Mechanics
The iOptron GEM28 consists of a compact mount head that can rotate and tilt on two axes and a lightweight 1.5” aluminium tripod with adjustable legs and a built-in bubble level.
The mount head has a simple Vixen-style dovetail saddle, which I would highly recommend you replace with an aftermarket dual Losmandy/Vixen saddle from ADM so you can use either size dovetail plate.
The GEM28 uses stepper motors and belt drives to move the mount smoothly and quietly on both axes, with minimal backlash or periodic error. The mount can track at four different speeds (solar, lunar, sidereal, or custom) and can slew at up to 6 degrees per second.
The mount also has a dual encoder mechanism that allows the user to manually point the telescope without losing alignment. Simply loosen the clutches and push the mount around, and the encoders keep track of where we’re pointed, even if we have been using the GoTo system. These same clutches are what we use to balance the mount.
The GEM28 requires polar alignment to achieve accurate tracking and GoTo pointing, as it follows the motion of the sky on an equatorial axis. A polar scope is built into the GEM28 to get you polar aligned. The mount also has a software polar alignment routine that can be performed using the hand controller or the iOptron Commander app and is compatible with the iOptron iPolar polar alignment tool.
A single 4.5 kg (10 lb) counterweight is provided with the GEM28. This is enough for smaller payloads, but you’ll probably need more counterweights for heavier telescopes. The good news is that with a standard 20mm shaft diameter, the GEM28 accepts most counterweights for small- to mid-sized mounts, and a counterweight shaft extension is also available if you need it.
The GEM28 is usually offered with a lightweight aluminium tripod that can support the mount and the load of a telescope with some stability. The tripod has adjustable legs that can extend from 31 to 43 inches and a built-in bubble level that helps with the initial levelling of the mount. The tripod also has an accessory tray that can hold the hand controller or other items. The tripod can be further extended by an optional 8-inch extension bar that can provide more height or clearance for larger and longer telescopes, such as long refractors.
The GEM28 is rather lightweight, at only 10 lbs for the mount head and 11 lbs for the tripod. This often makes for top-heavy setups.
The stock tripod is fine for visual use, but with a very top-heavy rig (particularly the case with astrophotography), it can be a little wobbly. In addition to the light weight of the mount and tripod themselves, the tripod is also inherently unsteady because its legs are only 1.5” in diameter, as opposed to the 1.75” and 2” legs normally used with mounts of this size and weight capacity rating.
We would recommend a different, larger tripod or pier, such as the beefier and sturdier 1.75” LiteRoc, for larger astrophotography setups over the stock 1.5” aluminium unit provided with the GEM28. The larger iOptron TriPier is also available for the GEM28.
Electronics
The iOptron GEM28 features a GoTo system that can automatically point to over 212,000 objects in the sky, such as planets, stars, nebulae, galaxies, and more. The mount has a built-in GPS and WiFi module that can provide accurate time, location, and alignment data. The mount can be aligned using a polar finder, a polar scope, a polar alignment camera, or a polar alignment routine that can be performed using the hand controller or a smartphone app.
The mount can be controlled via the provided Go2Nova hand controller, a smartphone app, or a PC. The Go2Nova hand controller has a large LCD screen that displays the menu and information about the selected object. The hand controller also has buttons that can adjust the speed, direction, and mode of the mount, as well as access various functions and settings. It works well enough, but it’s not as intuitive as the controllers used by most other manufacturers. Thankfully, you don’t have to use it, as iOptron has made a dedicated smartphone app for their mounts that can connect to the GEM28’s onboard WiFi adaptor. You can also control the mount via another app, such as SkySafari, or plug it directly into a PC.
The iOptron smartphone app, iOptron Commander, can be downloaded for free from the App Store or Google Play. The app can connect to the GEM28 mount via WiFi and provide a graphical interface for controlling the mount and selecting objects. SkySafari will also work independently of iOptron Commander to control the GEM28 wirelessly.
A PC can also be used to control the mount via ASCOM drivers that can be downloaded from the iOptron website. The PC can be connected to the mount via a serial cable or the GEM28’s built-in WiFi adaptor. A PC can provide more options and features for controlling the mount and interfacing with other software, such as planetarium programmes, imaging software, or autoguiding software. The GEM28 mount also has an autoguider port that can accept a standard ST-4 cable from an autoguiding camera.
The GEM28 is also available as an upgraded variant, the GEM28EC. The main improvement of the GEM28EC is that it features a built-in guide scope and guide camera that automatically guide the mount internally, though you can guide with an auxiliary guide camera and scope for even greater accuracy. Both versions are also available bundled with iOptron’s iPolar system for polar alignment and are compatible with the iOptron iGuider Autoguider, which attaches to the side of the declination axis of the mount.
Using the GEM28 for Visual Observation
The iOptron GEM28 is a versatile and convenient mount for visual observation with a variety of telescopes.
To use the mount for visual observation, you will need to attach a telescope to the mount with a dovetail bar. You will also need to level the mount and align it with the celestial pole, either using a polar scope, a polar alignment camera, or a software polar alignment routine.
For visual use, planetary imaging, and other non-critical applications, the manufacturer-specified 28 lb rating is the absolute maximum capacity of the GEM28. The LiteRoc tripod is arguably a bigger bottleneck than the mount itself, but either way, an 8” reflector, 6” refractor, or 9.25” Schmidt-Cassegrain is the biggest you can go with this mount.
The GEM28 is easy to set up and use for visual observation, as it is compact and lightweight and does not require any counterweights or balancing.
The mount also has a built-in GPS module that can provide accurate time, location, and alignment data rather than having to enter information yourself. The GEM28 includes various “quick align” options that make it pretty easy to get set up for observing, and polar alignment is of course less critical for non-imaging applications. After alignment is completed, you can then select and automatically point the GEM28 to any object in the sky or manually point the telescope by unlocking the GEM28’s clutches.
Using the GEM28 For Astrophotography
The iOptron GEM28 is a capable and user-friendly mount for astrophotography with a camera and a lens or a small- to medium-sized telescope. To use the mount for astrophotography, you will need to attach a camera and a lens or a telescope to the mount, either using a Vixen- or a Losmandy-style dovetail bar. You will also need to level the mount and align it with the celestial pole, either using a polar finder, a polar scope, a polar alignment camera, or a polar alignment routine. You can then use the hand controller, a smartphone app, or a PC to select and GoTo any object in the sky or manually point the camera or the telescope at your target.
The GEM28 can hold up to about 15-20 lbs for astrophotography purposes (the rule of thumb being that a mount can hold ⅓ to ⅔ of its weight rating for astrophotography). This corresponds to an 8” SCT or Ritchey-Chretien, a 4” refractor, or a 6-8” Newtonian reflector. Larger telescopes will be unsteady and induce tracking errors in the mount. With autoguiding (a must for good images) and accurate polar alignment, the GEM28 tracks extremely well thanks to its stepper-driven belt drives, which should guide at well under 1 arc second in most situations.
Like most quality German equatorial mounts, the GEM28 is compatible with ASCOM drivers, and while you could use the hand controller or a smartphone/tablet app to control it for imaging, the best option is usually via a PC connected to the mount’s USB port or connected over WiFi. With a PC and automated image sequencing software, you can control your guiding, where your mount is pointed, automatically plate solve to find and centre a target, and make adjustments to your motor focuser and filter wheel if you have one. If you’re operating the GEM28 in this manner, there is no need to perform a star alignment with an app or hand controller—just assemble, polar align, and let the software take care of the rest.
Should I buy a Used iOptron GEM28?
The iOptron GEM28 is relatively new, not the most popular product, and isn’t exactly widely available on the used market. However, if you can find a used GEM28 in good condition at a reasonable price, it may be worth considering.
The good news is that this mount is fairly durable and does not have many moving parts or complex electronics made out of cheap materials that can fail or wear out. However, you should check the condition and functionality of the mount before buying it if possible.
Aftermarket Accessory Recommendations
In addition to the guide camera/scope, which is a must for using the GEM28 for astrophotography with any telescope, you may want to invest in the iOptron iPolar for your GEM28. This tool is similar to the QHY PoleMaster and dramatically speeds up polar alignment if you’re unfamiliar with the process.
As mentioned earlier, ADM’s dual saddle for iOptron’s GEM28 mount is a must, both for its more solid connection with your telescope’s dovetail bar and also for the Losmandy D-style dovetail plate compatibility it offers.
Of course, you’ll also need a power supply for the GEM28, such as the Celestron PowerTank Lithium or a generic equivalent such as one from Westinghouse. An AC adaptor will also do the trick, but even if you have access to an outlet at your main location for observing/imaging, the flexibility of a portable battery is nice to have.
iOptron does offer a hard carrying case for the GEM28, but I would recommend just coming up with your own solution, as it’s quite expensive and won’t accommodate bulky add-ons like extra counterweights or a battery.
