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Skywatcher manual Focus Knob Mod.

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I own a Skywatcher ST120 that has the standard focuser as supplied with the scope. I am quite happy with the focuser in many respects, but I have found that sometimes a very delicate touch is needed to obtain precise focus, so I have been trying to think of ways to give me just a little bit more control. I have found references to other people using clothes pegs or screwing a plastic jar lid to the focus knob. I decided I wanted the simplicity of the clothes peg with the control of the plastic lid. Here’s what I came up with:


After spending an hour rummaging in my garage I eventually found the materials I needed.


1.      An old tube of silicon (the type you but in dispenser/gun

2.     An old punctured cycle inner tube.


The barrel of the silicon tube is precisely the right diameter to be a push fit over the existing focus knob. A piece of the tube was cut off of the desired width.

several pieces of the inner tube were cut off, of a similar width.

Cycle  inner tube is surprisingly strong, firm and stretchy. The pieces were then stretched and placed, one after another, forming layers, onto the plastic ring of Silicon tube until the desired thickness was built up. The final piece of inner tube was cut slightly wider than the rest so that once applied, the overhanging edges automatically covered the edges of the pieces beneath giving a slightly better appearance.


The resulting item pushes onto the existing Knob firm enough to grip and give control, but not so tight as to cause damage or be hard to remove. The advantages to this is that it is easily removed from one side and placed on the other depending on the orientation of the scope. The inner tube rubber, even in several layers, is firm to the touch and yet provides a good grip.

The original knob had a diameter of 45mm. The push on collar has a diameter of 60mm which can easily be increased by adding more layers of cycle inner tube if required.


Cost = Nothing. All materials were waste and would have been thrown away (I do like recycling) and no adhesives or fillers required.

I have just tried it on a daytime target and it does give me that slightly better control. 







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