Balance is a matter of leverage. A counterweight doesn't simply offset the weight of your telescope; it offsets the turning force, or torque, produced by the telescope's weight acting at a distance from the mount's right ascension axis. Move the counterweight farther from that axis and the same amount of weight produces more balancing torque.
That simple principle is the reason for iOptron's #8457 counterweight shaft extension. If you've added a heavier camera, filter wheel, guide scope, or other accessories and your existing counterweight has reached the end of its shaft before the telescope is balanced, the extension gives you another 4.7" (120mm) of usable length. Moving the weight farther out may provide the additional leverage you need without having to purchase and carry another counterweight.
The extension is designed for the iOptron CEM26, HEM27, and GEM28 mounts. It has the same 0.79" (20mm) diameter as the original counterweight shaft and attaches to its threaded end using an M16 × 2mm connection. The extension measures 4.7" (120mm) in usable length, or approximately 5.3" (135mm) including its attachment thread. Compatible counterweights slide along the extended shaft just as they do along the original.
More Leverage Without More Counterweight
Suppose you've been using a small refractor with a lightweight camera and then decide to add a larger cooled camera, filter wheel, and guiding equipment. The additional weight on the telescope side of the mount increases the torque that your counterweight must balance. Normally, you would slide the counterweight farther down its shaft until the two sides are in equilibrium.
But what happens when the counterweight reaches the end of the shaft and the telescope side is still heavier?
You can add another counterweight, increasing the total mass on the counterweight side, or you can extend the shaft so the weight you already have can move farther from the right ascension axis. The #8457 provides that additional reach. For portable observers who already carry a telescope, mount, tripod, camera equipment, and a collection of accessories, avoiding another counterweight may be particularly attractive.
There is a mechanical tradeoff, however. Moving a weight farther from the axis increases its rotational inertia, meaning the mount has more resistance to changes in rotational speed. When the choice is available, using sufficient counterweight closer to the mount is generally preferable to positioning a lighter weight unnecessarily far down the shaft. The extension is most useful when you genuinely need the additional reach or want to avoid carrying more weight than necessary.
It is also important to distinguish balance from carrying capacity. An extension does not increase the manufacturer's payload rating or make an oversized telescope suitable for a smaller mount. It simply changes the position at which the counterweight can act.
Check Your Mount Before Ordering
Many CEM26 and GEM28 packages already include a 4.7" (120mm) counterweight shaft extension as standard equipment. If your existing shaft has a removable section at its outer end, you may already own the #8457 or its equivalent.
This makes the extension particularly useful as a replacement for a missing or damaged original, or for a compatible mount that did not include one. Check your existing counterweight shaft and the accessories supplied with your mount before ordering.
The #8457 is intended for the CEM26, HEM27, and GEM28. Larger iOptron mounts may use different shaft diameters and threaded connections, so this extension should not be selected solely because it resembles the one already installed on another iOptron mount.
Observing Tip: Balance Before You Start Tracking
Whenever you change the weight of your imaging equipment, check the telescope's balance before beginning your observing session. A heavier camera, additional guide scope, or even a substantially different eyepiece can change the balance enough to require an adjustment.
With the mount powered off and the telescope securely supported, install the counterweight shaft extension and ensure it is fully threaded into place. Install the counterweight with its retaining clamp secured, and make sure an appropriate safety stop is fitted at the outer end of the shaft. Never remove the safety stop or allow a counterweight to rest against an unsecured shaft end.
Follow the mount manufacturer's balancing procedure, first establishing proper declination balance and then adjusting the counterweight position for right ascension balance. The telescope and counterweight should remain controllable when the appropriate axis is disengaged, without either side swinging unexpectedly downward.
Once you've found the correct position, a small piece of removable tape on the shaft can make it easier to return the counterweight to approximately the same location the next time you assemble the equipment. Check the balance again before engaging the drives, especially if anything in your telescope arrangement has changed.
FAQ
- Which mounts use the #8457 extension?
- The iOptron CEM26, HEM27, and GEM28 mounts, including compatible variants using the same counterweight shaft system.
- Does my CEM26 or GEM28 already include an extension?
- Many current packages include a 120mm extension as standard equipment. Check your existing counterweight shaft before purchasing an additional one.
- Can I use my existing counterweights?
- Yes, provided they are designed for the same 20mm counterweight shaft diameter. The extension matches the diameter of the original shaft.
- Is extending the shaft better than adding another counterweight?
- It depends on your equipment. Moving an existing weight farther out increases its balancing torque without adding mass, but also increases rotational inertia. Additional weight positioned closer to the mount may be preferable for some setups.
Technical Specifications
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Manufacturer: iOptron
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Part Number: 8457
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Compatibility: CEM26, HEM27, GEM28
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Diameter: 0.79" (20mm)
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Usable Extension Length: 4.7" (120mm)
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Overall Length Including Thread: 5.3" (135mm)
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Attachment Thread: M16 × 2mm
What's Included
- One iOptron #8457 counterweight shaft extension bar
Mount, counterweights, and original counterweight shaft are not included. Check that you have the appropriate safety-stop hardware for your completed shaft arrangement.
Final Thoughts
There's nothing particularly complicated about the #8457. It's a short piece of metal with a threaded connection, but it solves a problem that can become frustrating when you're trying to balance a telescope with a full complement of imaging accessories.
If the counterweight you already own is almost sufficient but has run out of room on the shaft, another 4.7" of leverage may be exactly what you need. And if your CEM26 or GEM28 originally included this extension and it has gone missing, the #8457 provides a straightforward replacement.
Sometimes a small mechanical accessory is a better answer than adding more weight or changing equipment. The important thing is knowing whether you actually need it.
