iOptron CEM70 – Center Balanced Equatorial Mount: By Guest Host: Mario Motta

I was considering making a take-apart mount but finally realized I could not build one light enough with all the features I desire, so, I purchased an iOptron CEM70G mount. (guilt for an amateur telescope maker!)” Mario Motta

My story:

Up to this point I have always built my own equipment, such as my 32-inch f/6 reflector telescope in Gloucester, Massachusett, which is my main telescope for imaging, and in a dome attached to my house.

At the end of this year I will be retiring, and per my wife’s wishes will be spending winter months in Naples Florida at our second home. However, in my Florida location I can’t build a dome for a number of reasons. This is due to (hurricanes, building restrictions, etc.)

I was considering making a take apart mount but finally realized I could not build one light enough with all the features I desire, so, I purchased an iOptron CEM70G mount. (guilt for an amateur telescope maker!)

The head weighs only 30 pounds , tripod another 30 pounds, for a manageable weight, yet, tracks very well (3 arc sec error periodic error), is very sturdy, can carry a 70 pound weight load, so it can handle up to a 14-inch scope easily. It has a built-in polar scope alignment guide scope. It even has WiFi, and 4 USB ports. 

Why the center mount instead of a german equatorial or a fork? At 43º North latitude (my 32-inch sits in a handmade fork) or german equatorial which works fine.

Look at the following images as following, “German equatorial”and see that at 43º, the center point of gravity pushes through the main mount, and weight of scope and dec axles pushes down the polar axle, a fork also works the same way.

However…at 26º N latitude, the weight of the scope and counter weight is very far forward, putting all the stress on the forward polar bearing.

Not very stable: A fork overhangs badly.

Now let’s see what a center mount does:

At 43º North latitude, it works well, but all the thrust is on the rear bearing and a german equatorial may be best. Now see what it looks like at 26º N latitude. (see image center mount) Here the weight of the scope is directly over the center of the polar axis, the weight is evenly distributed on both bearings, thus can handle a heavier load with less stress, overall an ingenious design. In reality this is nothing new, what this is… is a miniature English “cross axle mount”.

I built one for a 16-inch scope in the 1980’s and it worked very well.  See the following photo following the M42 image:

In summary, if far north, fork or german equatorials are best, but if closer to the equator, a center mount or cross axle is best.

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