Hello Everyone, my name is Jason. This is my first time posting on CN and admittedly I am also new to backyard astronomy / astrophotography. Being a double newbie sure begs the question, am I qualified to make a bold statement such as "poorly designed focus motor mounting plate"?
Although this wonderful hobby is new to me, I have over 20 years of Software Engineering experience, half of that with companies that manufacture in-house devices. I have enough of a mechanical aptitude to have rebuilt cars, engines and just about anything I can get away with taking apart. That said, my newness to this industry has me turning to this forum to see if anyone else has noticed what I have noticed. I think these design flaws might be causing some of the issues I have seen others speak about in other online places.
I bought a new 9 1/4 EdgeHD along with plenty accessories including the Celestron focus motor. I figured what good is it to remotely control a telescope if you cannot remotely control the focus?
Considering the scope and add-on focus motor are both manufactured by Celestron, I didn't expect to find the issues I am seeing. Although I overlaid text descriptions of the issues on the included images, I will describe the problem in text so the search engines can index the problem. I only have experience with the 9.25, I am willing to bet the same problem might apply to smaller sized scopes that use the same focus motor mounting plate.
The stock "focus knob mechanism" for the EdgeHD 9.25, uses a rubber knob slid onto a brass tube that has a bearing connected to it. This lets the end user direct the angular momentum into the tubes mechanical focusing mechanism. Except for a round gasket separating the two, the bearing sits directly on the metal of the rear of the OTA, however the bearing sticks up past the mounting location for the plate by approximately 1 mm. IMO, this is purely by design. The plate that covers the bearing (on the stock part) has a recessed area machined out of it that the bearing sits inside of. This gives a mechanical engineer an easy way to control the amount of pressure on the bearing. Less recession, more pressure, more recessing less pressure.
The Issue with the Focus Motor Kit is the fact that the motor mounting plate that replaces the 9.25 scopes and under, does not have this recessed area machined out of it. This causes 100% of that mounting plate to be contacted on the bearing and 0% contacted on the rear of the OTA except via the three original screws. Not only does it make the screws appear too short to some people, but you cannot properly tighten down the motor mounting plate without over-torqueing the bearing of the focuser's brass/bearing mechanism. This also places all of the torque from that electric motor on those three screws and the bearing alone.
Unless they properly machine a corrected mounting plate, Celestron's focus motors are likely to damage some people's OTA's. They might wobble out the threads for those three screws, a screw might come lose and cause other damage IMO.
As all companies hope to accomplish re-use, sometimes the desire to make "one size fits all" components can sacrifice overall quality. The Celestron focus motor kit was design to fit a larger diameter focus knob tube than found on the 9.25 inch scopes and under. For the smaller scopes, you have to use an insert, or "adapter sleeve" as it is called in the manual. The sleeve has a slit cut that provides a way for the setscrew to clamp down upon the smaller diameter focus knob mechanism.
The Issue from what I can tell is that as you clamp down that sleeve with the set screw, the round shape starts to become oblong. This less than a perfect circle shape causes the focus motor to rotate the focus mechanism's shaft in a non-perfect circular motion. (If you hold a flashlight into that hole and put magnification over your eyes you might be able to see what I have been looking at. Even when you have mounted the entire mechanism as intended and directed, you can see the end of that brass tube's center screw rotate out of axis.
To confirm this wasn't the motor itself, I removed the unit, re-sunk the set screw (to prevent damage) and ran the motor off the scope. It turns perfectly circular and makes a lot less noise at that.
When I first started writing this thread, I had hoped that just maybe they failed to fully machine the mounting plate that I received. If so, this would take care of part of the problem. However, as I re-read problems other people demonstrated on YouTube and Amazon, I have a feeling that this is purely a design flaw.
As badly as I want that focus motor to work, I removed it from my telescope. I just don't see how they overlooked this.