Simple DIY alternative to motorised focusers for visual observers
The very effective flexible slow motion controls for my mount inspired me to think about applying these principles to eliminating some manual focusing vibrations at high magnifications. No matter how careful, even when using the 7:1 reduction on a buttery smooth focuser, I was finding it impossible to avoid some momentary vibrations that made it a slow process to find the exact focus point at higher magnifications. I was looking for a non-powered solution to improve my visual observing experience.
So, my solution is a flexible slow motion control for the focuser knob. It consists of a simple clasp that slips over the fine focus knob, a section of marine grade shock cord and a left over lightweight plastic knob. The clasp is fashioned out of left over PVC pipe, some wood and sections of plastic tubing. A rubber band gives the clasp sufficient holding power to stay on the focusing knob during the observing session whilst still allowing the clasp to be easily slipped on and off the focuser knob. It took some experimentation with different sizes and lengths of shock cord to get a balance between flexibility to avoid transmitting vibrations and sufficient torque to enable to smooth turning of the focuser knob. 8 mm cord seems to be the best fit.
The flexible slow motion control has completely eliminated all vibration when focusing, even at the highest magnifications. Having a completely still object just makes it so much easier to find proper focus. It is easy to slide on and off and sits quite comfortably on the telescope all night.
However, I would expect that the knob like this might behave as swinging pendulum, affecting the positioning of scope - of course this will depend on the overall rigidity of the mount and scope on it.
The overall weight is only 21g so it cannot generate enough momentum to act as a troublesome pendulum. The flexible shock cord also acts to dampen out swinging motion almost immediately.
Of course it is another item to accidentally knock and generate some temporary vibration. This generates less vibration than a similar bump to the telescope as again the flexible shock cord has dampened some of the effects of the impact.
Over time I have experimented further with varying the length of the shock cord and think I have now found the optimum length of 110mm. That is about 60% of the length of the original version in the photos. At this length there is no transmission of hand movements/vibrations and it will lift vertically the weight of a binoviewer and two eyepieces without twisting on itself. I did not have a binoviewer when I first made it.
It works so well that it has become a permanent accessory for the telescope. I don't use the telescope without it.
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