There are several types of telescope mounts in use today. They are variations of the two main types: equatorial mounts and altazimuth mounts. Either type can be computerized to point to and track an object, but they do it differently.
For astrophotography, the only real choice is the equatorial mount. This is because the long exposures necessary for dim astronomical objects will require your target to be tracked accurately for several minutes to capture the maximum detail with pinpoint stars. Without accurate tracking, the image will be blurry, and the stars will appear blobby or elongated. Equatorial mounts only need to track one axis, and they are designed to be pointed to the celestial pole (either northern or southern) in one axis, and the other axis rotates to approximate the rotation of the earth. Orbiting objects such as satellites, the International Space Station, comets, and asteroids can be tracked also with some tweaks. The idea is that if this axis moves at exactly the same speed as the earth’s rotation, the object will remain centered. No mount’s mechanism is perfect, so autoguiding is usually necessary to give precision tracking for longer exposures. In order to achieve the best tracking, the polar axis must be pointed as closely to the pole as possible. This is the best type of mount for astrophotography.
The other main type of mount is the altazimuth mount. It is so named because it moves in the altitude and azimuth axes. In simple terms, think of movement in the left and right orientation (azimuth) and the up and down orientation (altitude). This type of mount can be computerized and point to objects once it is calibrated to known star positions. No polar alignment is necessary, but you must point precisely to a known star and tell the mount it is pointing to that star. Usually, 2 or 3 stars are necessary to calibrate the mount. Once this is done, the mount can continue to move itself in both axes to keep the object in view. In contrast to equatorial mounts, altazimuth mounts require constant movement in two axes to track an object over time. However, since neither axis is aligned with the axis of the earth, the result will be the rotation of the field around the center of the object, resulting in circular blurs of the stars and rotation of the object over time. There are devices called field rotators that can compensate for this, but it is simply not the best way to track an object for the duration of the exposure.
Dobsonian telescope mounts are a type of altazimuth mount and are popular for beginners because of their simplicity. Fork mounts are essentially altazimuth mounts but can easily be converted to an equatorial type of mount with the addition of a wedge that allows the altitude axis to be aligned with the pole star. The azimuth axis can then track the rotation of the earth.
Both equatorial and altazimuth telescope mounts allow you to point your telescope at objects in the sky, but they do so in different ways.
Advantages of equatorial mounts:
- Only one axis rotation is necessary to track objects
- Better for astrophotography
Disadvantages of equatorial mounts:
- Polar alignment is a bit complicated, but newer technology makes it easier
- More expensive
Advantages of altazimuth mounts:
- Simpler and easier to use. No polar alignment is necessary
- Less expensive
Disadvantages of altazimuth mounts:
- Two-axis rotation is necessary to track objects
- Not as good for astrophotography because of field rotation
Which type of mount is right for you?
For beginners who just want to point to objects, altazimuth mounts are simpler and cheaper. Computerized altazimuth mounts will allow tracking the object for long periods. If the mount is not computerized, nudging in two axes will be necessary to follow the object.
If astrophotography is your goal, you really need an equatorial mount or equivalent that only needs to track an object synchronous with the earth’s rotation. Many common computerized fork mounts can serve this purpose with an optional wedge that tilts the mount to point to the pole star.