Celestron CGEM II Mount Review: Partially Recommended

Technical Specs

The CGEM II mount has a stated weight capacity of 40 lbs. or 18 kilograms, although this number is sometimes difficult to interpret as the manufacturers of these mounts are not very specific as to their intended usage. If you’re doing visual astronomy or planetary imaging, you can load it up to the limit or even exceed it by a few kg by adding more counterweights, as the positioning errors are usually too small to notice.

However, when you are doing more accurate tasks, specifically deep-sky astrophotography, the mount’s precision of tracking is more critical, as the images you take will show streaking right away if you have positioning errors. Adding more weight to the mount makes positioning errors larger. For this reason, we recommend using a maximum of ½ to ⅔ of the stated capacity for astrophotography.

The CGEM II uses a dovetail mounting saddle that accepts Vixen V-style dovetails as well as CGE-style and Losmandy D-style dovetail bars, clamping to them securely rather than using a set screw (the smaller Advanced VX uses a dual saddle with knobs and cannot actually fit a Losmandy bar). With any telescope and mount that can fit them, we generally recommend the Losmandy style dovetails as they are more stable due to the larger surface area to grip onto.

The CGEM II also comes with a Nexstar+ hand control, which is highly praised among visual astronomers, but I will discuss that later in the visual section. The tripod included is the same 2” unit provided with the Advanced VX or the Sky-Watcher EQ6Ri Pro. The legs are extendable, which is useful for visual astronomy if the mount is a bit short for your telescope.

The problem with having such a sturdy mount is that it can get quite heavy. This mount is no exception, and just the head of the mount (not including the tripod) weighs 18 kg (40 lbs), with the tripod alone weighing 11 kg (24 lbs). This is something to consider if you have health problems that make you unable to lift heavy objects. You can set the tripod up first and then place the mount head-on, which can alleviate some of the weight concerns.

The CGEM II uses the same ¾” diameter counterweight shaft as most other mounts of its size and the smaller Celestron and Sky-Watcher mounts. This will allow it to accept a variety of counterweights, such as the standard 7- and 11-lb weights for smaller mounts, as well as its provided 17 lb weights. A DC power cord is provided to run off a 12v power supply, but you can also purchase an AC adapter.

Mechanical Flaws

The CGEM II mount, being a relatively inexpensive design, has some issues that are found inside the mount. These issues are magnified when guiding the mount because you can see exactly what is going wrong. The problems are with the gears themselves. The servo motors and their cheap plastic gearboxes can and almost always will produce a large periodic error. This means that as the gears go through a full rotation, different problems in the gears will cause the tracking to speed up or slow down in a repeating pattern. 

This is seen whether the mount is guided or unguided. The issue is so bad when you are not guiding the mount that even with a perfect polar alignment, you can practically not expose for longer than one minute without the images being streaked. Guiding takes care of most of these large-scale issues, as they are easy for the software to predict, but another issue is backlash in the gears.

The CGEM II’s azimuth alignment (left-right) is quite stiff when you are trying to polar align, but generally you can still get a good enough alignment with the celestial pole.

A polar scope is not provided with the CGEM II by default, though Celestron does sell one for both it and the Advanced VX mounts. The All-Star Polar Align can be used for alignment without a polar scope or the ability to view Polaris, but a polar scope or PoleMaster is preferable and faster to use for imaging compared to the All-Star Polar Align, especially if you lack any provision to look through your telescope.

Software Compatibility

The CGEM II has no EQMOD support, which is a handy feature for astrophotographers using Synscan-based mounts from Sky-Watcher or other mounts from many other manufacturers. The updated NexStar+ hand controller does somewhat alleviate this issue because the hand controller has a MicroUSB port, making it easy to connect to if you want to control the mount from the computer.

However, you can still connect to the CGEM II just fine with ASCOM, an open-source mount control software, meaning you can guide and slew like any other ASCOM-compliant mount from a PC or laptop. You must connect the CGEM II to a PC through the hand controller. This means that you have to power the mount on, go through entering the date and time if you do not have the time clock enabled, and then align the mount’s pointing. 

As a consequence of its lack of EQMOD compatibility and requiring a connection through the hand controller to a PC, you must use the mount’s ST-4 autoguider port and a guide camera that also has one. Additionally, response times to commands from an autoguider as well as astronomy software talking to the mount via its hand controller can be slightly slower than one would be able to get with a mount that accepts EQMOD and doesn’t require the hand controller or ST4 port for operation.

Using the CGEM II for Visual Astronomy & Planetary Imaging

The CGEM II is an excellent performer for visual astronomy as well as planetary imaging. It can not only withstand a reasonable amount of weight, but it can also track quite well at low powers. The hand controller, the Nexstar+, is much improved compared to the Sky-Watcher and Orion alternatives because of its easy-to-understand and use interface, built-in polar alignment features, and its internal clock.

Along with these quality features, the hand control lights up at night, has nine slew rates for the mount, and has over 40,000 targets in its database to ensure you never run out of targets to find.

What this means for visual use, as well as planetary imaging, which has similarly lax tolerances for tracking quality, is that you will have almost no troubles, apart from the heavy mount and tripod. You can polar align easily with built-in All-Star Polar Align software to boot, which requires no craning your neck to view through the polar scope and no laptop to take with you, though we’d still recommend using a polar scope if possible.

Note that you must always polar align the mount, but generally, if the mount is well polar aligned, the pointing system can predict where it is using the quick align feature in the software instead of star alignment. The mount’s azimuth alignment (left-right) is quite stiff when you are trying to polar align; however, generally, you can still get a good enough alignment with the celestial pole.

Using the CGEM II for Deep-Sky Astrophotography

The CGEM II mount, while being a great performer for visual use or low-accuracy planetary imaging, starts to show its weaknesses and quality issues when you use it for high-accuracy imaging. I have personally used this mount for imaging for over two years, and I know the struggle of using this mount to take photos.

You can use the hand controller of the CGEM to operate it for astrophotography, but as with any astrophotography setup, you’re going to be controlling your camera and guiding with a PC anyways, so connecting it to a PC via the hand controller’s PC Direct Mode and a MicroUSB controller is a good idea. After polar aligning with a polar scope or PoleMaster, you can bypass the entire GoTo alignment procedure by simply plate-solving your way to a target and then shooting with a program like NINA or Sequence Generator Pro.

Any guide camera that can connect to an ST4 port and be controlled by software like PHD2 can be used to autoguide the CGEM II. The CGEM II’s servo motors, ST4-only autoguiding, and the periodic error in its gears combine to make for slower response times when your autoguider attempts to correct tracking issues. As such, it can be difficult for it to carry telescopes longer than 1000–1200 mm in focal length with any reasonable accuracy and a long exposure time. Even with an 800mm or lesser focal length, I do not recommend using more than 2 or 3-minute exposures with the CGEM. Yes, even with these problems, you can produce some excellent images with this mount if you persevere and are willing to throw out some images, but there are definitely major issues to overcome if you are serious about imaging with this mount.

Should I buy a Used Celestron CGEM II or CGEM?

Older CGEM mounts may have slightly lower quality tracking and guiding than the newer CGEM II, but are otherwise the same, apart from the dovetail saddle. The original CGEM only fits Losmandy D-style and Celestron CGE-style dovetail plates. However, you may or may not even be using Vixen-style plates anyway, and an aftermarket saddle from ADM can be installed. Older units also may have a hand controller that is missing, dead, or requires a serial port adapter to plug into a PC; replacing it with a new unit is fairly straightforward and economical. The tripod is the same as the Advanced VX, CG-5, and many others, which also makes replacing it easy, along with the counterweights. Missing knobs or a missing counterweight shaft are also fairly easy to find spares of.

Alternative Recommendations

The CGEM II is not exactly our favorite due to its poor quality motors and similar pricing to other mounts that use harmonic drives or steppers and lack its annoying hand controller interface issues. Here are some alternatives we recommend in lieu of the CGEM for both visual astronomy and astrophotography use:

Under $2500

• The Sky-Watcher EQ6Ri Pro offers essentially the same weight capacity and form factor as the CGEM II, but with belt-driven stepper motors for higher accuracy, a retracting counterweight shaft, a polar scope provided by default, a built-in carry handle, and the ability to be controlled over WiFi or via an EQDIRECT cable and EQMOD.
• The Sky-Watcher AZ-EQ5i has belt-driven steppers like the EQ6Ri with the added bonuses of the ability to be converted to an alt-azimuth mount and hold two telescopes, and can be moved manually around the sky thanks to its FreedomFind encoders. However, it has lower weight capacity than the EQ6Ri Pro.
• The Sky-Watcher HEQ5i Pro features stepper motors and a highly compact equatorial mount head and tripod, albeit fairly low capacity of only 15 lbs for imaging and ~20 lbs for visual use.
• The ZWO AM5 Harmonic Drive Equatorial Mount and Tripod offers a unique harmonic drive design with huge payload capacity despite its diminutive size, though its low weight and primarily astrophotography-intended design make it a little wobbly and hard to use for visual observers.
• The Celestron Advanced VX is essentially a shrunken CGEM II with nearly as high of a capacity for visual use but all of the same problems for astrophotography, hindered further by additional mechanical issues. However, it offers vastly better value for the price to visual observers.

$2500+

• The Sky-Watcher AZ-EQ6i features a USB port for even simpler PC connection than the EQ6Ri, can be aimed manually thanks to its FreedomFind encoders, and can be used in alt-azimuth mode too.
• The Celestron CGX is only a bit bulkier than the CGEM II, but has belt-driven worm gears and stepper motors making for accurate tracking and guiding, higher weight capacity than either the CGEM II or any of the EQ6 variants, and a USB port to easily connect to a PC.
• The Losmandy G11 offers improved weight capacity over the CGEM II and EQ6 mounts with high-quality machined parts, though the interface is somewhat clunky and outdated.
• The iOptron GEM45 is great for holding small telescopes for astrophotography thanks to its superb tracking and guiding, but it lacks weight capacity and is confusing for visual observers to use.

Aftermarket Accessory Recommendations

A polar scope or Polemaster is required for accurately polar aligning the CGEM II for deep-sky astrophotography and speeds up setting up for visual observation too. You will also want either an AC power adapter or a power supply such as the Celestron PowerTank Lithium Pro, a generic lithium battery, or a larger marine battery if you have long imaging sessions over multiple nights and need power for a laptop and other items too.

ADM’s counterweight shaft extension for the CGEM II will allow you to use less counterweight to provide the same torque for balancing, reducing the load on the mount’s gears and tripod for a more stable setup.