A lot of good astronomy equipment stays indoors because setting it up becomes a project. By the time you've carried out the tripod, assembled a heavy equatorial mount, installed the counterweights, and mounted the telescope, a quick evening under the stars can begin to feel like more trouble than it's worth.
The iOptron GEM28 was designed with that problem in mind. It uses the familiar German equatorial arrangement, with the telescope on one side of the mount and counterweights on the other, but the mount head weighs only 10 pounds. Despite that modest weight, iOptron rates it to carry up to 28 pounds of telescope and accessories, excluding counterweights.
That's a 2.8-to-1 payload-to-head-weight ratio, an unusual combination for a conventional German equatorial mount. It means you can enjoy the advantages of a computerized tracking system without having to carry a particularly heavy mount head outside every time the sky clears.
This G281A3 package includes the standard GEM28 mount with its built-in iPolar electronic polar alignment camera, the Go2Nova 8409 hand controller, a heavy-duty LiteRoc stainless steel tripod, a 10-pound counterweight, and the necessary counterweight shaft and extension. The mount also features built-in Wi-Fi, computer control, and Permanent Periodic Error Correction for astrophotography.
It's a practical arrangement for the astronomer who wants a capable equatorial mount but doesn't want the mount itself to become the reason for staying indoors.
A Traditional German Equatorial, Built Light
Many astronomers prefer the traditional German equatorial design because it's familiar, straightforward to balance, and suitable for a wide variety of telescopes. The right ascension axis is aligned with the celestial pole, allowing the mount to follow the apparent movement of the stars as the Earth rotates.
When properly aligned, the telescope can track an astronomical object by moving primarily around that single axis. That's one reason equatorial mounts remain popular for long-exposure astrophotography.
The drawback has traditionally been weight. Substantial gears, bearings, counterweight shafts, and mounting castings can make an equatorial head difficult to transport, particularly when compared with the small telescopes it may be carrying.
iOptron borrowed some of the weight-saving design techniques developed for its center-balanced CEM mounts and applied them to a more conventional German equatorial configuration.
The GEM28's all-metal head incorporates cast and CNC-machined components, 35mm steel right ascension and declination shafts, and 55mm ball bearings.
Both axes use 88mm aluminum worm wheels with 144 teeth, driven by brass worm gears through synchronous belt transmissions. The motors are precision stepper designs, operating with 128x microstepping and a published motor resolution of 0.17 arcsecond.
These are substantial mechanical components for a mount head weighing only 10 pounds.
The synchronous belt transmission also eliminates the need for conventional spur gears between the motors and worm drives. That helps reduce one possible source of mechanical backlash while providing smooth motor operation.
The GEM28 includes iOptron's spring-loaded gear engagement system with large, positive-lock switches. These make it easier to engage or disengage the drive when balancing the telescope, even with gloves on during a cold evening.
One caution is important: these switches are not ordinary friction clutches. Always support the telescope and counterweight system before disengaging either axis. Allowing an unbalanced load to swing suddenly can damage the precision worm mechanism.
Understanding the 28-Pound Payload Rating
The GEM28 has a published instrument capacity of 28 pounds, excluding the counterweight. That makes it suitable for a variety of smaller refractors, compact photographic telescopes, and appropriately sized catadioptric instruments.
But a mount doesn't respond to weight alone.
A short, compact telescope weighing 15 pounds places different demands on a mount than a long refractor of exactly the same weight. The longer telescope provides more leverage, catches more wind, and may take longer to settle after being touched.
The location of cameras, focusers, guide equipment, and other accessories also affects the complete system.
iOptron recommends optical tubes smaller than approximately 6 inches in diameter for this mount, with the telescope's length also influencing suitability.
For astrophotography, it's sensible to leave some practical margin below the maximum rated capacity. A well-balanced 70mm to 100mm refractor, for example, can make an attractive portable imaging arrangement when the complete equipment weight and tube length are appropriate.
The supplied 10-pound counterweight is sufficient for many lighter telescope arrangements. Heavier configurations may require additional counterweights to achieve proper balance.
The goal isn't simply to place the heaviest possible telescope on the mount. It's to assemble a system that tracks reliably, remains stable in ordinary conditions, and is enjoyable enough to use regularly.
Permanent Periodic Error Correction
One important feature of the standard GEM28 is its Permanent Periodic Error Correction system, usually abbreviated PPEC.
Every worm-driven equatorial mount has some amount of periodic tracking error. As the right ascension worm rotates, small variations in the mechanical drive cause the mount to move slightly ahead of or behind the ideal sidereal tracking rate.
These variations repeat as the worm turns, making them predictable enough that the mount can be trained to compensate for them.
iOptron specifies less than plus or minus 12 arcseconds of periodic error for the standard GEM28, with a published worm period of approximately 600 seconds.
PPEC allows the mount to record corrective movements and store that information in its electronics. During later observing sessions, the recorded corrections can be played back to reduce the effect of the worm gear's repeating mechanical errors.
Training uses an autoguiding arrangement. A guide camera and compatible guiding software monitor a star while the mount records the corrections made during the training process.
Once a correction curve has been recorded, it remains stored even after the mount is switched off. There's no need to repeat the training from scratch every time you set up.
There is, however, an important detail worth understanding: PEC playback is not automatically enabled when the mount is powered on. According to iOptron's operating instructions, the default setting is PEC Playback Off. Select PEC Playback On through the controller when you want the recorded correction to operate.
The recorded curve can be replaced with a new training session if you want to improve the result.
PPEC can be particularly useful when taking unguided exposures with an appropriate telescope and focal length. For guided imaging, it may reduce the periodic correction work required of the guider, although actual results depend on guiding settings and the complete system.
It doesn't correct poor polar alignment, wind, mechanical flexure, or atmospheric seeing. Those factors can still affect the finished photograph.
GEM28 or GEM28EC: What's the Difference?
iOptron also offers the GEM28EC, a related model with a high-resolution optical encoder on the right ascension axis.
The two mounts share the general compact German equatorial design, 28-pound rated payload, lightweight construction, and broad latitude adjustment range.
The principal difference is how they address periodic tracking error.
The standard GEM28 uses Permanent PEC. You train the mount using guiding corrections, store the resulting correction curve, and enable playback when you want to use it.
The GEM28EC instead uses a high-resolution right ascension encoder to monitor actual axis movement and provide Real-Time Periodic Error Correction. It doesn't require the same conventional PEC training process.
iOptron publishes a periodic-error specification of less than 0.3 arcsecond RMS for the encoder-equipped GEM28EC, measured on a bench over ten minutes. That figure describes the manufacturer's periodic-error test, not a guaranteed total guiding result under the night sky.
Both models can benefit from autoguiding for demanding long-exposure astrophotography.
The standard GEM28 is the more straightforward choice for an observer who wants the mount's lightweight mechanical design, computerized pointing, and conventional trainable periodic error correction without paying for the additional encoder system.
Electronic Polar Alignment with iPolar
Accurate polar alignment is one of the foundations of successful equatorial tracking, particularly for astrophotography.
The GEM28 G281A3 includes iOptron's iPolar electronic polar alignment camera integrated into the right ascension axis.
Rather than looking through a small optical polar scope and positioning Polaris on a reticle, you connect iPolar to a compatible computer and use its software to examine the stars surrounding the celestial pole.
The software identifies the polar region and guides you as you adjust the mount's altitude and azimuth controls.
One advantage is that Polaris itself doesn't necessarily have to be visible. If a building, tree, or other obstruction hides the pole star but leaves enough surrounding stars visible, iPolar can still assist with alignment.
It can't work through a completely obstructed sky, of course. The camera still needs a suitable view of stars near the pole.
The GEM28 accommodates observing latitudes from 10° to 70°, making it useful at locations farther north than some compact center-balanced mounts can reach.
iPolar does require a compatible computer during electronic polar alignment. Once you've completed that procedure, the mount's GoTo and tracking functions can operate independently through the included hand controller.
For portable astrophotography, the ability to complete polar alignment without crouching behind a tiny optical scope can make setup considerably more comfortable.
Go2Nova GoTo and Built-In Wi-Fi
The included Go2Nova 8409 hand controller gives the GEM28 access to a database of approximately 212,000 celestial objects.
Its four-line LCD display provides access to star catalogs, galaxies, nebulae, star clusters, planets, and other objects, along with the mount's setup and control functions.
After proper polar alignment and any necessary pointing calibration, you can select an object from the database and have the telescope slew toward it automatically.
For visual observing, that's especially convenient when searching for faint objects that may be difficult to locate manually from light-polluted skies.
The mount offers a maximum slew speed of 6° per second, allowing it to move between widely separated targets relatively quickly.
Built-in Wi-Fi provides another control option through compatible software and mobile applications. A USB connection is also available for computer control and firmware updates, with support for compatible ASCOM applications.
For astrophotographers, this makes it possible to incorporate the GEM28 into a computer-controlled imaging arrangement that coordinates target acquisition, camera exposures, and guiding.
The mount also has an ST-4-compatible autoguider port for connecting appropriate guiding equipment.
Keep in mind that the built-in Wi-Fi is for mount control. It does not turn the GEM28 into a wireless hub for your imaging camera, guide camera, or other accessories. Those devices require their own appropriate connections.
An optional external GPS receiver is available for automatically obtaining time and geographic location information. Without GPS, the necessary settings can be entered through the hand controller.
The Included LiteRoc Tripod
A lightweight mount head is only useful if the tripod underneath it provides a stable foundation.
The G281A3 package includes iOptron's heavy-duty LiteRoc stainless steel tripod rather than the smaller 1.5" tripod found in some other GEM28 configurations.
The LiteRoc uses two-section stainless steel legs with approximately 1.75" upper sections and 1.5" lower sections. Its die-cast metal accessory tray also serves as part of the leg-spreader assembly, helping brace the tripod when properly installed.
The tripod weighs approximately 15 pounds, which is more than the mount head itself. That additional weight is purposeful: a rigid tripod helps resist vibration and unwanted movement while supporting the telescope.
Its adjustable height allows the observing position to be changed for different telescopes and users. For astrophotography, keeping the legs relatively short when practical generally provides a more rigid arrangement.
The tripod folds for transportation, so it can be carried separately from the mount head and counterweights.
That separation is one of the reasons the GEM28 is practical as a portable system. Instead of carrying one exceptionally heavy assembly, you can make several manageable trips between your storage area and observing location.
Powering the GEM28
iOptron specifies a regulated 12V DC power source rated for 5 amps.
The GEM28's published power consumption is approximately 0.6 amp while tracking and 0.9 amp during GoTo movement. That relatively modest operating draw makes the mount a good candidate for portable battery power.
An AC power adapter accepting 100–240V input is included with the G281A3 package. The supplied adapter is intended for indoor or otherwise appropriately protected use and should not be exposed to rain or heavy dew.
For a remote observing site, a compatible regulated 12V battery or portable power supply can operate the mount without household electrical service.
Remember to account for the entire imaging system when choosing battery capacity. A cooled astronomical camera, dew heaters, computer, and other accessories may consume substantially more power than the mount itself.
Under the Night Sky
One of the most enjoyable uses for a portable equatorial mount is taking a modest refractor to a location where the Milky Way is visible overhead.
During summer, the Sagittarius region contains an impressive concentration of nebulae and star clusters.
The Lagoon Nebula, M8, and nearby Trifid Nebula, M20, are particularly attractive imaging subjects. Both can fit into a relatively wide field with an appropriately chosen telescope and camera.
Farther north along the Milky Way, the Eagle Nebula, M16, and Swan Nebula, M17, provide another pair of emission nebulae worth exploring.
Later in the evening, Cygnus climbs high in the sky, bringing the North America Nebula, NGC 7000, and the enormous Veil Nebula complex into favorable observing positions.
A short refractor can frame broad regions of these nebulae, while a telescope with more focal length can concentrate on individual structures.
Emission nebulae can be much easier to photograph from a dark location, particularly when using appropriate filters. They also provide excellent opportunities to learn guiding, exposure planning, and image processing.
For visual observers, the GEM28 can just as easily track the Moon, Jupiter, Saturn, or a favorite double star. Its usefulness isn't limited to photography.
And because the head weighs only 10 pounds, the equipment is considerably less intimidating to take outside when an unexpectedly clear evening arrives.
Observing Tip: Make the Most of Permanent PEC
If you plan to use the GEM28's Permanent Periodic Error Correction, begin with good mechanical preparation.
Balance the telescope carefully with all of its normal imaging accessories attached. Make sure the tripod is firmly supported, the gear switches are properly engaged, and polar alignment is accurate.
Choose a night with reasonably steady seeing and configure a compatible guide camera and guiding program. iOptron's PEC training procedure uses corrections received through the mount's guide port or compatible ASCOM control.
A star near the celestial equator and reasonably close to the meridian is a useful choice for examining right ascension tracking error because that region makes the effect of RA movement easier to measure.
Follow the Go2Nova controller's PEC recording procedure, allowing the mount to complete the training sequence. The resulting correction data is stored in the mount's electronics and remains available for later sessions.
Before using the stored correction on another night, select PEC Playback On from the controller's PEC menu. The mount normally starts with playback disabled after power-up.
If you're using autoguiding, compare the results with PEC playback enabled and disabled. Guiding software settings and the mount's periodic corrections can interact, so the best arrangement should be determined from actual tracking results.
One final point: PEC corrects repeating mechanical error in the right ascension drive. It doesn't compensate for a tripod settling into soft ground, a snagged cable, or poor polar alignment. Those basic setup details still matter.
Know Before You Buy
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No carrying case in this package. The G281A3 includes the GEM28 mount, iPolar, LiteRoc tripod, hand controller, counterweight, and other specified accessories. The GEM28 hard case is sold separately.
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28 pounds is the rated payload. Telescope length, diameter, wind exposure, and imaging requirements influence practical performance. iOptron recommends optical tubes smaller than approximately 6" in diameter.
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Permanent PEC requires training. The mount stores a recorded correction curve, but PEC playback must be enabled when you want to use it.
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Autoguiding remains useful. PPEC does not correct every source of tracking error during demanding long-exposure imaging.
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Vixen-style dovetail saddle. The included 3.8" saddle is designed for Vixen-style mounting plates, not Losmandy D-style dovetails.
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iPolar requires a computer. The electronic polar alignment camera operates with compatible software. GoTo and tracking can operate from the hand controller afterward.
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GPS is optional. Time and geographic coordinates can be entered manually or acquired with a compatible external GPS receiver.
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Additional counterweights may be required. One 10-pound counterweight is included, but heavier telescope configurations may need more.
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Use the gear switches carefully. Always support the telescope and counterweights when disengaging the drive. Sudden uncontrolled movement can damage the worm mechanism.
What's Included
- 1x iOptron GEM28 German equatorial mount head with internal iPolar electronic polar alignment camera
- 1x Go2Nova 8409 hand controller
- 1x iOptron heavy-duty LiteRoc stainless steel tripod
- 1x Die-cast metal tripod accessory tray and spreader
- 1x 10 lb (4.5 kg) counterweight
- 1x Stainless steel counterweight shaft, 20mm x 200mm
- 1x Stainless steel counterweight shaft extension, 20mm x 120mm
- 1x 100–240V AC power adapter
- Hand-controller connection cables
- USB cable for mount computer connection
- USB cable for iPolar
- Manufacturer's Quick Start Guide and documentation
Telescope, mounting dovetail, autoguiding equipment, carrying case, GPS receiver, field battery, and any additional counterweights are sold separately. Package contents may vary with manufacturer revisions.
FAQ
- Should I choose the GEM28 or GEM28EC?
- Both share the general lightweight German equatorial design, 28 lb rated payload, and 10°–70° latitude range. The standard GEM28 uses trainable Permanent Periodic Error Correction, with a published periodic-error specification of less than ±12 arcseconds. The GEM28EC adds a high-resolution RA encoder for Real-Time Periodic Error Correction, with a manufacturer bench specification below 0.3 arcsecond RMS. The EC version doesn't require conventional PEC training. Both can benefit from autoguiding for demanding long-exposure imaging.
- How does the GEM28 compare with the CEM26?
- The GEM28 uses a traditional German equatorial layout, carries up to 28 lb, and has a latitude range from 10° to 70°. The CEM26 uses iOptron's center-balanced arrangement, carries up to 26 lb, and has a published latitude adjustment range from 0° to 60°, with additional hardware required for certain low-latitude configurations. The best choice depends on your observing location, telescope, and preferred mount design.
- Does the GEM28 include a carrying case?
- Not in the G281A3 package. The iOptron GEM28 hard case, part G288, is available separately. Other GEM28 package configurations may include a case.
- Does Permanent PEC run automatically?
- No. Once recorded, the correction data remains stored in the mount, but iOptron's manual specifies that PEC Playback defaults to Off when the mount is powered on. Enable playback through the controller when you want to use the correction.
- Do I still need to autoguide?
- That depends on your telescope, focal length, exposure duration, and required image quality. PPEC can reduce periodic tracking error, but autoguiding remains useful for correcting additional drift and tracking disturbances during demanding long exposures.
- What telescopes can the GEM28 carry?
- The rated instrument capacity is 28 lb, excluding counterweight. Smaller refractors, compact astrographs, and appropriately sized catadioptric telescopes are natural candidates. Telescope length and diameter are important, and iOptron recommends optical tubes smaller than approximately 6" in diameter.
- Does the mount include iPolar?
- Yes. The G281A3 includes iOptron's integrated iPolar electronic polar alignment camera. A compatible computer is required to operate the polar alignment software.
- Does the GEM28 have Wi-Fi?
- Yes. Built-in Wi-Fi provides mount control through compatible applications. USB and ASCOM computer control are also supported. Cameras and other imaging accessories require their own appropriate power and data connections.
- What is the LiteRoc tripod?
- The LiteRoc is iOptron's heavy-duty stainless steel tripod, with approximately 1.75" upper leg sections and a metal accessory tray that also helps brace the tripod. It is a more substantial tripod than the smaller 1.5" model included with some GEM28 configurations.
- What power source does the mount require?
- The GEM28 requires regulated 12V DC power with a source rated for 5 amps. An AC adapter is included. A suitable regulated 12V battery can be used for field observing.
- Does the package include a telescope?
- No. The G281A3 is a complete mount-and-tripod package, but the optical tube, appropriate Vixen-style dovetail, camera, and other observing accessories are purchased separately.
Optional Accessories
iOptron GEM28 Hard Case (#G288, sold separately): A rigid protective case with custom-fit foam for transporting and storing the GEM28 mount head. The G281A3 package does not include a carrying case.
iOptron External GPS Module (#8438, sold separately): Automatically obtains time, date, and geographic coordinates for compatible mounts, reducing manual setup when traveling between observing locations.
iOptron 5.5" MiniPier (#8040, sold separately): Raises the mount above the tripod to provide additional clearance for long refractors, star diagonals, cameras, and other optical accessories.
iOptron AccuAlign Optical Polar Scope Kit (#G2833, sold separately): Replaces the electronic iPolar camera with a traditional illuminated optical polar scope for observers who prefer visual polar alignment.
Additional iOptron Counterweights (sold separately): May be necessary when balancing telescope and imaging configurations heavier than those supported by the single included counterweight.
Final Thoughts
There are larger equatorial mounts, and there are lighter tracking systems, but the GEM28 occupies an appealing middle ground.
Its 10-pound head is easy enough to carry outside without much planning, yet its 28-pound rated payload makes it suitable for a useful range of telescopes. The traditional German equatorial design is familiar to experienced observers, while GoTo pointing and electronic polar alignment make it approachable for someone assembling a more capable imaging system.
The mechanical construction is substantial for a mount this light, with steel shafts, large bearings, belt-driven worm gears, and a positive-lock engagement system.
Permanent PEC gives the astrophotographer a way to improve the mount's repeating mechanical tracking error, while built-in Wi-Fi and USB connectivity allow it to become part of a computer-controlled imaging setup.
This G281A3 configuration also includes the heavier LiteRoc tripod, which is an important consideration. A lightweight mount head doesn't accomplish much if the tripod beneath it isn't up to the job.
The GEM28 won't eliminate the need for careful balance, proper polar alignment, or good autoguiding practices. No equatorial mount can do that. But it makes the physical process of getting a capable telescope outside considerably easier.
And sometimes that's the feature that matters most. A telescope that's easy to set up has a better chance of being used on the next clear night.
