At the scale of a permanent observatory, mount errors stop being abstract specifications. Put a C14, large Ritchey-Chrétien, CDK, or substantial refractor on a camera with an image scale below an arcsecond per pixel and small mechanical errors become visible in the stars. The mount has to carry the system, track it precisely, manage all of its cables, and return to the same reliable behavior night after night.
The iOptron CEM120EC2 is the dual-encoder version of the 115 lb CEM120 platform. The mount head weighs 57 lb (26 kg), carries up to 115 lb (52 kg) of payload excluding counterweights, and adds high-resolution position encoders to both the right-ascension and declination axes.
The RA encoder works with iOptron's Real-Time Periodic Error Correction to reduce the repeating tracking error of the worm drive. iOptron specifies encoder-corrected periodic error below 0.15 arcsecond RMS over the mount's 240-second worm period. The second encoder gives the mount direct positional feedback on declination as well, rather than relying solely on commanded motor movement and the gear train.
That does not make polar alignment, atmospheric refraction, flexure, guiding, or careful observatory setup disappear. What it does is give the mount accurate position feedback on both axes before the rest of the imaging system ever gets involved. For the largest telescopes and finest image scales in the CEM120 family, that is what separates the EC2 from the standard CEM120 and the single-encoder CEM120EC.
Key Features
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High-resolution encoders on both axes. The CEM120EC2 adds position feedback on both RA and DEC, with a published encoder resolution of 0.035 arcsecond.
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Real-Time Periodic Error Correction. The RA encoder continuously measures and corrects the periodic component of the worm drive rather than relying on a previously trained PEC curve.
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Less than 0.15 arcsecond RMS periodic error. iOptron specifies encoder-corrected PE below 0.15 arcsecond RMS, measured on the bench over one complete 240-second worm cycle.
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115 lb payload capacity. Carries up to 52 kg (115 lb) of telescope and imaging equipment, excluding counterweights.
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57 lb mount head. Approximately a 2:1 payload-to-mount-weight ratio.
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Center-balanced equatorial design. The CEM architecture concentrates the combined mount, payload, and counterweight mass more directly over the support.
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Push-to capability. With the mount powered on, the gear switches can be released and the telescope moved manually while the dual encoders maintain positional information.
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216mm worm wheels on both axes. RA and DEC each use a 360-tooth worm wheel driven by a 26mm worm.
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Large steel axles and bearings. Both axes use 80mm steel shafts supported by 125mm bearings.
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Precision stepper motors. 128 micro-steps provide 0.07-arcsecond motor resolution.
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17.2-inch Losmandy-D saddle. Provides substantial clamping length for large OTAs, long dovetail plates, and side-by-side systems.
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Built-in Ethernet and Wi-Fi. Designed for permanent observatory networking and remote operation.
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USB and RS-232 control. Direct wired connections are also provided for local computer control.
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Advanced through-mount cabling. USB, 12V power, guide, auxiliary, and iPolar connections can pass through the mount to the telescope side.
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32-channel GPS. Supplies location and UTC information to the mount.
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Automatic zero-position search. Fixed reference sensors provide a repeatable mechanical starting point for automated operation.
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Programmable parking. Horizontal, vertical, current-position, and user-defined Alt/Az parking positions are available.
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Power-down position memory. The mount can retain positional information after shutdown when the system has not been physically moved.
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Configurable meridian treatment. The CEM120EC2 can stop or automatically flip, with settings allowing up to 14° of travel past the meridian.
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ST-4 autoguider port. Traditional autoguiding is supported along with software-based pulse guiding.
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Optional internal iPolar. iOptron's CEM120-specific electronic polar scope can be installed for dedicated camera-based polar alignment.
- Two-year limited warranty.
Why Two Encoders?
The CEM120EC already has a high-resolution encoder on right ascension, which is where periodic worm error directly affects tracking. So what does the second encoder on the EC2 actually add?
It gives the mount direct position feedback on declination as well.
Without a high-resolution DEC encoder, the control system commands the declination motor to move a certain amount and relies on the mechanical drive to produce that movement. With the EC2, the mount can also measure the resulting position of the declination axis itself.
That does not mean the DEC encoder eliminates atmospheric seeing, wind, polar-alignment drift, mechanical flexure, or every reason an autoguider might issue a correction. It means the mount has positional feedback on both of its primary axes rather than only one.
For an observatory system working at fine image scales, that gives the control system more information about what the mount itself is doing.
Real-Time PEC on Right Ascension
The right-ascension encoder also changes how the CEM120EC2 deals with periodic error.
A traditional worm-driven mount has small mechanical errors that repeat as the worm rotates. On the standard CEM120, those errors can be characterized and reduced with Permanent Periodic Error Correction.
The EC2 instead measures the RA axis directly while it tracks. Real-Time PEC uses the encoder feedback to correct the repeating component as it happens rather than replaying a previously recorded correction curve.
iOptron publishes an encoder resolution of 0.035 arcsecond and specifies periodic error below 0.15 arcsecond RMS when measured on the bench over the full 240-second worm cycle.
Those numbers should be understood for what they are. The 0.035-arcsecond figure is encoder resolution, not a promise that the telescope points to or tracks every star to 0.035 arcsecond. Likewise, the 0.15-arcsecond figure describes encoder-corrected periodic error under iOptron's bench measurement, not total tracking performance under the night sky.
You May Still Want to Guide
Dual encoders do not make the rest of the universe disappear.
The mount cannot use its encoders to correct atmospheric refraction, differential flexure, movement in the optical train, wind loading, seeing, or drift caused by an imperfectly aligned polar axis. Those effects happen outside the worm gears the encoders are monitoring.
That is why guiding can still make sense with the CEM120EC2, particularly at the long focal lengths and fine image scales this mount is capable of supporting.
The difference is that an external guider is not starting with an ordinary uncorrected gear train. The mount already has high-resolution positional feedback on both axes and Real-Time PEC on RA, leaving the guider to respond to the residual movement of the complete telescope, atmosphere, and imaging system.
There is no single guide cadence or aggressiveness setting that is correct for every EC2. Telescope focal length, camera scale, seeing, guider type, payload, software, and local conditions all matter. Tune the guiding system to the stars produced by your own observatory rather than chasing one set of numbers from someone else's installation.
Unguided Imaging Needs More Than Encoders
The CEM120EC2's dual encoders make unguided imaging an understandable question, but encoders alone are only part of that problem.
They can tell the mount where its axes are. They cannot, by themselves, tell it how atmospheric refraction changes across the sky, how the telescope structure flexes as gravity changes direction, or exactly how a small polar-alignment error alters the apparent motion of a target over several hours.
Premium unguided observatory systems that work at demanding image scales often combine high-resolution encoders with a detailed sky model that measures those position-dependent errors and changes the tracking rates accordingly.
The CEM120EC2 does not automatically build that kind of many-point model as part of its native mount operation.
Depending on focal length, image scale, exposure time, and alignment quality, unguided imaging may still be useful. But we would buy the EC2 for its dual-axis position feedback and exceptionally low RA periodic error, not on the assumption that a guider will never be needed.
Push-To Without Losing Position
One unusual benefit of having high-resolution encoders on both axes is that the CEM120EC2 can remain aware of its position even when you move the telescope manually.
With the mount powered on, iOptron allows you to release the RA and DEC gear switches, move the telescope by hand to another part of the sky, and then re-engage the drives. Because both axes are encoded, the control system can retain positional information through that movement.
That gives a very large equatorial mount some of the immediacy of a manual push-to system without throwing away the alignment information every time the gears are disengaged.
It may not be the feature that sells an EC2 to a remote imager, but for an observatory that is also used visually or interactively, it is a useful consequence of having encoders on both axes.
The CEM120 Mechanical Platform
Under the encoders is the same substantial mechanical foundation used throughout the CEM120 family.
Both RA and DEC use 216mm diameter, 360-tooth worm wheels driven by 26mm worms. The right-ascension worm period is 240 seconds. Each axis turns on an 80mm steel shaft supported by 125mm bearings, and precision stepper motors use 128 micro-steps.
Those dimensions matter because observatory-class telescopes create more than simple downward weight.
A long refractor creates leverage. A large RC or CDK presents a substantial cross-section to the wind. A C14 with a large camera, rotator, filter wheel, reducer, focuser, and off-axis guider hangs considerable equipment behind the optical tube.
The CEM120 platform is large because those systems need a large mechanical foundation, not simply because the published payload number looks impressive.
Why Center-Balanced?
The CEM design reorganizes the geometry of a traditional German equatorial mount so more of the combined mass sits directly over the pier or tripod.
The telescope still needs counterweights and the axes still need to be balanced. "Center-balanced" does not mean counterweight-free.
Instead, iOptron's layout reduces how far the mount structure itself is cantilevered away from the support. On a small telescope that may not sound especially important. On a permanent system whose total installed moving mass can easily exceed 150 pounds once the payload and counterweights are included, the geometry becomes much more meaningful.
115 Pounds Gives You Room to Build
The CEM120EC2 carries up to 115 lb (52 kg) of payload, excluding counterweights.
That rating is still dependent on the size and length of the optical tube. A compact 70 lb telescope and a very long 70 lb telescope do not place identical loads on the mount.
The greater advantage of the capacity is often not putting a 115 lb telescope on the saddle. It is having mechanical margin once a more reasonable optical tube becomes a complete observatory instrument.
A C14 leaves substantial capacity for a large camera, filter wheel, rotator, focuser, reducer or corrector, off-axis guider, dew equipment, and all of the plates and hardware needed to connect them. Large RC and CDK systems can be configured around the imaging job rather than around the last few available pounds of mount capacity.
The long 17.2-inch Losmandy-D saddle also gives large dovetail plates plenty of clamping surface and makes substantial side-by-side systems practical when the complete load remains within the mount's mechanical limits.
Built for the Observatory
The CEM120EC2 makes the most sense once the mount becomes part of the observatory rather than something you carry outside at dusk.
Ethernet LAN and Wi-Fi are built into the mount, along with USB and RS-232 control. In a permanent installation, a hard-wired Ethernet connection can make the mount another stable device on the observatory network rather than relying on a long USB cable or wireless link for primary control.
Automatic zero-position search provides a known mechanical reference at startup. Programmable park positions let you choose where the telescope belongs when a roof or dome closes. Power-down position memory helps preserve the mount's state when it has been shut down without being physically disturbed.
Meridian behavior is configurable as well. The mount can stop or automatically flip, with settings allowing up to 14° of tracking beyond the meridian when the telescope, camera, pier, and observatory have enough clearance.
Those features are easy to overlook on a specification sheet, but they become some of the most important parts of a mount once nights are automated and nobody is standing beside it watching every slew.
Through-Mount Cable Management
A large imaging system can have nearly as much wiring as optical equipment.
Main camera, guide camera, filter wheel, focuser, rotator, dew heaters, flat-field equipment, and other accessories all need data, power, or both. External cable loops can change tension as the telescope moves and eventually become snag points during a slew or meridian flip.
The CEM120EC2 includes an extensive through-mount cable system. Available paths include two USB 2.0 ports, three USB 3.0 ports, two 12V outputs rated to 1A, two higher-current DC outputs rated to 5A, ST-4 guiding, 6P6C, AUX, and a dedicated USB connection for the optional iPolar.
That allows much of the imaging train to connect near the saddle rather than sending a loose bundle of cables across the moving RA and DEC axes.
For a permanent installation, that is not just tidier. It removes potential cable movement from an imaging system where fractions of an arcsecond matter.
Under the Night Sky
The CEM120EC2 belongs with telescopes that can actually take advantage of this level of mount.
A large SCT, Ritchey-Chrétien, or CDK brings small galaxies, compact galaxy groups, planetary nebulae, globular clusters, and fine structure inside larger targets into useful image scale. Objects such as NGC 891, Stephan's Quintet, the Cat's Eye Nebula, M51, and the smaller galaxies scattered through spring skies become more than tiny features in a wide field.
At those focal lengths, tracking becomes part of image resolution. A periodic error that would be nearly invisible through a short refractor can spread a star across multiple pixels through a long-focal-length telescope.
The EC2 cannot improve the seeing or make the optical system sharper than physics allows. What it can do is reduce the mount itself as a source of uncertainty, with direct positional feedback on both axes and encoder-corrected periodic error on RA.
There is also enough capacity for dual systems. A wide-field refractor can record an entire nebula while a longer-focal-length instrument works a small structure within it, allowing the same clear hours to serve two very different image scales.
From the Cloudy Nights Community
Experienced CEM120EC2 owners tend to make an interesting point about guiding: once the mount is behaving properly, too much guider intervention can become counterproductive.
Owners working with large SCTs and other long-focal-length systems have reported excellent guiding after tuning their software to make relatively gentle corrections. Individual RMS figures vary substantially with seeing, payload, image scale, and installation, so we would not turn those owner numbers into a specification.
The more useful lesson is that an encoder mount should not necessarily be guided the same way as a conventional mount with larger uncorrected gear error. Let the encoders do the work they were designed to do, then use the guider to correct what remains in the complete telescope and sky.
CEM120, CEM120EC, or CEM120EC2?
All three versions share the same basic 115 lb CEM120 mechanical platform. The difference is how much direct positional feedback the mount has.
CEM120: conventional precision worm gearing with native periodic error below ±3.5 arcseconds, Permanent PEC, and no high-resolution tracking encoders.
CEM120EC: adds a high-resolution encoder to right ascension and uses Real-Time PEC to reduce RA periodic error to less than 0.15 arcsecond RMS under iOptron's bench measurement.
CEM120EC2: uses high-resolution encoders on both RA and DEC, adding direct positional feedback to the declination axis as well as the encoder-corrected RA system.
If your imaging program assumes conventional guiding and you primarily want the CEM120's mechanical platform and observatory infrastructure, the standard CEM120 is the economical route.
If your priority is removing RA periodic error at the mount, the CEM120EC provides the most significant encoder benefit with one high-resolution RA encoder.
The EC2 is for the installation where you want positional feedback on both axes and the most complete version of the CEM120 control system.
Polar Alignment
The CEM120EC2 does not include an optical polar scope.
For a permanent observatory, polar alignment is normally performed through the imaging or guide camera using software. Once the pier is accurately aligned and nothing physically moves, that alignment should require little ongoing attention.
The mount also includes iOptron's Polar Iterate Align routine for locations where the celestial pole cannot be seen directly.
If you prefer a dedicated alignment camera, the optional #3339A-120 iPolar installs internally in the CEM120 family and provides electronic plate-solving polar alignment without requiring Polaris itself to be visible.
What's Included
- iOptron CEM120EC2 center-balanced equatorial mount head
- High-resolution RA and DEC encoders
- 17.2-inch Losmandy-D dovetail saddle
- Go2Nova hand controller and connection cable
- 38.1mm × 540mm stainless-steel counterweight shaft
- Two 22 lb (10 kg) counterweights
- 12V/5A AC/DC power adapter for indoor use
- Mount accessories and connection hardware
Not included: pier or tripod, optional iPolar electronic polar scope.
The CEM120EC2 ships in two boxes, with the counterweights packaged separately.
Frequently Asked Questions
How much can the CEM120EC2 carry?
iOptron rates the mount for 115 lb (52 kg) of payload, excluding counterweights. Telescope length, diameter, and overall geometry still matter along with actual weight.
How much does the mount head weigh?
The CEM120EC2 weighs 57 lb (26 kg).
How many high-resolution encoders does it have?
Two: one on right ascension and one on declination.
What does 0.035 arcsecond mean?
That is the published resolution of the high-resolution encoders. It is not a claim that the telescope has 0.035-arcsecond absolute pointing or total tracking accuracy.
What is the periodic error?
iOptron specifies encoder-corrected periodic error below 0.15 arcsecond RMS, measured on the bench over the full 240-second RA worm period.
Does it use conventional PEC training?
No. The encoder-equipped CEM120 models use Real-Time Periodic Error Correction rather than the trained Permanent PEC system used by the standard CEM120.
What does the DEC encoder add?
It gives the control system direct high-resolution position feedback on declination instead of relying solely on commanded motor movement through the gear train. The EC2 therefore has encoded position information on both axes.
Do I still need to guide?
For demanding long-exposure work at fine image scales, guiding can still be useful. The encoders monitor the mount's axes, but an autoguider can respond to residual motion caused by polar-alignment error, atmospheric refraction, flexure, wind, and other effects elsewhere in the complete system.
Can the CEM120EC2 image unguided?
The dual encoders remove important mechanical errors inside the mount, but practical unguided exposure length still depends on focal length, image scale, polar alignment, refraction, flexure, and the availability of a suitable sky-modeling and tracking system. We would not buy the EC2 on the assumption that guiding will never be necessary.
Can I move the telescope manually without losing position?
Yes. One benefit of the dual encoders is push-to operation. With the mount powered on, the gear switches can be released, the telescope moved manually, and the drives re-engaged while the encoder system preserves positional awareness.
Can it carry a C14?
Yes. A C14 OTA leaves substantial room within the published 115 lb rating for a camera, filter wheel, focuser, guider, rotator, dew equipment, dovetails, and other accessories. Consider the complete system and its mechanical geometry rather than OTA weight alone.
Can it carry a large RC or CDK?
Many large observatory-class RC and CDK systems fall within the CEM120EC2's published payload range. Tube diameter, length, center of gravity, accessory load, and pier stiffness should all be considered along with total weight.
Can I control it remotely?
Yes. Ethernet LAN, Wi-Fi, USB, and RS-232 are built into the mount, and automatic zero search, programmable parking, power-down position memory, and configurable meridian behavior support permanent and automated observatory operation.
Does it have through-mount cabling?
Yes. Multiple USB, DC power, guide, auxiliary, and iPolar connections are routed through the mount so cameras and accessories can connect near the telescope rather than running loose cables across the axes.
Does it include a power supply?
Yes. iOptron's current specifications state that a 12V/5A AC adapter intended for indoor use is included.
How do I polar align it?
You can use polar-alignment software through the imaging or guide camera, the mount's Polar Iterate Align routine, or install the optional internal #3339A-120 iPolar electronic polar scope.
What pier should I use?
A permanent observatory pier is the natural choice for most CEM120EC2 installations. iOptron also offers substantial support options including its Permanent Pier and Tri-Pier 360. At this scale, pier stiffness and the foundation beneath it are part of the tracking system.
Accessories
iOptron #3339A-120 Internal iPolar Electronic Polar Scope (sold separately): installs inside the CEM120 family and provides dedicated camera-based polar alignment.
iOptron Permanent Pier or Tri-Pier 360 (sold separately): substantial support options appropriate for the size and installed mass of a CEM120EC2 system.
Off-axis guider and guide camera (sold separately): a natural choice for long-focal-length systems because the guide camera sees motion through the main telescope's optical path rather than through a separate guide scope.
Additional CEM120 counterweights (sold separately): available when the installed payload requires more counterweight than the two included 22 lb weights provide.
Low-latitude counterweight #7326LL (sold separately): iOptron specifies a special counterweight arrangement for CEM120-family installations below approximately 10° latitude.
Final Thoughts
The CEM120EC2 is the version of the CEM120 platform for an observatory where positional feedback on both axes matters.
Its 115 lb payload, large worm wheels and bearings, long Losmandy saddle, center-balanced geometry, through-mount cabling, Ethernet networking, automatic zero search, and programmable parking give it the mechanical and operational foundation for a substantial permanent imaging system.
The dual high-resolution encoders add another layer. Right ascension gets Real-Time PEC and extremely low encoder-corrected periodic error, while declination gains its own direct position feedback. Together they let the mount know far more about what its axes are actually doing than a conventional open-loop gear system can.
That still does not remove the atmosphere, polar alignment, flexure, wind, or every reason to use a guider. The encoders solve problems inside the mount; a complete imaging system still has to deal with everything outside it.
If the standard CEM120 gives you the capacity and the CEM120EC gives you the RA tracking correction you want, the CEM120EC2 completes the idea by putting high-resolution positional feedback on both axes.
