Common disasters in selecting telescope mounts

Mounts are important. So much so that choosing a telescope mount can either make or break your entire hobby. For beginners, most telescopes come bundled with telescope mounts. These come in two types, alt-azimuth and equatorial. Equatorial mounts are intended to help you track stars – that is, you can more easily keep what you’re looking at in the field of vision as the Earth continues to turn. In this blog I’ll give you some information about both types of telescope mounts, the similarities, the differences and what you should look for when choosing what setup you need.

Mount types

Mounts for telescopes come in two varieties. Both have their advantages and disadvantages.

Type 1: Alt-azimuth mounts

The “alt-azimuth” mount is the familiar type we use for cameras and video. It’s named after the directions it rotates in. Up-down is called “altitude” and left-right is called “azimuth”. Astronomers have to make up complicated words for everything. They come in different types, including single arm “fork” mounts, dual arm “fork” mounts, and Dobsonian mounts shown here.

Simple alt-az mount

saxon F767AZ Newtonian Reflector

Single-arm fork mount

Celestron Nexstar 6 SE Computerised Cassegrain

Dual arm fork mount

Celestron CPC Deluxe 800 HD Computerised Cassegrain

Dobsonian mount

saxon 12″ Dobsonian Telescope

The main advantage of the alt-az is its simplicity. Not only are they easy to use, they’re also simple to set up.

When alt-azimuths break your telescope

Of course, there are a couple of downsides to working with an alt-az.

  • Stars move in directions that alt-az mounts aren’t comfortable with. As the Earth turns, your star moves out of view. With an alt-az, you have to adjust two different controls to get it back.
  • Trying to control the telescope in the dark is a pain. You end up bumping the scope and wobbling the image badly. It gets really old really fast.
  • A no-no for astrophotography. If you’ve ever taken a time exposure of someone with a sparkler, you’ll know what a moving dot of light looks like on a long-exposure photograph. This is exactly what we don’t want with an astrophotograph, and an alt-az just can’t do this – even if you adjust both axes.
Equatorial and alt-azimuth mounts
An equatorial mount (left) and an alt-azimuth mount (right)

In the photo above, the scope on the right is my own (ancient) Kowa TSN2 birding scope. It’s on a standard tripod video-style head, which is an example of an alt-azimuth, as it goes left-right and up-down.

Did you know?

Time exposures have a problem called “field rotation”. This means that even if you track your target perfectly, the stars will turn to streaks around the middle of the field. Long-exposure photography with an alt-az is just no good at all.

Alt-azimuths come in “goto” varieties, where a computer controls where the mount is pointing. After setting it up, and helping the mount align itself with the sky, all you have to do is use a handbox or your mobile phone to tell the mount what you want to look at. It does the rest, finding your target and tracking it as it moves through the sky.

Type 2: Equatorial mounts

An equatorial mount solves this problem. It can perfectly track the stars as they move during the night, and you only need to adjust one axis. That means you can keep adjusting your scope with a single knob and stay watching your target all night if you want. Of course, if you want to do any long-exposure photography, like longer than a few minutes in a photo, you absolutely have to have an equatorial mount.

In the picture above, the big black telescope (a saxon Hyperion 1021) is sitting on the equatorial mount it is sold with, the saxon EQ3.

How do equatorial mounts work?

The idea of the equatorial mount is to completely cancel out the rotation of the Earth. In my wobbly diagram below you can see that both the Earth’s axis and the polar axis of the mount are pointing at the South Celestial Pole (SCP). As the Earth turns, the stars appear to move in the sky. But the equatorial mount turns on its polar axis the other way, completely cancelling the apparent motion of the stars through the telescope. It’s like riding on a merry-go-round, but you’re on an office chair that spins so that you’re always facing the same way, so you don’t get dizzy.

Equatorial mounts diagram

Of course, if the office chair is spinning differently to the merry-go-round, all that good work is lost and you’re going to get terribly dizzy.

How do I get it set up?

Once you get your equatorial mount set up right, it’ll track properly. But setting up a scope with an equatorial mount can be tricky. In my experience, this is the cause of more swearing than any other aspect of backyard astronomy. No matter if you’re a beginner with a simple equatorial mount or an amateur like me with a big high-tech mount, polar alignment can be a pain.

I’ll cover how you get this done – near enough – in a future blog.

So which one?

So when would you use an alt-azimuth, and when would you use an equatorial mount?

People who would find that an alt-az suits them best include

  • beginners who don’t want to be confused by too much all at once
  • astronomers who want to hop between targets
  • people who want to use their telescope for land viewing
  • travellers who don’t want a complicated setup every night

People who would prefer an equatorial mount include:

  • enthusiasts who want to watch a single object for a long time
  • astronomers who want to use catalogue information to find targets
  • astrophotographers who want to take long exposures

Bill is Optics Central’s expert on astrophotography, telescopes and bird watching. You’ll find him in the Mitcham store on Fridays and Saturdays. Come in for advice on how to get the best out of your current telescope, what your next telescope should be, how to take photos of the sky, or even how to see some rare birds.

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