Altaz mounts can be regarded as the oldest form of mounting for a telescope. They have two axes of rotation, one level with the horizon and the other moves in elevation (altitude). ‘Alt’ derives from altitude and ‘Az’ from azimuth, the latter being the position measured from north, usually through east along the horizon.
In another article we’ve provided a beginner’s guide to equatorial mounts, which are technically altaz mounts tilted so that one axis is aligned with Earth’s polar axis. Unlike equatorial mounts, altaz systems don’t track the stars, but they are quick and easy to set up.
Unpowered versions require no alignment whatsoever. The simplest forms are push-pull variants and Dobsonians (for more info, read our guide to Dobsonians), where you either manually move the scope and point it at your target or have simple slow-motion controls to move the mount.
What is a ‘Go-to’ mount?
With the advent of computerised, motorised ‘Go-To’ systems, altaz mounts have become more sophisticated. Some now support Go-To, and can track targets in the night sky once an alignment process has been completed.
Initially this technology was limited to high-end, high-cost systems, but no longer. Single- and dual-arm altaz mounts are now widely available, for example the Meade ETX, Celestron NexStar and Sky-Watcher SkyMax series.
iOptron has also launched an unusual take on the design with the iOptron AZ Pro, where the mount has been designed as a cube.
Such systems have helped bring Go-To technology to a wider audience and to beginners who don’t mind the extra cost involved compared with basic mounts and telescopes.
Unlike equatorial mounts, altaz mounts don’t follow the sky smoothly in an arc as they aren’t lined up with Earth’s polar axis. This leads to ‘field rotation’ in altaz systems, which poses a problem for anyone interested in deep-sky astrophotography.
Field rotation occurs because the altaz view remains lined up in relation to the horizon, whereas your celestial target moves with the sky, so when you stack multiple images they won’t align. For visual purposes, this doesn’t matter: you just keep on adjusting the mount to keep your target in view.
If your primary interest is simply viewing the night sky, or perhaps taking short videos of Solar System targets, a Go-To altaz setup could be an ideal choice.
All is not lost, however, if you do want to try out deep-sky astrophotography with an altaz mount, as two things can help:
Many modern DSLRs and CCDs can capture good detail in short exposures, take a lot of them and then stack them. Stacking software such as the free DeepSkyStacker can deal with the rotation you will see in the images. Read our guide on how to use DeepSkyStacker for astrophotography.
The equatorial wedge
Some manufacturers produce equatorial wedges for their altaz systems. This add-on effectively turns the mount into an equatorial one, making long-exposure imaging possible.
Single-arm mounts can usually hold telescopes up to 8 inches in aperture and are typically supplied with an adjustable tripod. There will be a mounting bracket for either a Losmandy or Vixen-style mounting bar, and often ports for connecting a Go-To hand controller and an external power supply.
Some will have extra ports for other accessories. Most mounts also have a battery compartment for cable-free operation. Computerisation has revolutionised altaz mounts: some can now be controlled wirelessly, using an app on a tablet or smartphone.
Whether you want a straightforward push-pull system, one with Go-To, or one with dual arms, there is an altaz option out there for you.
Different types of altaz mounts
Single arms are the ‘standard’ type of altaz mount but, as is also the case with equatorial mounts, there are others:
The simplest altaz design, push-pulls have no drives on either axis. You manually move the telescope on a simple tripod. These make great, simple to operate and relatively cheap systems for beginners. Birdwatching spotter scopes on a tripod can also be used for visual astronomy in the same way.
Dual-arm fork mounts
For larger aperture telescopes, turn to a dual-fork mount. By supporting the optical tube between the two arms the weight is spread evenly between them for greater stability. Many dual-arms support equatorial wedges, meaning they can be used for deep-sky astrophotography.
The TTS 160 Panther altaz mount from Track The Stars is a portable system that has all the same features and connectivity as other altaz Go-To systems, and with the addition of an optional ‘telescope rotator’ it can also be used for deep-sky astrophotography.
Paul Money is BBC Sky at Night Magazine’s Reviews Editor.