20 Mount Types
GSS supports three types of mount:
- German Equatorial
- Polar Equatorial
- Altitude Azimuth
German Equatorial – The German equatorial mount, often called a “GEM”, for short, has a has a T-shape, where the lower bar is the right ascension axis and the upper bar is the declination axis. The right ascension axis is aligned with the celestial pole. Celestial objects are tracked by rotating this axis at the sideral rate.
The counterweight balances the telescope, reducing flex in the mount and allowing low power motors to be used.
A German equatorial mount has two possible orientations for each point on the celestial sphere. A meridian flip swaps between the two orientations, pier East and pier West, ensuring that the counterweight is below the telescope and preventing collisions with the pier.
Polar Equatorial – The Polar equatorial mount, in a dual telescope or fork configuration can allow arbitrary rotation around both the right ascension and declination axis. The right ascension axis is aligned with the celestial pole. Celestial objects are tracked by rotating this axis at the sideral rate.
There is no counterweight for a polar equatorial. mount. Dual telescopes balance each other, and a canted pier means that collisions are avoided. A single telescope is automatically balanced when mounted centrally in a fork which also allows free movement.
A Polar equatorial mount has two possible orientations for each point on the celestial sphere. Hardware and cable wrap constraints may prevent slewing to both orientations. Hardware limits, in certain cases may make some celestial coordinates unreachable in either orientation. A polar flip swaps 
between the two orientations, normal and through pole.
The schematic above shows the celestial sphere with the celestial pole in the lower half. Lines of constant right ascension radiate out from the celestial pole and lines of constant declination approximate to circles around the celestial pole. For an example hardware limited polar mount the red line defines the reachable parts of the celestial sphere.
Points 1, 2, 3 and 10 are only reachable using “normal” declination values. Points 4 and 9 are reachable using “normal” and “through pole” declination values. Polar flip will work for points 4 and 9. Points 5, 6 and 8 are only reachable using “through pole” declination values. Point 6 is unreachable.
Altitude Azimuth – The altitude azimuth (AltAz) mount is a simple two-axis mount for supporting and rotating a telescope about two perpendicular axes: one vertical and the other horizontal. Celestial objects can only be tracking by simultaneously rotating both axes.
A celestial object’s right ascension and declination map to a unique azimuth and altitude for a given observatory location and time.
An Alt Az mount could move to the object’s azimuth and altitude by rotating clockwise or counterclockwise from the starting position. However, cable wrap constraints may not allow one of the rotation directions.
Most Alt Az mounts allow some azimuth movement in excess of 180 degrees from the zero position, usually North. Thus, there will be a range of azimuth positions around the South that can be reached in two ways.
An Alt Az flip will swap between these positions.