CGEM Final Thoughts

I’ve been agonizing over the Celestron CGEM mount now for several months. I’ve done extensive testing using widely accepted professional modeling software as well as relying on my own technical skills and tools, such as precision bubble levels and laser pointing measures, to measure the CGEM’s suitability for astrophotography.

My final conclusions is that the CGEM is NOT suitable for any serious astrophotography. It may, however, be acceptable to those on a budget and are fully willing to accept major compromises and serious degradations in the obtainable images without extensive DESTRUCTIVE post processing.

My conclusion is that the apparent design specifications with the orthogonality of the four axis of the Celestron CGEM telescope mount seems to be firmly and uniformly limited to about one-quarter degree tolerance in their manufacturing process. These tolerances can be, and indeed are, cumulative and can add up to serious misalignments of the Right Ascension axis due to the two axis being askew.

I’ve been trying to find a good and reliable way to align the RA axis with the Earth’s axis and found that even when using the drift alignment method the polar alignment is not achievable to within less than one-minute of one-degree. This is because there is no real accessible way to level the mount in the beginning of the procedure. And it becomes, at best, long drawn out iterative Newtonian method that would require hours to align each time the mount or scope was moved.

I reported earlier that the bubble level was off by a one-half of a bubble, this was in relation to the base of the azimuth axis. I also found that the altitude axis is also non-perpendicular to the azimuth axis. Further more the altitude axis is also non-perpendicular to the right ascension axis and finally, let me gather my breath,  the declination axis is non-perpendicular to the right ascension axis.

All four of these non-perpendicularities or non-orthogonalities interact with each other such that it makes it nearly impossible to obtain an accurate polar alignment with a portable mount. If one worked at it long enough, and knew exactly what they were doing, a permanent mount might be able to be tweaked such that the only pertinent non-perpendicularity that degraded the performance was the RA-DEC non-orthogonality.

It should be noted that while the OTA-DEC non-perpendicularity is constant and can be easily accommodated the RA-DEC non-perpendcularity is NOT the same, and results in the DEC axis carving out a Lissajous ( figure-eight) pattern. The CGEM hand controller apparently accommodates only the former error. Also if the altitude axis is non-orthagonal to the RA axis the altitude adjustments will cause the two axis being skewed and thus preventing precise polar alignment with the polar drift method. The severity of this skewness depends on your latitude.

Since there are NO surfaces or reference points that can be reliably used in order to obtain or adjust the mount accurately level or true on the Celestron CGEM telescope mount; this effectively makes it absolutely worthless for the precision necessary for the purposes of any serious astrophotography.

Here is a photo of the precision bubble level used for verifying the non-perpendicularities, an automotive caster/camber wheel alignment tool. The accuracy, by single interpolation, is better than 1/8 degree.

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Again I will iterate; the CGEM can be a terrific mount for VISUAL use but is very deficient for the purposes of astrophotography.