Altitude-Azimuth Mount

An altitude-azimuth mount, also known as alt-az, is a two-axis mount that provides intuitive motion for pointing a telescope and uses the horizon as a frame of reference.

The altitude axis refers to the angle above or below a line parallel to the ground and pointing at the horizon. A 0° altitude angle is pointing at the horizon, a 90° altitude angle is pointing straight-up at the zenith, and a -90° angle is pointing directly downward at the nadir.

The azimuth axis refers to the angle of motion left-or-right. Specifically, it it refers to the angle from North (0°) clockwise. Thus East is 90°, South is 180°, and West is 270°.

Alt-az motion is intuitive for most people as this is the motion that we typically move our heads in. Though our heads can tilt right or left, mostly, we turn our heads left or right and up or down to view something, using the same kind of motion as an alt-az mount.

A common example of an alt-az mount would be a typical camera tripod mount.

The location of an object in the night sky can be expressed in alt-az coordinates. It is possible to describe the position of the moon, say, as 30° above the horizon at an azimuth of 110°. However, these coordinates are relative to the observer's location and the time. Alt-az coordinates are most frequently used to describe transient phenomena and relative positions, particularly referring to the horizon. For example:

• At sunset tonight, the moon will be about 20° above the Eastern horizon.
• I saw an Iridium Flare at about 60° above the horizon to the South-South-East, around 200°
• Venus never rises more than about 45° above the horizon.

Some software programs will provide alt-az coordinates, calculated based on the observer's location and time/date.

Computerization, Tracking, and GoTo

Because objects in the sky do not follow straight-line paths, but, rather, curved arcs, Alt-Az motion is not well suited to automated tracking. A given object will move across the sky in the altitude and azimuth axes at rates that differ from one another and vary depending on the object's position in the sky. In fact, objects will even reverse-direction in the altitude angle as they pass the meridian overhead. While difficult, it is not entirely impossible to automate such a mount.

In order to provide GoTo functionality and tracking, an alt-az mount must be computerized. (It is theoretically possible to do it through intricate clockwork, but the author of this entry doesn't know of it ever being done). To accomplish this, the on-board computer of the mount must contain an accurate database of object locations and essentially maintain a model of the sky based on this. At startup-time, the computer would need accurate values for a set of essential input parameters including:

• current date and time
• current time zone
• current Longitude, Latitude, and Altitude

Once these variables are initialized, it can then calculate the position of objects in the sky. At this time, it will need alignment points to synchronize its model with the observed sky. This is typically done through the identification of two or more bright stars. In most GoTo systems, once the initial entries are set, the mount will give the user a choice of bright stars that should be visible based on the date/time/location. The user selects one and the mount moves to where it believes that star should be - usually this requires the mount be set up with a specific initial altitude and azimuth position, usually pointing at the horizon, straight North (altitude 0, azimuth 0). When the mount stops slewing, the user must verify whether or not the mount is on-target. If not, the user will adjust position using the telescope's controls until it is, then he or she will acknowledge that the mount is on-target. Most such mounts require a minimum of 2 such alignment stars, many require three or more.

Once aligned, the mount will continuously re-compute positions and move the altitude and azimuth axis motors in order to follow the object based on the position the mount expects it to be in. If the initial input parameters or the alignment points are inaccurate, the calculations will not be accurate. Since each calculation requires the result of the previous calculation, the accuracy tends to drift. This can somewhat be reduced by using a larger number of alignment points (some mounts allow for several additional points) or occasionally re-synchronizing the mount on a target.

Use for Astrophotography

Alt-az mounts are not typically well-suited for AP. This is because the sky does not move in the same frame of reference as does the mount. As mentioned previously, the rates of motions will vary, as will direction. This requires more precise control and can only be realistically done using autoguiding and software that is designed to autoguide an alt-az mount. Even then, however, the issue of frame-rotation arises.

Frame Rotation

As an astronomical body moves across the sky, it will appear to rotate in the field of view of an alt-az telescope, due to the different frames of reference for motion. As an example, when the constellation Orion is rising in the East, its brightest star, Betelgeuse, a noticeably red supergiant in Orion's shoulder, will appear to be on the left, north of the three stars of the belt. At the same time, the blue giant Rigel appears in one of the feet, which will to be the left, or South, of the belt. However, when Orion is setting in the West, Betelgeuse, still North of the belt as that does not change, will appear to the right and Rigel, to the South, appears on the left. This is because North and South are relative to the poles, but right and left are relative to the user's position and orientation.

For astrophotography, this is a problem, as objects will appear to twist over time in the field of view. The most common techniques for overcoming this issue in an alt-az mount are either shorter exposure times (short enough that there is no noticeable rotational motion), or the use of equipment to rotate the camera with the motion of the sky. The equipment to do this, however, tends to be expensive and, coupled with the inherent inaccuracy in tracking, makes alt-az motions less desirable for long-exposure imaging.

For short-exposure imaging, such as Lunar and Planetary astrophotography, alt-az mounts are usually sufficient, though their accuracy issues can, in some cases, make keeping them on-target more difficult.