Setting Up an Alt-Az Mount

It’s almost unreasonable to write about “setting up and aligning” an Alt-Az Mount. The whole point of the Alt-Az mount is simplicity, and their popularity is largely due to the fact that they don’t require any “setting up”. You just put the scope on the mount and look at things.

Nevertheless, there may be some initial problems to solve as you first work out how to mount your particular scope on your particular mount, and it’s also worth understanding how to balance your scope on the mount — and why you should.

Note: this little article is about traditional, completely manual alt-az mounts like the Televue Gibraltar mount shown here. Motorized fork mounts are also alt-az mounts, but they actually do require alignment, and I’m treating them in a separate article.

Most alt-az mounts have a very simple mounting capacity, typically a couple of holes in a steel plate. Sometimes the mount will have a dovetail-shaped slot instead of a flat plate, and will provide a matching dovetail plate that you must fasten to your telescope. This combination allows faster attachment and removal in the future. The problem of attaching your scope to a block of metal (the mount or the dovetail) remains the same, though.

And most optical tubes designed for this kind of mount have a system of rings — one or two metal clamps that surround and firmly hold the tube and that are, themselves, drilled with threaded holes. This scope uses a pair of rings,

while this one uses a single ring that is much wider to achieve similar stability. A single wide ring like this is called a clamshell.

If the holes line up with the holes in your mount, all you need are a couple of machine screws. If the available holes don’t line up, you can buy or build an adapter. Many telescope accessory stores sell adapters between popular mount styles and popular ring systems. But if you are at all handy with tools, or have a friend who is, it’s usually just a question of drilling some holes in a hunk of metal.

Hint: it is really handy to make friends with a machinist.

This may seem like a lot of trouble, but you’ll only have to solve the problem of how to attach the tube to the mount once. You’ll be mounting and dismounting it hundreds of times, and you’ll spend many hundreds of hours using the system, so it is worth taking the time now to work out a convenient and secure attaching mechanism.

Put all the accessories you would normally use on the scope (dew shield, finder, diagonal, and typical eyepiece) so the weight distribution is like what it will typically be.

Loosen the clutch or friction mechanism that holds the up-down motion so the scope can move freely, and gently let go of the scope. It will probably tip severely forward or backward. Hold on to it, letting it down gently, so it doesn’t crash into the mount and chip the paint.

Adjust the balance by somehow moving the entire telescope forward or backward in the mount. Usually you can get the motion you need by loosening the scope’s mounting rings and sliding the tube forward or backward in the rings. If this is not enough motion, you may have to look for other ways to move the balance around, like using different mounting holes to connect the scope to the mount.

Your objective is that the scope will stay level when you release it, even with the friction clutches loose. Then when you point it at an object in the sky, with moderate friction dialed in on the friction controls, it will stay where it is pointed.

On a theoretically ideal and properly balanced alt-az mount, the tube would be perfectly centred around the axis of altitude rotation, as shown here. This mount is balanced and will remain balanced even when the tube is pointed upward at the sky.

The problem is that most mounts are not built that way. On most mounts, such as the one shown above, the centre of gravity of the OTA is held somewhat above the axis of altitude rotation, as shown in this diagram. (Getting the weight centred is why the U-shaped mount saddle in the photos above hangs below the axis. But it makes assumptions about the size and weight balance of the OTA.)

Even when the tube is perfectly matched to the mount and centred, the accessories you add to the OTA (finder and eyepieces) will usually put the centre of gravity above the axis of rotation.

The tube in this diagram is balanced when horizontal, but as the tube is tilted to point upward, there will be more weight above the axis of altitude rotation than below it, and an imbalance will result.

It’s easier to see why that is so if we consider an absurdly exaggerated example. This optical tube is balanced while horizontal. However, if you imagine tilting it to point upward, it’s easy to see that the weight of the tube will immediately make the mount want to fall to the tube side.

So what do we do about this?

On most mounts, there are limits to what you can do, but there are some things:

• Find a way to mount your OTA as low as possible in the mount, to minimize the distance between the altitude rotation axis and the centre of gravity of the tube.
• When balancing your OTA in the mount, bias the weight slightly forward. I.e., so that the tube has a very slight tendency to tip forward when horizontal. This will counter the tendency for it to want to tip backward when pointed upward.

  Unfortunately the amount of forward bias you need depends on the altitude at which you will be viewing, so no single adjustment can compensate for all possible viewing targets. However, if you are doing most of your viewing within a small range of altitudes, which is often the case, you can compensate this way.

• Use the mount’s friction controls to reduce the tube’s ability to tip when tilted.

Now you should tighten the friction clutches that hold the mount in the up-down and left-right directions just enough so you can still move the mount but so that it stays where you put it.

Different mounts do this differently. The up-down and left-right friction controls on this Televue Gibraltar mount are shown here as an example.

When you are ready to point your telescope at an object, you will appreciate the simplicity of the alt-az mount. You just point it. Grab the telescope itself, or the handle on the mount (if it has one), and gently move the scope left or right, and up or down, until you are pointed at your target. There is nothing to it, and this is the main reason alt-az mounts are popular. Some alt-az mounts have “slow motion” controls to make the last part of the trip more precise (the mount I’ve been using in the photos in this article does not).

The ability to quickly assemble an alt-az mount, quickly put the telescope on it, and quickly point it at something to observe, is why they are so popular as “grab and go” mounts, for having a quick look at the sky with a small, portable telescope.

Tracking Objects

Once you are observing an object, you will notice it gradually drifting out of your field of view. This is apparent motion caused by the rotation of the earth. The higher the magnification you use, the faster your target will drift.

You keep the object in view by gently “nudging” your scope in the necessary directions, either by hand or using the slow motion controls if you have them. Because an alt-az mount does not mimic the way the sky moves you will have to adjust in both directions. For example, if an object in the south is still rising, you will have to adjust slightly right and slightly up to keep it in view, and repeat this adjustment every minute or two.

This constant nudging is why manual Alt-Az mounts are not useable for astrophotography.