Rethinking the Equatorial Mount: iOptron’s CEM40


iOptron CEM40 center-balanced equatorial mount. Credit: iOptron

iOptron’s center-balanced equatorial mounts (CEM) have a quirky-looking design and boast impressive specifications and payload capacity. Does the CEM40 deliver on its promise?

Pros: Accurate tracking, very stable, portable.

Cons: Needs a computer for polar alignment.

The iOptron CEM40 is unlike a traditional German equatorial in that the declination axis is not collinear with the counterweight arm, with the axis and arm instead offset to opposite ends of the right ascension (RA) axis.

The effect of the offset is, even at low latitudes, the center of mass of the telescope and the counterweight are both over the center of the tripod or pier. The result is the CEM40 can carry a payload greater than German equatorials of a similar weight. Its specified payload is 40 lbs (18 kg).

If you are new to iOptron mounts, there are a few things you will need to get used to. First, the RA and declination axes are free-running when they are unlocked. This makes it easy to attain perfect balance, but it also means that you must support the weight of the telescope whenever you unlock an axis; there is no gradual declutching. You must also unlock the RA axis when the mount is stored, so that any shocks in storage or transport will not damage the gears, and remember to lock it again when you remove it from its case.

The CEM40 is compact for portability. Credit: iOptron

You also need to keep in mind iOptron’s “zero” position, which is distinct from the “park” position. In the zero position, the telescope points to the celestial pole and is exactly vertically above the RA axis, and setting it is an essential part of the initial alignment setup. There is a handset option to seek the zero position, that is quite accurate, but many users set the unloaded mount to the zero position with a spirit level and make alignment marks with adhesive tape to have a visual reference.

Polar alignment is simple with the included iOptron iPolar electronic polarscope. After initializing the software (very well described in the documentation), the software will plate-solve what the polarscope sees, then place a cross on the image at the location of the pole and a dot where the RA axis is pointing. You merely adjust altitude and azimuth until the dot and cross coincide, then secure the mount to its pier or tripod.

Once you have set zero position and attained polar alignment, you are nearly ready to go. A GPS dongle sets your time and location in the iOptron Go2Nova®8407+ handset. A one-star alignment will compensate for any error in the zero position, and a full three-star alignment will also compensate for any cone error. If I take care with my initial setup, I usually find that a one-star alignment is sufficient to get the target onto my CCD chip. Tracking is excellent; unguided three-minute exposures at 714mm focal length show no sign of trailing. Periodic error is specified to be a maximum of 7 arcseconds, and the mount has permanent periodic error correction so you can compensate for this.

If you wish to guide, there is a standard ST4 guider port in a block on the dovetail saddle. The block also houses a USB 2.0 port and a 5.5mm 12V power socket. You can thread cables through the mount and this ability, combined with the on-board sockets and ports, makes for very tidy cable-management. If you wish to run the CEM40 remotely, the RA drive housing makes a perfect platform for a Raspberry Pi computer.

I have the basic version of the mount. There is also a CEM40-EC option that has an encoder in the RA axis and a specified periodic error of <0.25 arcseconds, and an iGuider option that has a built-in guidescope in the dovetail saddle. For me, it’s a keeper.

MSRP: $1998


About Stephen Tonkin

I first tried to use binoculars for astronomy in 1957, when my father took me outside to see if we could spot Sputnik. I was hooked! In 2011, I started The Binocular Sky website, to promote this aspect of astronomy. This led to me being invited to write a monthly Binocular Tour for BBC Sky at Night Magazine, for which I also write equipment reviews and articles on practical astronomy. I also teach astronomy courses, am a STEM ambassador, and do practical astronomy outreach with people of all ages. I am a speaker on the UK astronomy society circuit.

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