Altazimuth mount

Lore & Background

When used as an astronomical telescope mount, the biggest advantage of an altazimuth mount is the simplicity of its mechanical design. The primary disadvantage is its inability to follow astronomical objects in the night sky as the Earth spins on its axis. An equatorial mount only needs to be rotated about a single axis at a constant rate to follow diurnal motion, whereas altazimuth mounts must be rotated about both axes at variable rates, achieved via microprocessor-based two-axis drive systems. This imparts an uneven rotation to the field of view that also requires microprocessor-based counter rotation. On smaller telescopes, an equatorial platform is sometimes used to add a third 'polar axis' to overcome these problems, providing an hour or more of motion in right ascension. The design does not allow for the use of mechanical setting circles, though modern digital setting circles have removed this shortcoming. Another limitation is gimbal lock at zenith pointing: when tracking at elevations close to 90°, the azimuth axis must rotate very quickly; at exactly 90°, the speed is infinite. Thus, altazimuth telescopes cannot track smoothly within a 'zenith blind spot', commonly 0.5 or 0.75 degrees from the zenith.

Reader's Guide

In the largest research telescopes, the mass and cost of an equatorial mount is prohibitive, and they have been superseded by computer-controlled altazimuth mounts. The simple structure of an altazimuth mount allows significant cost reductions, despite the additional cost of more complex tracking and image-orienting mechanisms. An altazimuth mount also reduces the cost of the dome structure covering the telescope, as the simplified motion allows a more compact enclosure. Among amateur telescopes, altazimuth mounts are cheap and simple to use, making them ideal for beginners. John Dobson popularized a simplified altazimuth mount design for Newtonian reflectors because of its ease of construction, using non-machined parts such as plywood, formica, plastic plumbing parts, and modern materials like nylon or Teflon. 'GoTo' telescopes often use a mechanically simpler altazimuth mount with a motion controller to manipulate both axes simultaneously, compared to a more mechanically complex equatorial mount that requires minimally complex control of a single motor. A gun turret is essentially an altazimuth mount for a gun, and a standard camera tripod is also an altazimuth mount.

Did You Know?

• A standard camera tripod is an altazimuth mount.
• John Dobson popularized a simplified altazimuth mount design using non-machined parts like plywood and Teflon.

Countering Earth's Rotation in Long-Exposure Imaging

The altazimuth mount serves as one of two primary mechanical solutions—alongside the equatorial mount—for the fundamental challenge of keeping a telescope locked on a celestial target while the Earth spins beneath it. In the world of astrophotography, where exposures can stretch across minutes or hours to accumulate enough photons from faint stars, nebulae, or distant galaxies, any drift in the field of view would render the image a useless smear. The altazimuth mount addresses this by rotating the telescope in the opposite direction to the apparent diurnal motion of the sky, effectively freezing the target in the focal plane. This function is critical not only for amateur astronomers chasing aesthetically pleasing deep-sky images but also for professional research programs that have historically relied on photographic plates to catalog hundreds of thousands of previously invisible objects. Whether the mount is driven by a simple clock mechanism or by modern computer-controlled motors, its core purpose remains the same: to hold a fixed aiming point steady long enough for the sensor or film to build up a coherent image.

Inherent Tracking Limitations and Error Correction

No telescope mount system, including the altazimuth configuration, is immune to the small but consequential errors that creep into long-exposure work. Three principal sources of drift are identified across all mount types: imperfect motor drives that cannot maintain perfectly constant rotation rates, the mechanical sag of the telescope tube under its own weight as it shifts through different pointing angles, and the bending of starlight as it passes through layers of Earth's atmosphere, a phenomenon known as atmospheric refraction. For an altazimuth mount, these errors compound over the duration of an exposure, gradually pulling the target away from the center of the field. The standard remedy is a guiding system: a secondary reference point, typically a bright guide star, is monitored throughout the entire exposure, and corrective adjustments are applied whenever the star begins to wander from its designated position. In the case of moving targets such as comets, the telescope must be continuously re-centered on the object rather than on a fixed background star, adding another layer of complexity to the tracking task.

From Manual Corrections to Automated Computer Control

The history of guiding an altazimuth-mounted telescope through a long exposure reveals a clear trajectory from labor-intensive manual work to fully automated computer control. In the earliest days of astronomical photography, an observer would stand at—or even ride inside—the telescope for the entire duration of the exposure, making continuous small corrections to keep a crosshair locked on a guide star. This was physically demanding and introduced human variability into the tracking. The advent of computer-controlled systems transformed this process: the altazimuth mount could now be driven by automated electronics that monitored the guide star through a co-mounted guide scope or an off-axis guider, a device employing a prism or optical beam splitter to let the observer see the same image the main camera was recording. Today, both professional observatories and amateur equipment incorporate these automated guiding routines, freeing the operator from the tedious task of constant manual correction and allowing exposures to run unattended for the extended periods needed to capture faint deep-sky objects or to build up images in non-visible wavelengths such as infrared.

Engineering Demands of the Mounting Platform

The altazimuth mount does not operate in isolation; its performance is deeply tied to the broader engineering challenges that early and modern astrophotographers have had to solve. Because long exposures demand that the optical train remain rigidly fixed relative to the sensor or photographic plate, the telescope structure must be stiff enough to resist sagging out of focus as the mount rotates it through the sky. Equally important is the drive mechanism: clock drives or motor systems must rotate the mount at a remarkably constant rate, since any wobble or speed variation translates directly into image blur. Accurate pointing over extended periods was another hurdle, particularly in the nineteenth century when the daguerreotype process was too slow for anything but the brightest targets and the wet-plate collodion method limited exposure time to the period a plate could remain wet. The altazimuth mount, whether operated by hand or by computer, must satisfy all of these mechanical and optical requirements simultaneously to produce the sharp, detailed images that have driven both professional sky surveys and the amateur pursuit of beautiful astronomical photographs.