Celestron CGEM Mount Motor Drives

This is the next installment of my CGEM evaluations, I realize that this topic is not actually about photography in general but it is very pertinent to those who may be evaluating systems for the purposes of astrophotography. I chose to buy the Celestron CGEM after reading numerous evaluations on many websites and was fully aware of its limitations. I thought that I would be able to save some cash and utilize my technical background and attempt to make some improvements by using sweat equity.

Unfortunately I am finding out that there are basic design flaws that will most likely prevent me from taking this mount from consumer grade into the level of precision that might be called laboratory or research grade. Most of what I thought that I might be able to improve upon has already been exploited by bean counter engineering in that nearly every penny that could be squeezed out of this particular design has already been done at the expense of chipping away at the margins of quality. A sad and, unfortunately, common story with many products, quantity and profitability being extracted at the expense of quality.

I found that this also is indeed the case with the DC axis drive motors in the CGEM telescope mount. The potential for more precise control by using DC motors by using feedback control system circuitry is high when compared to the CGEM’s sister Atlas mount from Orion. The Atlas mount uses stepper motors and relies on the assumption that the motor moves exactly as commanded without any feedback. This is fine as long as the motors are never overloaded and starts, stops, and reversals are ramped slow enough that an overload never occurs.

This has lead to a debate about which mount motors design is superior. I side with DC motors and controls, that is when the purpose of choosing this system was one of power and precision rather than one of bean counting. After evaluating this mount and mount design I now feel that the simplicity of the stepper motor design is far superior to the Celestron CGEM design when cost is the only factor being considered. Let me explain, one of the possible downsides to stepper motors is that they move in discrete motions, the myth is that a DC motor should operate with more consistently and their motions are much smoother.

Well I’m about to burst that bubble because what I have seen is that the CGEM DC motors do NOT operate smoothly. In fact they actually oscillate quite considerably in their rotational speed, even when completely unloaded and being supplied with a very stable DC voltage source. Table 1 shows the relationship with the selected motor speeds, with the hand controller, to the actual speeds of the motors and worm drives, there is a 50:1 gear reduction between the motor and worm drive.

Rate Setting Speed (deg/s) Sidereal Multiple Motor RPM Worm Drive RPM
9 5 7000 140
8 2 3000 60
7 1 1500 30
6 64 400 8
5 16 100 2
4 8 50 1
3 4 25 0.5
2 1 6.25 0.125
1 0.5 3.125 0.0625

Table 1

While measuring the motor speeds with an oscilloscope attached to the motor encoders I noticed that the speeds varied quite a bit making it difficult to reliably and accurately detect the encoder frequency. I initially contributed this variation to the worm drives and the interface to the gear reduction system. To isolate the motor and gear reduction system I removed the motor assembly for further testing. To electrically isolate the motor from being controlled by the mounts electronics I used separate regulated power supplies capable of suppling 10 amps, one variable from 0-13V to supply motor power and the other regulated to 5.0V for powering the encoder circuit. The results were enlightening.

Even when supplied from a very stable well regulated DC power source the motor still has a variation in its rotational speeds by as much as 25%. That ain’t chump change. Figure 1 shows the results of a simple voltage input to unloaded motor speed test.

graph Figure 1

 During this test I noticed some peculiar aspects in the qualities of this motor. For one during most of the operational speeds this motor is most likely going to be used are not achievable with a simple DC voltage source. The minimum DC voltage that caused the motor to rotate are the published rates corresponding to 7, 8, and 9 on the hand controller and are given in units of rotational speed in degrees per second. For slower rotational speeds the motor must be actively controlled by using short impulses controlled by the encoder feedback circuits and are given in units of multiples of sidereal time. This mode of control is most likely being accomplished using position control rather than speed control. What this means is that the DC servo motor is being utilized in the SAME functional equivalent mode of a stepper motor! See the following videos for a more in-depth view.

Here is a photo highlighting the encoder wheel, note that it is of the reflecting type:
encoder wheel

Here is a video that uses the FFT function of the oscilloscope to show the frequency domain plot of the square wave of one encoder channel signal. Note that the subharmonics shown are continuous and NOT discretely quantized as is reasoned by most all modern physicists. See how they vary continuously and in opposition to each other. See my paper located on my home page for a more detailed treatment of harmonics and subharmonics.

An additional video highlighting both the speed variation as well as only two of the four encoder transitions that are useable. Again this highlights that whenever or wherever there was an opportunity to cut costs for profit it was taken, and it was taken at the expense of quality.

In conclusion my initial estimation of being able to improve on this design has swirled around the toilet for the last time. In my judgement the stepper motor design is at least, if not more, accurate, reliable, and precise as the compromised DC servo motor drive. If, however, quality and quality control was not considered such a expendable area in which to exploit profits by Celestron the DC servo motor design should have been easily able to outperform the much simpler rudimentary stepper motor design.

Overall I give the Celestron CGEM two thumbs down for its consistent lack of quality and quality controls. (Even if it was not actually manufactured by Celestron) For anyone seriously considering an astrophotography telescope mount look elsewhere! (Losmandy, Astro-Physics Inc, Software Bisque, among others)

7 thoughts on “Celestron CGEM Mount Motor Drives

  1. Duncan Radbourne


    Your research is very interesting , I have a CGEM DX which in their advertising I seem to remember celestron claim to have improved motor control and use motors with strange shaped armatures to reduce gogging etc .
    Did you use this new ( ish) kit in your evaluation ?



    1. ddady Post author


      Thanks for the response. From what I could tell the CGEM DX has only three differences from the standard CGEM:

      1) Tripod with higher load capacity
      2) Larger diameter counter weight bar
      3) Motor control with upgraded power transistors for driving the motor

      While these upgrades do allow for a higher load capacity they would have minimal, if any, effects on the accuracy and precision of the overall design, quality, or manufacturing quality controls.


  2. Heinz

    Hi Dady,

    i also noticed that various speed while slewing and was skipping it to the shitty excentric decoders. However did you also see this variable speed when you were using a lower speed?? I thought this would only affect faster position speeds and not the guide speed.


    1. ddady Post author

      I noticed a very large variability in motor speeds at all hand controller settings. The variability was so large that my oscilloscope had difficulty triggering or even detecting the encoder frequency. This is why I isolated it from the motor controller. The actual encoder wheel appeared to me to be symmetrical and the wheel was rotating true, see photo in post. There could also be some variability in the encoder itself but I’m fairly certain that the majority of the variability is due to the strong magnetic fields with relatively few numbers of poles in the basic servo motor design. Either way the motor controller will see this variability and end up chasing its own tail when trying to regulate the erroneous motor positions or speeds.

      For instance in a good quality HURST BLDC motor, that I do have experience with, there is also some motor (or encoder) frequency variability but not anywhere near the amount present in the very low quality servo motors used in the CGEM mount. There is also a major design trade off compromise by using the same motors and drive systems for the both tracking and slewing. But it seems as if many other manufactures have solved this with varying degrees of success. Imagine expecting the engine on your automobile to be able to use a single gear ratio. How well would your engine run at one-half RPM without a massive flywheel?

  3. Heinz

    ok understand,… for me it was suspect that 256×4 encoder would run the stepping but could be. I thinkits more used to goto,…

    However the more I read your blog the more I am confused because I thought I could improve a lot by diong some tuning, but now I think CGEM will never reach the Losmandy, even when it is a Dinosaurier and much too old from design.

    1. ddady Post author

      Unfortunately that is the same conclusion that I am converging towards. A great visual mount but a barely useable astrophotography mount that should never be advertised as such. At the very least it should be advertised with a disclaimer that it is only suitable for very limited short exposure astrophotography.

      1. Heinz

        lets shout out what it is – shit hehe
        In this century this should not happen anymore.

        Well the chief of skywatcher who hates Celestron, its own company will ne happy to see how they destroy the brand.

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