hfhenrichs Shouldn't this be prohibited by brakes?
I think I remember reading that only the hour angle motor of your mount has a brake, Huib. You can reduce the likelihood of an accident by just moderately balancing the load on the declination axis. You still won't have to worry about "third axis" imbalance, since that is usually too small to cause a run-away problem. Just enough balance so to overcome the gear ratio.
If you (and other new strain wave gear owners) are not familiar with those gears, allow me to explain the need for brakes.
(The particular gearing is originally trademarked as Harmonic Drive™ motors, whose patent has run out, but Trademark has not. Strictly speaking, the gear used in the AM5 is a strain wave gear, but not a Harmonic Drive™. I tend to use the two terms interchangeably, but they are strictly not; just that my mount actually use a gear from the Harmonic Drive company.)
The gears have very high ratios (in mounts, typically 100:1), and are usually very hard to turn from the output shaft end. However, a very heavy load can start the mount moving when power is not applied to the servo or stepper motor to counter the movement.
The rub comes in because once the mount starts moving, it will also rotate the armature of the motor, which then acts like a flywheel, through a 100:1 reduction. It is as if the small flywheel has 100x larger mass -- that makes it a very large equivalent flywheel. That flywheel effect makes it very hard to stop once it starts moving. Unlike a legacy German mount, that momentum will move it past its equilibrium point.
The usual trick is to place a hand as far away from the motor's axes as possible (e.g., at the end of the OTA) to achieve a larger moment arm to stop the movement.
My mount does not have a brake in either axis, and since I do get more than average power outages, I had built a 15 volt DC uninterruptible power supply (UPS) using a SLA battery and an Epic PowerGate module.
If you are afraid that the lack of a brake on the declination axis of your mount can cause you problems if there is a power cut somewhere, you can try to build a UPS and wire it as close as possible to the mount and use strain relieves on that so that the power plug never detaches (I use copious amount of Gaffer tap, ha ha; my instrumentation is never pretty since I aim for function instead of form).
In general, never mount or dismount a heavy payload without a stain wave mount in a fully powered state. Remove the payload before removing the power. Apply power before mounting a payload.
That being said, I need a hefty payload to back drive my RST-135. I don't know about your mount. In my case, a fully complemented FSQ-85 (flattener, large filter wheel, large camera, electronic focuser, etc) can back drive the mount, especially since I have the OTA on a dual saddle plate. Anything under 5 kg is not able backdrive it, no matter how imbalanced within the limits of my dovetail clamps.
Chen
