There comes a point in astrophotography when finding the object is no longer the difficult part. The telescope is accurately pointed, the camera is focused, and the subject is sitting exactly where you want it. Now the mount has to follow that object for several minutes while the camera collects light, ideally without allowing the stars to move noticeably across the sensor. At longer focal lengths, even very small tracking errors can become apparent in the finished photograph.
The iOptron GEM45EC addresses one of those errors with a high-resolution optical encoder on its right ascension axis. Rather than relying entirely on the accuracy of its worm gears or a previously recorded periodic error correction curve, the GEM45EC continuously measures the position of the RA axis and makes corrections as the mount tracks. iOptron calls this Real-Time Periodic Error Correction (RPEC), and specifies less than 0.3 arcsecond RMS periodic tracking error under its bench test conditions.
What's particularly interesting is that iOptron has incorporated this technology into a traditional German equatorial mount head weighing only 17.5 pounds (7.9 kg). The GEM45EC carries up to 45 pounds (20 kg) of telescope and accessories, excluding counterweights, giving it substantial carrying capacity without requiring a mount head that takes two people to lift.
This #7620ECA package includes the GEM45EC mount head, high-resolution RA encoder, built-in iPolar electronic polar scope, GPS, computerized Go2Nova hand controller, and aluminum hard carrying case. It does not include a tripod, pier, or counterweights. For someone who already has a compatible support, or is building an imaging system around a permanent pier, that avoids purchasing equipment they may not need.
Why an Optical Encoder Matters
A conventional equatorial mount follows the stars by slowly rotating its right ascension axis to compensate for the rotation of the Earth. The motor drives a small screw-shaped gear, called a worm, which turns a much larger toothed wheel connected to the telescope's RA axis. This arrangement provides the enormous reduction in speed needed for sidereal tracking.
Unfortunately, even precisely machined worm gears contain minute imperfections. As the worm turns, these variations can cause the telescope to move very slightly ahead of or behind its intended position. Since the same portion of the worm comes around with each revolution, the error repeats in a predictable pattern. Astronomers call this periodic error.
On the standard GEM45, iOptron specifies periodic error of less than ±7.5 arcseconds over the 400-second worm period. That mount includes Permanent Periodic Error Correction (PPEC), which allows the owner to record a correction curve and have the electronics repeat those corrections as the worm rotates.
The GEM45EC approaches the problem differently. Its high-resolution optical encoder monitors the actual position of the right ascension axis and compares that position with where the axis should be. When a discrepancy is detected, the electronics can adjust the drive to compensate. Because the system measures the movement directly, it does not need a previously trained PEC curve.
iOptron specifies less than 0.3 arcsecond RMS periodic error for the GEM45EC with its encoder operating, measured on a test bench over the worm's 400-second cycle. For perspective, one arcsecond is just 1/3600th of a degree. Reducing the repeating error to such a small value represents a substantial improvement in the mount's specified mechanical tracking performance.
That specification should not be confused with a guarantee of identical tracking accuracy beneath the night sky. The encoder monitors the RA axis, not the position of a star on your camera's sensor. Poor polar alignment, atmospheric refraction, telescope flexure, wind, and other disturbances can still produce movement in the image. Nevertheless, removing much of the worm's periodic error gives the rest of the imaging system one less problem to overcome.
Does the GEM45EC Still Need an Autoguider?
That depends on the telescope, camera, exposure length, and the accuracy you're trying to achieve.
With a short focal length and careful polar alignment, an encoder-equipped mount may allow useful unguided exposures. As the focal length increases and the camera records smaller portions of the sky per pixel, errors that were previously invisible become much easier to detect. A slight amount of drift that makes no practical difference at 300mm may be troublesome at 1,500mm.
An autoguider provides a different kind of information from the encoder. Instead of measuring the rotation of the mount's axis, it watches an actual star and detects its movement through the optical system. This allows guiding software to respond to errors that the RA encoder cannot measure directly.
For serious long-exposure imaging with a long-focal-length refractor, Ritchey-Chrétien, or Schmidt-Cassegrain telescope, I would still plan on autoguiding. An off-axis guider is particularly useful with these systems because it samples the same optical path as the main camera, reducing the potential for differential flexure between separate guide and imaging telescopes.
The GEM45EC provides ST-4-compatible guiding and USB computer control through ASCOM. Encoder-equipped mounts can also behave differently from conventional mounts when receiving guiding corrections, so it's worth following iOptron's current guidance for configuring your guiding software and RA corrections rather than assuming that settings developed for a standard GEM45 will necessarily be optimal.
There is a legitimate purchasing decision here. An experienced imager with a carefully tuned autoguiding system may find that the standard GEM45 already provides the tracking performance needed for their images. The GEM45EC is most attractive to someone who specifically wants its real-time encoder correction, whether for experimenting with unguided exposures or for reducing the mount's inherent periodic tracking error.
A Traditional German Equatorial with Substantial Capacity
The optical encoder is the principal difference between the GEM45EC and the standard GEM45, but the underlying mechanical design is worth examining as well.
Both axes use 4.33" (110mm) diameter aluminum worm wheels with 216 teeth, driven by 0.67" (17mm) brass worms through synchronous belt transmissions. The 1.77" (45mm) steel axis shafts are supported by 2.68" (68mm) bearings. These are substantial mechanical components for a mount head weighing less than 18 pounds.
The worm gears provide the controlled movement necessary for accurate slewing and tracking, while the bearings support the telescope as its position changes across the sky. The synchronous belts transmit power from the stepper motors to the worm assemblies without the additional meshing gear stages used in some mount designs.
Large quick-lock gear engagement levers allow the axes to be disengaged for balancing and re-engaged for normal operation. They are designed to be manageable even when wearing gloves, although the telescope and counterweight assembly should always be supported when releasing the drives.
The GEM45EC has a 6" dual-format dovetail saddle that accepts both Losmandy D and Vixen-style mounting plates. That flexibility is useful when moving between smaller refractors and larger optical tubes, since you don't necessarily have to change saddles to accommodate a different dovetail format.
The 45-pound payload rating allows a substantial imaging system, but telescope dimensions remain important. A long refractor presents more leverage and wind resistance than a compact telescope of the same weight. The encoder improves right ascension tracking accuracy; it does not increase the mount's mechanical capacity or eliminate vibration caused by an oversized optical tube.
Internal Cable Management
A large astrophotography system can accumulate a surprising collection of cables. The camera needs power and a data connection, the autoguider needs to communicate with the mount or computer, and additional equipment may include a filter wheel, electronic focuser, and dew heaters.
Those cables become a problem when the telescope slews across the sky. An exposed cable loop can catch on a tripod leg, become stretched during a meridian flip, or pull against the camera as the telescope changes position.
The GEM45EC provides an internal cable-management arrangement with a 12V accessory power connection rated for 3 amps, an ST-4 guiding connection, and USB 2.0 pass-through wiring to the saddle end of the mount. This allows some connections to be made close to the telescope while their corresponding cables exit through the lower portion of the mount.
The benefit is fewer long cables hanging freely around the moving telescope. The USB connection is a pass-through rather than a complete powered USB hub, so an imaging system with several USB accessories may still require a separate hub or computer mounted near the telescope.
Proper cable routing remains important even with internal connections. Before beginning an unattended imaging session, check that every cable has adequate clearance through the telescope's expected movements and that nothing can be pulled tight against the camera or other accessories.
iPolar, GPS, and GoTo Control
Accurate polar alignment is especially important when using a mount with very low mechanical tracking error. A well-corrected right ascension drive cannot prevent declination drift caused by poor polar alignment.
The GEM45EC includes iOptron's iPolar electronic polar scope built into the right ascension axis. It uses a small camera to recognize the star field surrounding the celestial pole, then software on a connected computer shows you how to adjust the mount's altitude and azimuth controls.
Unlike a conventional optical polar finder, iPolar does not require Polaris itself to be visible. It does, however, need enough unobstructed sky near the pole to identify the surrounding stars. The system is particularly convenient for observers who regularly transport their equipment and must establish polar alignment at a new location each night.
The GEM45EC also includes a 32-channel GPS system that supplies location and time information automatically. Its latitude adjustment range extends from 14° to 68°, accommodating observing locations across a broad range of northern and southern latitudes.
The supplied Go2Nova hand controller has an eight-line, 21-character LCD display with a built-in heater for cold-weather operation. Its database contains more than 212,000 astronomical objects, including familiar deep-sky catalogs, stars, and solar system targets.
Automatic zero-position searching helps establish the mount's reference position, while configurable meridian handling allows tracking beyond the meridian by a selected amount, up to 20°. As with any German equatorial mount, the actual clearance between telescope, camera, counterweights, and support must be checked before allowing extended tracking past the meridian.
Wi-Fi is optional rather than built in. iOptron's iStarFi adapter provides wireless mount communication through compatible software, while USB allows conventional computer control.
A Head-Only Package for Your Existing Support
The #7620ECA includes the GEM45EC mount head and its accessories, but not a tripod, pier, or counterweights. The included stainless steel counterweight shaft measures 1.10" (28mm) in diameter and approximately 11" (280mm) long. Compatible counterweights must be purchased separately according to the telescope and equipment being carried.
For someone who already owns a suitable iOptron Tri-Pier or another compatible support, this arrangement avoids paying for a second tripod. It also makes sense for an observer building a more permanent installation, where the mount will eventually sit on a properly designed pier rather than a portable tripod.
The included aluminum carrying case protects the mount head during storage and transportation. At 17.5 pounds, the GEM45EC remains manageable for one person to carry, despite its substantial payload capacity. The counterweights, telescope, and supporting pier or tripod are transported separately.
The mount operates from a 12V DC power source with a rated 5A supply. iOptron specifies approximately 0.6 amp while tracking and 0.9 amp during GoTo slews, making portable battery operation practical with an appropriately sized power system. Cameras, computers, and dew control equipment will add to the total electrical load.
What Owners Say on Cloudy Nights
The GEM45EC has generated some interesting discussions on Cloudy Nights, particularly among astrophotographers trying to decide whether the high-resolution encoder is worth the additional cost over the standard GEM45.
One GEM45EC owner described using the mount as a portable imaging system, where the ability to collect relatively short unguided exposures was particularly useful. He regarded the encoder-equipped version as attractive for mobile observing and reported adjusting his guiding approach when using the encoder.
Other experienced imagers have reached different conclusions. In discussions comparing the GEM45 and GEM45EC, some observers have argued that a well-tuned autoguider on the standard GEM45 can already deliver excellent results, reducing the practical advantage of paying for an encoder. Others have discussed the need to configure guiding carefully with encoder-equipped mounts.
These are useful perspectives because they address the actual decision a buyer faces. The encoder offers a measurable improvement in the mount's specified periodic tracking error, but the finished photograph depends on much more than the worm drive. Seeing conditions, polar alignment, image scale, guiding configuration, and the rigidity of the complete imaging system all have a part to play.
The best reason to choose the GEM45EC is therefore a specific interest in what its encoder can contribute to your observing or imaging arrangement, rather than assuming that the more expensive mount must automatically produce better results in every situation.
Under the Night Sky
A mount with substantial carrying capacity and accurately controlled tracking gives an astrophotographer the freedom to explore objects that don't fill the field of a short, wide-field refractor. Many galaxies and planetary nebulae are comparatively small in apparent size, making them particularly rewarding subjects for telescopes operating at longer focal lengths.
M82 in Ursa Major is a good example. Its bright, elongated body is crossed by dark dust features, while long exposures through suitable narrowband filters can reveal the enormous hydrogen-alpha filaments extending away from the central starburst region. Recording those faint structures requires much more than accurate tracking, but stable imaging equipment makes the job easier.
Nearby, the Whirlpool Galaxy, M51, offers a different challenge. Its spiral arms and interacting companion provide enough structure to reward nights of careful imaging, particularly when the atmosphere is steady enough to support fine detail.
The Whale Galaxy, NGC 4631, and neighboring Hockey Stick Galaxy, NGC 4656, make a striking pair in Canes Venatici. Their elongated shapes, distorted structures, and surrounding faint features reward a suitable field of view and substantial total exposure time.
In Cassiopeia, the Bubble Nebula, NGC 7635, provides an opportunity to examine a delicate shell of gas surrounding a massive star. Narrowband imaging can reveal the brighter portions of the bubble and the surrounding emission nebula. For a more difficult project, the faint outer halo surrounding the Ring Nebula in Lyra can become an interesting target when using appropriate filters and very long integration times.
Abell 39 in Hercules presents an entirely different appearance: a faint, nearly spherical planetary nebula with a comparatively uniform shell. It is not an especially bright target, and photographing it successfully requires good sky conditions, careful processing, and patience.
The GEM45EC cannot make a small telescope gather the light of a much larger instrument, nor can its encoder eliminate poor seeing. What it offers is a way to reduce a significant source of mechanical tracking error while the rest of the imaging system does its work.
Observing Tip: Set Up Your Off-Axis Guider Before Dark
If you're using a long-focal-length telescope with an off-axis guider, one of the most useful things you can do is establish the correct spacing between the main imaging camera and guide camera before your first night outside.
An off-axis guider uses a small pickoff prism to direct part of the light entering the main telescope toward a separate guide camera. Since both cameras receive light through the same optical system, the guider avoids much of the differential flexure that can occur when a separate guide telescope moves slightly differently from the main optical tube.
The two cameras need to be parfocal, meaning that when the main camera is correctly focused, the guide camera is also focused. You can establish this during daylight by pointing the telescope at a sufficiently distant object, focusing the main camera, and then adjusting the guide camera's position until its image is also sharp. Final adjustment on stars may still be needed, particularly with systems that have limited focusing travel.
Once the guide camera is correctly positioned, secure its adjustment and make a reference mark if the arrangement is likely to be disassembled. That can save considerable time on subsequent nights, when you'd rather be collecting images than trying to find focus in a small guiding camera.
With the GEM45EC, also review the mount's recommended encoder and guiding settings before beginning a long exposure sequence. An encoder can reduce mechanical periodic error, but a poorly configured guider can still introduce unnecessary corrections. Start with careful polar alignment, establish a sound guiding calibration, and judge performance from the actual star shapes in your images rather than a single guiding statistic.
FAQ
- What is the difference between the GEM45 and GEM45EC?
- The GEM45EC adds a high-resolution optical encoder on the right ascension axis for Real-Time Periodic Error Correction. The standard GEM45 uses conventional periodic error correction and supports autoguiding. Both have the same 45-pound rated payload capacity and broadly similar mechanical construction.
- Can the GEM45EC take photographs without autoguiding?
- Yes, under suitable conditions. Exposure length depends on polar alignment, telescope focal length, camera image scale, atmospheric conditions, and other factors. For demanding long-exposure work, particularly at longer focal lengths, an autoguider remains advisable.
- Does the optical encoder increase payload capacity?
- No. The encoder reduces periodic tracking error in right ascension. The maximum rated payload remains 45 pounds (20 kg), excluding counterweights, and the practical choice of telescope still depends on its dimensions and weight distribution.
- What do I need to purchase separately?
- A compatible tripod or pier and counterweights for the included 28mm counterweight shaft. The mount head, iPolar, GPS, Go2Nova controller, power adapter, cables, and aluminum hard carrying case are included.
- Does the GEM45EC have built-in Wi-Fi?
- No. Wi-Fi mount control requires iOptron's optional iStarFi adapter. USB computer control is provided as standard equipment.
Supplied Accessories
- iOptron GEM45EC German equatorial mount head with high-resolution RA optical encoder
- Built-in iPolar electronic polar scope
- 6" Losmandy D/Vixen dual-format dovetail saddle
- Go2Nova computerized hand controller with heated eight-line display
- GEM45 GPS module
- 1.10" (28mm) diameter stainless steel counterweight shaft, approximately 11" (280mm) long
- Aluminum hard carrying case for the mount head
- Two coiled control cables
- USB cable
- 100–240V AC power adapter
- Quick Start Guide
Tripod, pier, and counterweights are not included with the #7620ECA head-only package.
Optional Accessories
iOptron 5 kg Counterweight (#8027): Designed for the GEM45's 28mm counterweight shaft. The required number of counterweights depends on the telescope and accessories being carried.
iOptron 2.5 kg Counterweight: Provides a smaller increment of counterweight mass when balancing lighter optical systems or making finer adjustments.
iOptron Tri-Pier (#8034): A substantial freestanding pier-style support for observers who want a stable mounting platform. Confirm the appropriate GEM45 attachment hardware before ordering.
Off-Axis Guider and Guide Camera: Particularly useful for long-focal-length astrophotography, allowing guiding through the main optical path of the telescope.
iOptron iStarFi Wi-Fi Adapter: Adds wireless mount control through compatible computer and mobile applications.
Final Thoughts
The GEM45EC is an interesting instrument because it combines two things that don't always appear together: a traditional German equatorial mount with substantial carrying capacity and a sophisticated real-time tracking correction system, all in a head weighing less than 18 pounds.
The optical encoder is the reason to consider this version over the standard GEM45. It monitors the actual right ascension axis and compensates for the repeating mechanical errors that conventional mounts must handle through trained correction curves or guiding. That doesn't make autoguiding obsolete, but it does provide a different and potentially valuable approach to accurate tracking.
I wouldn't recommend the GEM45EC merely because someone owns a longer telescope. If the standard GEM45 can carry the equipment comfortably and a well-tuned guiding system already produces satisfactory images, the less expensive mount may be the better use of the money. On the other hand, an observer interested in encoder-based tracking, experimenting with unguided imaging, or reducing periodic error at its source has a genuine reason to consider the EC version.
The head-only configuration is also attractive for someone who already has a suitable support. With iPolar, GPS, internal cable management, a dual-format saddle, and an aluminum carrying case included, the GEM45EC provides the essential components of a capable imaging mount without requiring the purchase of another tripod.
For the right astrophotographer, the appeal is not simply that the GEM45EC can carry 45 pounds or that it has an impressive periodic error specification. It's the opportunity to put a carefully controlled tracking system underneath a telescope while keeping the entire arrangement manageable enough to use regularly.
