The first German Equatorial Mount (GEM) is attributed to Joseph von Fraunhofer, financed by the Imperial Observatory at Dorpat that had made a 9.5" refractor (by Fraunhofer), which arrived on November 10th, 1824.   REFERENCE

The GEM design is generally adequate for "small" telescopes (such as 24" or less) due to being compact and fairly straight forward.   But as the weight of the telescope/payload increases, stability becomes an issue for GEM designs.   There are more stable designs evolved since the 1940s, but for small scale (backyard or portable) ameteur DSO imaging the GEM remains among the top option. 

1824 ~2009

DEC: Declination
RA: Right Ascension (or also called Polar Axis)

Mounts are the most essential aspect for astronomical imaging.   Motorized mounts are used to automatically compensate for the spin of the Earth, allowing the sensor to remain fixed on a target.   Fixed targeting is necessary since imaging requires hours of exposure to collect data that is processed into final images.   Large mounts are used to simply hold the weight of imaging equipment (payload) and move it in a continuous smooth motion without vibration.  Mounts must also be correctly Polar Aligned so that their internal virtual sky model matches reality, which allows them to find targets and stay centered on them.

The "mount" itself is the head or top portion that contains the motorized components.  Mounts are then attached to either a pier or a tripod.   Piers are permentant posts, generally using concrete to secure them into the ground.  Tripods allow more portability but are less rigid.  Tripods can be braced with a "spreader" to increase their stability and reduce vibrations.

Some of the top consumer mounts are:

More casual consumer mounts are under $2000, by vendors such as Celestron, Orion, Vixen, Sky-Watcher, Meade.  More expensive mounts are generally larger and heavier, allowing them to support more payload, but are also easier to Polar Align (or have features that better assist with Polar Alignment).  These mounts also have better components for smoother tracking, and generally provide more consistent stable results for more years.  But these expensive mounts are generally for use at permenant observatories or dark sites (i.e. mounted on a pier).  Less expensive mounts are generally lighter and more portable, but may require more tuning and maintence to function well.   In other words, more expensive mounts are like expensive cars: more refined, better components, last longer, but less expensive cars can still get the job done (especially with diligent attention to maintenance).

Computerized mounts under $1000 are generally regarded for "visual-only" observations, since either they don't have the capacity/stabiliy for heavy payloads or don't have good components for sustained smooth tracking.  These pricing guidelines are generally for DSO imaging.  Planetary imaging, however, can get by with less expensive mounts (perhaps in the $200-$500 range).

The two GEM mounts I use are smaller Celestron AVX and larger CGEM-DX.  The following are notes on my experience with these mounts.

Essentials for Celestron mounts:

Celestron AVX

Ordered April 23rd, 2014.  The Celestron AVX is a medium entry-level mount that is portable and rated for equipment payloads of up to 30lbs.  SkySync GPS ordered May 4th, 2014.  Mount was HyperTuned around March 2015.

TPI Spreader (makes the AVX tripod much more rigid and stable):
AVX ADM Kit (the larger knobs make setup and alignment easier):

Celestron CGEM-DX

Ordered January 28th, 2016.  The Celestron CGEM-DX is a high entry-level mount that is rated for equipment payloads of up to 50lbs.   While the mount is portable, the tripod and mount/head are ~86lbs which makes that portability not very practical.   A better characterization would be to call the mount relocatable. One important note is that the CGEM-DX shares counter weights with the CGE Pro, not the regular CGEM, because of the thicker counter weight bar.   Also, the CGEM-DX comes with a Losmandy style saddle.  ADM offers a dual Losmandy/Vixen saddle kit for the CGEM/CGEM-DX.


Polar Alignment:
Celestial coordinate system:
Equatorial coordinate system: