recommend a package that includes an AltAz mount because they will quickly understand how it is used. There is less to learn and understand about the mount, which leaves more of the new owner’s attention to be focused on the optical tube and the sky.
When using an AltAz mount to track an astronomical target, you would make small adjustments up or down, left or right, to keep your target in the eyepiece as the object moves through the sky. You might do this by pushing the optical tube, or there may be a handle that you use to move the optical tube.
Some AltAz mounts add a set of dials and gears that are referred to as slow-motion controls, as shown in the picture of this popular Explore Scientific Twilight 1 telescope package. Instead of pushing the optical tube or using a lever, you can adjust the scope’s position on each axis by turning these two dials. Many people prefer having slow-motion controls on their AltAz mount, but they are not required.
AltAz mounts do not require an alignment process to use the mount. Just point it where you like.
Altitude Azimuth Coordinates – A Method to Find Your Targets
While an alignment process is not required to use an AltAz mount, you can make use of the altitude and azimuth coordinates of celestial objects to help find them. If we look at the stars as they move through the sky, their altitude above the horizon and their azimuth around the circle of the horizon change constantly.
To use this method, you will need an app that constantly figures out where your target is in terms of altitude and azimuth. You use these coordinates to point the scope at a target that you can’t see. I use Stellarium or SkEye on my Android phone, but there are many apps for Apple and Android phones, tablets, and computers using a variety of operating systems.
You can designate one leg as the zero-degree leg and point that at celestial zero, which is Polaris in the northern hemisphere. That sets that leg at zero degrees azimuth.
Most AltAz mounts don’t have an azimuth scale on them, but you can add one. The picture below is an AZ scale I added to my Explore Scientific Twilight 1 mount. It is just a 6” 360-degree circular protractor that I purchased.
You can also use a compass to estimate the azimuth direction, but you will have to account for how magnetic north differs from true north, Polaris, in your area. This is called the local magnetic declination, not to be confused with the declination we will discuss when we get to the equatorial mount.
You can read more about magnetic declination in this NOAA article. You can also find the variance for your location. For example, at my location, near New York City, the difference between magnetic North and true North is almost 13 degrees.
If I use a compass to find azimuth directions, my compass shows magnetic north as zero degrees. But in astronomy, Polaris marks true north, zero degrees. As I look in the sky and compare my compass to the position of Polaris, the star is almost 13 degrees to the right of magnetic North. So, if I want to use the compass to help me find things, I must adjust for this difference.
To get your optical tube to the correct altitude above the horizon, you can use a carpenter’s angle gauge to read the angle of your optical tube. These are often used on table saws to set the cut angle of the blade.
I use a digital angle gauge, as shown in the photo below. The one on the right is the popular Wixey Digital Angle Gauge. These are magnetic, so they stick to the steel tube of my Dobsonian telescope, as shown in the photo.
The Equatorial Mount and the Right Ascension/Declination Coordinate System
Again, let’s get the terms straight first. You will see these mounts described as equatorial, EQ, German equatorial, or GEM mounts. All these designations refer to the same type of mount.
The equatorial mount was specifically designed for astronomy. It is based on the fact that the earth rotates on an axis and that this axis is tipped in relation to its orbit around the sun. You can see an illustration of this in the picture of the globe.
For simplicity, think of the AltAz mount with the azimuth plane tipped to align with the rotational axis of the earth. On an equatorial mount, this is called the right ascension axis, or RA axis. I will discuss right ascension and declination in more detail shortly.
Why do this?
As you know, stars and deep-sky objects do not move through the sky. The earth rotates, which makes them appear to move. By tipping the mount to align with the axis of the Earth, the telescope will track targets in a straight line as they move across the sky, only having to adjust one axis to track them. This makes following targets much easier than with an AltAz mount.
At the time the equatorial mount was invented, there were no computers to control motors to move the mount for tracking. Often, it was an operator who was turning a dial to make the telescope follow the target. Having to only do this on one axis made it so much easier.
This also allowed for the development of very simple motor and gear arrangements, allowing the scope to move in sync with the rotation of the earth. These were sometimes called clock drives as the speed of movement was based on the rotation of the earth over 24 hours.
By design, equatorial mounts usually offset the telescope’s optical tube on an arm. So, they usually have a shaft and a counterweight to keep the optical tube from tipping over. This adds weight to the mount, making it heavier overall than a comparable AltAz mount.
In the picture of the Celestron Omni CG4 GEM mount, you see the counterweight on one end of the shaft and the slot on top where the optical tube will be placed. This is a manual EQ mount with slow-motion controls that you turn by hand to track your target. When set up correctly, you only turn one dial to smoothly track the object as it moves through the sky.
To make the equatorial mount work as designed, we must perform a polar alignment. In simple terms, we adjust the mount to match our latitude. For example, if you live in New York City, you would tip the right ascension axis to 40.7 degrees.
In the northern hemisphere, we now point the mount so that the right ascension axis is pointing at Polaris, the North Star. Once the polar is aligned, we can track our targets in a straight line.
Unfortunately, there is no bright, easily visible pole star in the southern hemisphere. Polaris Australis would be a good approximation, but it is a dim star (only mag. 5.4) and is not easily visible to the naked eye from most locations in the southern hemisphere. The constellation Crux (“the Cross”), often referred to as “the Southern Cross”, is often used to find an approximate pole position in the southern hemisphere.
When using an equatorial mount, rather than speak of altitude and azimuth axes, we now speak of the axis as being the right ascension, or RA, axis, which is measured in hours, minutes, and seconds. The declination (Dec) axis is measured in degrees.
The Right Ascension axis, functionally, would be comparable to the azimuth axis on an AltAz mount, with the Declination axis functionally comparable to the altitude axis.
Equatorial mounts usually have scales built into the axis so that you can set the mount to a certain right ascension and declination position to help you find your targets.
The key is that the horizon is no longer the reference point; it is the celestial equator, as shown in the diagram. This is not a familiar reference point for most people, and it is not something you can easily point to for identification.
These concepts and procedures will be unfamiliar to those new to astronomy. For this reason, first-time telescope users can find the equatorial mount very confusing and frustrating.
The Right Ascension/Declination Coordinate System
The Right Ascension and Declination system was created to better describe the position of the stars and other celestial objects in the sky relative to each other rather than relative to our point of view standing on the earth. It describes the position of all objects as if they were projected onto a celestial globe.
The zero point of the right ascension scale is measured from the position of the sun at the March equinox. This is the place on the celestial sphere where the Sun crosses the celestial equator from south to north at the March equinox. That currently creates a reference point located in the constellation Pisces.
The reason right ascension is measured in hours is that it describes how much time it would take that point to move in an hour, understanding that the point will move back to its original position in 24 hours.
Unlike the AltAz coordinate system, each celestial object has a fixed coordinate relative to all other objects. For example, if you were viewing a star that was at the coordinates of right ascension, RA, of 5 hours and declination, Dec, of 10 degrees, then a star at the coordinates of RA, 6 hours, and Dec, 10 degrees, would move into the eyepiece in 1 hour.
You could use a bright star as a reference point and know how long you would have to wait for a dim target that you can’t see to move into your field of view. Once it did, you would simply turn the dial that moves the right ascension axis to keep the target in the eyepiece.
There is a recalibration of the right ascension and declination coordinates of celestial objects that is done about every 50 years. This recalibration is to account for the inconsistencies in the rotation of the earth.
Object positions are typically given by right ascension and declination coordinates in star charts and computer programs. In addition, they will identify the epoch or reference date used for the right ascension and declination coordinates used. Currently, the common reference frame is based on the year 2000.
It is likely that a new set of RA/DEC coordinates will be published in 2050. The differences will be small so this is of little concern for most hobbyists.
A Mount for Visual Astronomy
While we could go much deeper into these mounts, the real question we want to answer is which mount would be the right one for you.
I am going to assume you are looking for a scope/mount for visual astronomy. That is to say that you are planning to put your eye to an eyepiece to look at the Moon, planets, and wonders of the universe.
If you are shopping for your first telescope and you are looking at a package that includes the optical tube and mount, you may see designations in the name of AZ or EQ. The AZ designation means that this package includes an AltAz mount, and the EQ designates an equatorial mount.
If you want simplicity and a very short learning curve, then the AltAz mount would be your best choice. Once you take it out of the box and set it up, you will know how to use it. I generally recommend AltAz-mounted scopes for beginners.
If you are willing to study the instructions, learn a new coordinate system, and perform a polar alignment every time you use the scope, then you might consider an equatorial mount. From what I’ve seen, the equatorial mounts that come with beginner telescope kits are usually of low quality, heavy, and confusing to use.
I’ve assisted many people who have their first telescope on an EQ mount. Often, it has been gathering dust for years because they could not figure out how to use the mount.
In addition, the right ascension and declination dials are very small on these entry-level equatorial mounts, making it very hard to use them to set the mount to a precise position to locate targets. I don’t recommend them for beginners.
Motorized Tracking and Astrophotography
If you are looking for equipment for astrophotography, the equatorial mount may be your best choice. Since the equatorial mount only has to track in one plane, it is easier for it to do the long exposure accurate tracking that is needed for astrophotography. A simple motor and gear system can be incorporated into the mount to provide tracking.
However, for those who are serious about astrophotography, the mounts are usually purchased separately, not as part of a beginner package. Good astrophotography mounts are fairly heavy to make them very stable, and they are expensive—in some cases, over $1000 just for the mount.
Computerized Mounts – GoTo
Both AltAz and EQ mounts are available as computerized mounts, often referred to as “GoTo” systems. Here, the complication of AltAz tracking is handled by the computer system so that the target stays in the field of view. The computer operates a series of motors and gears to perform the up-and-down and left-and-right movements required to track a target.
A computerized AltAz mount will track smoothly enough to satisfy visual astronomers. Our eyes are not as sensitive to the little jumps and tweaks that will occur as the computer tracks the target.
But again, the equatorial mount will usually track more smoothly, even when controlled by a computer. It is also the best mount for astrophotography and is good for visual astronomy as well.
As we have seen, there are two types of mounts for astronomy. For visual use, they both work, and both are available for manual or computerized use.
I usually recommend AltAz mounts for beginners because of their lighter weight and simple operation. But equatorial mounts are fine for visual purposes as well.
As you progress in the hobby, you may find that you will want more than one telescope and will likely have more than one mount. Depending on your objectives, you may have a mix of Alt-Az and equatorial mounts to take advantage of each.