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Celestron CGEM Series Telescopes
Celestron CGEM series telescopes were originally announced in October of 2008. Originally these telescopes combine the familiar 8, 9-1/4, and 11 inch Schmidt-Cassegrain optical tubes with a new German Equatorial mount. This series was later augmented with the addition of Celestron's new premium EdgeHD optics that provide an inherently flat field and tighter star images at the edge of field. Both of these characteristics are highly prized by astro-imagers using the new generation of large format CCD cameras.
The Celestron CGEM mount itself offers a 40 pound instrument capacity and permanent Periodic Error Correction, making it Celestron's entry level platform for serious astrophotography. The mount features a standard Losmandy or D-style saddle that will easily accommodate other telescopes. We have used this mount ourselves with a 127mm f/7.5 triplet refractor weighing twenty pounds and found it to be a very enjoyable combination.
Introducing the new CGEM Mount
Celestron has advertised the new CGEM mount as capable of carrying their higher-end SCT optical tubes (up to 11 inches) securely and vibration free, and suitable for both imaging and visual observing. From what we can see, this mount appears an evolution of the Synta EQ-6, which is sold in the US as the Orion Atlas EQ-G. The 40 pound instrument capacity of these two mounts appears to be the same, although that specification has yet to appear on the Celestron website. The evolutionary aspects include the addition of permanent periodic error correction (PEC) and an innovative method of polar alignment.
Permanent Periodic Error Correction
Periodic error is caused by minute imperfections in the worm gear of any German Equatorial mount. When a high or low spot on the worm gear is encountered, the drive momentarily runs either fast or slow. Periodic Error Correction reduces the amplitude of these worm drive errors by automatically playing back a series of corrections that are trained into the drive. The word permanent means that these corrections are retained in non-volatile memory such that the training process need not be repeated every time the mount is powered up. All higher end German Equatorial mounts have this capability, but the Celestron CGEM brings this feature to a new level of affordability.
All Star Polar Alignment
In order for any German Equatorial mount to work properly, its right-ascension axis must be aligned with the Earth's axis of rotation. This means that the right-ascension axis of the mount must be oriented due north and elevated at an angle equal to the latitude of your position on Earth. Getting this alignment within a few degrees is generally good enough for visual observing, but grossly inadequate for photography. Celestron's All Star polar alignment routine is designed to help you achieve polar alignment without the use of a polar axis finder scope. It is important to note that you must first achieve both a rough polar alignment and a computer go-to alignment before this utility can work. This routine can help you in situations where the north star is not visible, but it is not going to help you if you do not already know which direction is true north.
