How Mounts Work

Alt-azimuth mounts[edit]

Alt-azimuth GoTo mounts need to be aligned on a known “alignment star”, which the user will centre in the eyepiece. From the inputted time and location and the star’s altitude and azimuth the telescope mount will know its orientation to the entire sky and can then find any object.

For accuracy purposes, a second alignment star, as far away as possible from the first and if possible close to the object to be observed, may be used. This is because the mount might not be level with the ground; this will cause the telescope to accurately point to objects close to the initial alignment star, but less accurately for an object on the other side of the sky.

An additional reason for using two alignment stars is that the time and location information entered by the user may not be accurate. For example, a one-degree inaccuracy in the latitude or a 4-minute inaccuracy in the time may result in the telescope pointing a degree away from the user’s target.

When the user selects an object from the mount’s database, the object’s altitude and azimuth will be computed from its right ascension and declination. Then, the mount will move the telescope to that altitude and azimuth and track the object so it remains in the field of view despite Earth’s rotation. Moving to the location is called slewing.

When astrophotography is involved, a further motor has to be used to rotate the camera to match the field of view for long exposure photographs.

Equatorial mounts[edit]

For an equatorial GoTo telescope mount, the user must align the mount by hand with either the north celestial pole or the south celestial pole. Assuming the user is accurate in the alignment, the mount points the telescope to a bright star, asking the user to center it in the eyepiece. Since the star’s correct right ascension and declination is already known, the distance from what the user considered to be the celestial pole and the actual pole can be roughly deduced. Using another alignment star can further improve the accuracy of the alignment.

After alignment the telescope mount will then know its orientation with respect to the night sky, and can point to any right-ascension and declination coordinates.

When the user selects an object to view, the mount’s software looks up the object’s right ascension and declination and slews (moves) to those coordinates. To track the object so that it stays in the eyepiece despite Earth’s rotation, only the right-ascension axis is moved.