My posts concerning the CGEM telescope mount, curiously, continues to be very popular even though many of them are several years old. By back checking to the referring sites I can read the forum posts which links to my blog pages. I find it very interesting that there seems to be several persons vehemently disagreeing with my conclusions that the CGEM is not an acceptable mount for anything but introductory amateur astrophotography or strictly for use with wide angle lenses.
The interesting thing is when I travel to other posts giving advice (from the very same posters) to any would-be astrophotographer they nearly always advise against the CGEM and similar class of mounts. Instead they nearly always recommend much more expensive AP and similar class mounts for any serious astrophotography. The very same conclusions that I arrived at. Actually I also recommended a much cheaper route which is buying time on remote time-sharing systems which tend to use laboratory grade equipment that can be used for scientific purposes other than strictly making pretty graphic arts pictures.
There are several issues brought up with my criticisms. First, they seem to confuse non-orthogonality between the optical tube and the mount axis, called cone error, with non-orthogonality between the axis. Very different animals, cone error can be corrected, inter-axis orthogonality can only be properly corrected by line-boring in a machine shop.
Second, they seem to think that I was doing my testing with an overloaded mount. Wrong! My 9.25 optical tube was on the ground. When I did my testing it was loaded ONLY with the Orion auto guiding scope which weighed less than ten pounds.
Third, my criticism of comparing the mode of operation of the feedback control DC motors to that of stepper motors. Yes, many servo motors, DC and otherwise, use pulse width modulation to produce an effective DC voltage less than the power supply in order to control motor speed. These pulses are very often several hundreds to several thousands of pulses per second, outside the vibrational frequencies of the mount and scope. My point is that you CANNOT use speed control below the effective voltages in which the CGEM servos operates during guiding. Instead the motors are subjected to strong pulses every second or two, depending on the exposure time of the guide camera. These pulses cause detectable vibrations that reduce resolution, PERIOD!
Hence the usability of imaging unguided with PEC corrections during longer exposure times is a VERY GOOD INDICATOR OF MOUNT QUALITY, or in the case of the CGEM the indicator of the LACK OF QUALITY. The CGEM is NOT PEC CORRECTABLE, Celestron is NOT working on the so-called 8/3 error, despite the continued misinformation supplied by such web fanboy forums.
Fourth, the resolution of any lens, or antennas used in radio telescopes, is dependent on the aperture diameter and the frequencies being observed. So when a 2.5 inch camera lens produces higher resolution than a 9.25 inch diameter optical tube that is supposedly optimized for astrophotography – that is a VERY BIG DEAL!
I could go on and on but the facts remain the same, arguing with fanboys only leaves you as frustrated as trying to use the CGEM mount for astrophotography. Like I said many times before the CGEM is a superlative mount for visual use, but for astrophotography you must lower your expectations or look elsewhere, and that just about says it all.