A portable equatorial mount lives a considerably harder life than one permanently installed in an observatory. It gets carried outside, taken apart, packed into a vehicle, driven to a dark sky site, and assembled again on a different piece of ground. Sometimes there's a different telescope or camera sitting on top of it. Yet every time you set it up, you're asking the mount to follow the stars with enough accuracy to record faint astronomical objects during exposures that may last several minutes.
The iOptron CEM26EC was designed to bring a level of tracking precision normally associated with more substantial equipment into a mount head weighing only 10 pounds (4.5 kg). Its distinguishing feature is a high-resolution optical encoder on the right ascension axis that continuously monitors the axis position and allows the mount to correct periodic tracking errors in real time. Unlike conventional periodic error correction, the encoder does not require you to record and play back a correction curve for the worm gear. The corrections are made from actual measurements of the axis as the mount operates.
That's a significant engineering difference, particularly when you're trying to photograph small galaxies, planetary nebulae, or other objects that benefit from a longer imaging focal length. iOptron specifies periodic error of less than 0.3 arcsecond RMS for the CEM26EC, measured with the encoder operating on a test bench over a ten-minute period. That's an exceptionally small tracking variation for a mount in this weight class, although the actual performance of an imaging system will also depend on polar alignment, telescope balance, atmospheric conditions, and the mechanical behavior of the telescope and camera.
The C264A3 package includes the CEM26EC mount head with its encoder and built-in iPolar electronic polar scope, a 1.75" LiteRoc stainless steel tripod, 10 lb counterweight, Go2Nova computerized hand controller, and an aluminum hard carrying case for the mount head. It's a complete mounting package for observers who want precise tracking without having to transport a heavy observatory-class mount.
What the Optical Encoder Actually Does
To understand why the CEM26EC costs more than a standard CEM26, it helps to understand how a conventional equatorial mount tracks the sky.
An equatorial mount follows the stars by turning its right ascension axis at approximately the same angular rate as the Earth rotates, but in the opposite direction. In a traditional worm-drive mount, a motor turns a small screw-shaped gear, called a worm, which drives a much larger toothed wheel attached to the axis. The large gear reduction allows the motor to move the telescope very slowly and precisely.
Unfortunately, no worm gear is manufactured with absolute perfection. Minute variations in the worm's shape and its engagement with the wheel can cause the axis to move slightly faster or slower than intended. Since the worm turns at a regular rate, these tracking variations repeat with every revolution. Astronomers call this periodic error.
The standard CEM26 deals with periodic error using Permanent Periodic Error Correction (PPEC). The mount records a series of corrections over a complete worm cycle and plays them back on subsequent cycles. This can reduce the predictable component of the error, but it depends on a previously recorded correction curve rather than directly measuring the axis position continuously.
The CEM26EC takes a different approach. Its high-resolution optical encoder monitors the actual angular position of the right ascension axis as it moves. If the axis begins to get ahead of or fall behind its expected position, the electronics can correct the drive rate. iOptron calls this Real-Time Periodic Error Correction, or RPEC.
Because the encoder is measuring what the axis is doing, there's no periodic error correction curve for you to train before using the mount. Its correction system operates automatically, even after the mount has been transported or a different telescope installed.
iOptron's published specification of less than 0.3 arcsecond RMS periodic error was measured with the encoder engaged over a ten-minute test period. For perspective, one arcsecond is only 1/3600th of a degree. The number is a measure of the remaining periodic tracking variation under the manufacturer's test conditions, not a promise that every photograph will show stars tracked to within that amount.
Nevertheless, substantially reducing one of the principal mechanical sources of tracking error gives an astrophotographer a much better starting point, especially when using smaller camera pixels or longer focal lengths where a slight tracking irregularity becomes more visible in the finished image.
Does the Encoder Eliminate Autoguiding?
Not necessarily, and this is an important distinction when deciding whether the additional cost of the EC version makes sense.
The encoder measures and corrects the motion of the right ascension axis. It cannot directly measure the position of a star on your camera sensor, so it does not compensate for every source of movement that can affect a long exposure.
Even with nearly perfect mechanical tracking, a mount that is not accurately polar aligned can produce declination drift. Atmospheric refraction changes the apparent position of objects as they move across the sky. A telescope can flex slightly as its orientation changes, and a guide scope may not move in precisely the same way as the main imaging telescope. These are all factors that can influence the final photograph without being errors in the right ascension worm drive.
The CEM26EC can produce useful unguided exposures under suitable conditions, particularly with shorter focal length instruments and careful polar alignment. For demanding long-exposure imaging, however, an autoguider remains a valuable addition.
There is an interesting difference in how the CEM26EC can work with guiding software. The Go2Nova controller provides an RA guiding option that lets you choose whether the mount accepts external right ascension guiding corrections or filters those corrections and relies on the encoder to manage RA tracking. In the filtered mode, a guider can still help manage declination corrections while the encoder maintains the right ascension drive.
This gives an experienced imager some flexibility when tuning a guiding system. The best arrangement depends on your equipment, software, seeing conditions, and the nature of the tracking errors being measured. More guiding corrections are not always better, particularly if they are reacting to atmospheric movement rather than actual mechanical tracking errors.
The mount includes an ST-4-compatible autoguider port and USB computer control through ASCOM. An off-axis guider can be particularly useful at longer focal lengths because it monitors stars through the main telescope's optical path, reducing the potential for differential flexure between a separate guide scope and the imaging telescope.
High-Precision Tracking in a Ten-Pound Mount
Adding an encoder does not change the CEM26's basic mechanical payload capacity. The CEM26EC still carries a manufacturer-rated 26 pounds (12 kg) of telescope and accessories, excluding the counterweight, on a mount head weighing only 10 pounds (4.5 kg).
The reason this is possible is iOptron's center-balanced equatorial design. Instead of placing most of the telescope and mount weight off to one side of the tripod, the CEM arrangement positions the combined load more directly over the center of its support. This provides a favorable mechanical arrangement while allowing the mount head to remain relatively small and light.
Both axes use 3.46" (88mm) diameter aluminum worm wheels with 144 teeth, driven by 0.60" (15.2mm) brass worms through synchronous belt transmissions. The 1.38" (35mm) steel right ascension and declination shafts are supported by 2.17" (55mm) ball bearings. The spring-loaded gear engagement system helps maintain proper contact between the worm and wheel, while the large positive-lock controls allow the axes to be disengaged for balancing the telescope before observing.
The encoder adds precision to the right ascension tracking system, but it does not turn the CEM26EC into a substantially larger mount. A long telescope still places greater demands on the bearings and drive system than a short telescope of the same weight. Wind and the position of the telescope's center of gravity remain important considerations.
iOptron recommends optical tubes smaller than 6" in diameter for the CEM26 family. A compact apochromatic refractor, small astrograph, or suitably sized catadioptric telescope is a natural match. The additional tracking precision may be particularly attractive when an observer wants to use a somewhat longer focal length within the mount's mechanical limits, but it should not be interpreted as permission to ignore those limits.
The 3.8" adjustable saddle accepts Vixen-style dovetail plates rather than the wider Losmandy D format. If you already own a telescope with a Losmandy-format mounting plate, you'll need to account for that difference when planning the system.
iPolar Alignment and Field Operation
Accurate polar alignment is important on any equatorial mount, but it becomes especially worthwhile when the mechanical tracking itself is capable of such small errors. After all, there is little point in paying for an exceptionally accurate right ascension drive if poor alignment introduces substantial drift during an exposure.
The CEM26EC includes iOptron's iPolar electronic polar scope, built into the right ascension axis. It uses a small camera to identify the star field around the celestial pole and determines where the mount is pointing relative to the true pole.
Connect iPolar to a compatible computer running iOptron's software, and the program shows you how to adjust the mount's altitude and azimuth controls to bring the axis into alignment. Polaris itself does not have to be visible, provided enough of the surrounding polar sky remains unobstructed for the software to identify the field.
For a portable imaging system, this is a welcome convenience. Every new observing location requires polar alignment, whether you're in the backyard or at a dark sky site several hours from home. With iPolar, you can establish that alignment electronically without kneeling behind the mount to look through an optical reticle.
The supplied Go2Nova 8409 hand controller contains a database of more than 212,000 astronomical objects, with computerized GoTo and tracking available without using a computer for normal telescope operation. Built-in Wi-Fi also allows mount communication through compatible applications, while USB provides computer control and firmware update capability. The Wi-Fi connection is for the mount itself and does not replace the data connections required by imaging cameras or other USB accessories.
The CEM26EC operates from 12V DC power and draws approximately 0.5 amp while tracking and 0.8 amp during GoTo slews, according to iOptron's specifications. This relatively modest power consumption makes a suitable field battery practical when observing away from an electrical outlet. Remember to calculate the requirements of your entire imaging system, since a cooled camera, dew heaters, and computer may consume substantially more power than the mount.
The LiteRoc Tripod and Hard Carrying Case
The C264A3 includes iOptron's heavier 1.75" LiteRoc stainless steel tripod, weighing approximately 16.5 pounds (7.5 kg). The substantial steel legs and metal spreader provide a firm foundation for the mount, helping resist vibrations caused by focusing, handling, and light breezes.
This is particularly useful when imaging at a longer focal length. The encoder can reduce the tracking errors originating in the right ascension drive, but it cannot prevent a telescope from shaking because the tripod is being disturbed by wind. A stable support remains essential to getting the most out of the mount's precision.
The aluminum hard carrying case protects the mount head during storage and transportation. An optical encoder is a precision measuring device, and although it is housed within the mount, there's every reason to treat the instrument carefully when transporting it. The fitted case gives the CEM26EC a dedicated place in the car instead of leaving the mount head loose among counterweights, cables, and telescope accessories.
The case is designed for the head rather than the entire assembled mounting system. The LiteRoc tripod, counterweight, and telescope are transported separately, which also makes the equipment easier to divide into manageable pieces when carrying it to an observing site.
What Owners Say on Cloudy Nights
The CEM26EC has attracted attention among Cloudy Nights imagers who want better tracking accuracy without carrying a large equatorial mount. Several discussions have addressed the practical differences between the encoder-equipped version and the standard CEM26, particularly whether the additional expense can reduce the need for autoguiding.
In one discussion about astrophotography with long camera lenses, a CEM26EC owner described taking a series of five-minute unguided exposures using a Nikon 800mm f/5.6 lens and a Nikon D810a camera. He reported being pleased with the results, despite photographing from a wooden deck rather than a particularly rigid observing platform. His experience illustrates what can be possible when the mount, optical system, and alignment cooperate, although it should not be taken as a guaranteed unguided exposure length for every CEM26EC.
Another owner discussed using the CEM26EC with an Astro-Physics Stowaway refractor and reported satisfactory two-minute unguided exposures with round stars. He also described plans to use guiding equipment with a different refractor configuration, illustrating that the encoder and an autoguider are complementary tools rather than mutually exclusive approaches.
There are also CEM26EC owners who use PHD2 and other guiding software, with discussions about RA correction settings, guiding calibration, and how to interpret tracking performance. The recurring lesson is that an encoder can greatly reduce periodic mechanical error, but successful astrophotography still depends on the complete system and how it is set up.
For someone considering the CEM26EC, these experiences are valuable because they show both sides of the argument. The encoder provides a real technical advantage, and under suitable conditions it may make unguided imaging practical. It does not remove the need to understand polar alignment, telescope balance, or the other sources of movement that can affect a long exposure.
Under the Night Sky
One of the attractions of a more accurate tracking mount is the opportunity to work with smaller astronomical subjects that benefit from a longer imaging focal length. A wide-field refractor is wonderful for photographing large nebulae, but sometimes you want to move in closer and examine a galaxy's spiral structure or the intricate appearance of a compact planetary nebula.
Spring is a particularly rewarding season for galaxy photography. M51, the Whirlpool Galaxy, presents a pair of interacting galaxies with spiral arms that become progressively more distinct as exposure time, sky quality, and resolution improve. M65 and M66 in Leo provide another opportunity to photograph bright galactic nuclei, dust lanes, and surrounding structure, while M94 in Canes Venatici has a bright central region surrounded by much fainter outer features.
Planetary nebulae offer a different challenge. The Cat's Eye Nebula, NGC 6543, in Draco has a bright, compact inner structure that rewards good seeing and a suitably long focal length. The Eskimo Nebula, NGC 2392, in Gemini and the Blue Snowball, NGC 7662, in Andromeda are other small objects where image scale becomes particularly important.
At the opposite extreme, the Helix Nebula in Aquarius is large enough to favor a shorter focal length and a wider field. That makes it a useful reminder that longer isn't always better. The telescope and camera should be chosen to match the angular size of the subject rather than simply to take advantage of the mount's additional tracking precision.
For especially ambitious imagers, Stephan's Quintet near NGC 7331 in Pegasus provides a demanding target for a suitably equipped telescope under dark skies. The interacting galaxies are faint and relatively small, so the quality of the sky, camera sensitivity, telescope aperture, and total exposure time are every bit as important as the mount.
The CEM26EC doesn't make a small telescope behave like a much larger one. What it can do is reduce an important source of tracking error, giving your optical system a better opportunity to record the detail it is capable of resolving.
Observing Tip: Let the Encoder Do Its Job
When setting up a CEM26EC for guided imaging, take a little time to understand the mount's RA guiding options before changing correction settings in your autoguiding software.
The Go2Nova controller allows you to filter right ascension guiding commands so that the encoder maintains the RA tracking, or allow those commands if you want the guiding software to make RA corrections. These are different ways of operating the same mount, and one may work better than the other depending on your particular imaging system and conditions.
Begin with a careful iPolar alignment, proper telescope balance, and the guiding configuration recommended for your software. After calibration, examine the guiding graph over several minutes rather than judging the system from one or two corrections. Look for consistent drift, oscillation, or repeated corrections that might indicate an alignment or guiding configuration issue.
If declination shows a persistent drift, check polar alignment before making large changes to guiding aggressiveness. If right ascension appears to be chasing small atmospheric fluctuations, review the encoder's RA guiding setting and the exposure cadence being used by the guide camera.
It's also worth remembering that tracking error measured in arcseconds is not the same as image quality measured in pixels. A tracking variation that is practically invisible with a short camera lens may become noticeable at a long focal length and small image scale. Before committing to a long night of exposures, take a few test images and inspect the actual star shapes at full resolution.
FAQ
- What is the difference between the CEM26 and CEM26EC?
- The CEM26EC adds a high-resolution optical encoder on the right ascension axis with Real-Time Periodic Error Correction. Instead of relying on a previously trained periodic error correction curve, the EC version monitors axis position and makes corrections as it tracks. The payload capacity and basic mechanical size remain the same.
- Does the CEM26EC need an autoguider?
- Not for every photographic application. The encoder can make unguided imaging practical under suitable conditions, particularly with shorter focal lengths and careful polar alignment. For demanding long exposures or longer focal lengths, autoguiding can still correct errors outside the encoder's control.
- Does the encoder improve both axes?
- The high-resolution tracking encoder is installed on the right ascension axis. It does not provide the same direct position correction on declination, although the mount supports guiding corrections on that axis.
- Can I use the CEM26EC with a larger telescope?
- The payload rating remains 26 pounds (12 kg), excluding the counterweight. iOptron recommends optical tubes smaller than 6" in diameter. Tube length, balance, wind exposure, and focal length matter in addition to weight, so the encoder should not be viewed as increasing the mount's mechanical carrying capacity.
- Does this package include the hard case and LiteRoc tripod?
- Yes. The C264A3 includes the CEM26EC mount head with iPolar, the 1.75" LiteRoc stainless steel tripod, 10 lb counterweight, and an aluminum hard carrying case for the mount head.
Supplied Accessories
- iOptron CEM26EC center-balanced equatorial mount head with high-resolution RA optical encoder
- Built-in iPolar electronic polar alignment camera
- Aluminum hard carrying case for the mount head
- Go2Nova 8409 computerized hand controller
- 3.8" adjustable Vixen-style dovetail saddle
- 1.75" LiteRoc stainless steel tripod
- Die-cast metal tripod spreader and accessory tray
- 10 lb (4.5 kg) counterweight
- Stainless steel counterweight shaft with extension
- Mount and controller connection cables
- 100–240V AC power adapter for indoor use
Optional Accessories
Off-Axis Guider and Guide Camera: Particularly useful for longer focal length imaging, since an off-axis guider measures star movement through the main telescope rather than a separate guide scope.
iOptron iGuider: A compact external autoguiding option that can be useful with shorter and medium focal length imaging systems.
iOptron External GPS Receiver: Automatically supplies observing location and time information when setting up at different sites.
12V Field Battery: Provides portable power for the mount when an AC outlet is unavailable. Select a battery with enough capacity for the full imaging system if cameras, dew heaters, and computers will share the power supply.
Final Thoughts
The CEM26EC is an interesting telescope mount because iOptron has put a rather sophisticated tracking system into an instrument that remains remarkably easy to carry. Ten pounds is not much weight for a computerized equatorial head capable of supporting a useful astrophotography system, particularly one with a high-resolution encoder correcting right ascension tracking in real time.
The encoder is also the reason to think carefully before choosing the EC version over a standard CEM26. If you primarily observe visually or photograph large nebulae through a short refractor with a well-tuned autoguider, the standard mount may provide everything you need at a substantially lower cost. The EC version makes more sense when the accuracy of the drive itself matters enough to justify the additional expense, whether you're interested in experimenting with unguided imaging or reducing the periodic error your guiding system must handle.
I particularly like the fact that the C264A3 hasn't abandoned portability to achieve that improvement. You still have a 10-pound mount head, a substantial LiteRoc tripod, and a dedicated case for transporting the equipment. The encoder makes the mount more precise, but it doesn't make the system more difficult to live with.
For an astrophotographer who wants to travel to dark skies with a compact refractor or astrograph and make the most of limited clear nights, that's an appealing combination. Good tracking doesn't replace good observing technique, but it gives you one less mechanical limitation to work around when the stars finally come out.
