The Skywatcher Evoguide 50ED is a very capable imaging scope once paired with a field flattener. However, its focuser really lets it down. There’s a lot of backlash. You have to turn the focus ring at least 90 degrees when swapping from focus in to focus out or vice-versa. This is irritating to use manually and a potential show stopper for any auto-focus system. The focuser is just a mechanical device, so I thought I’d take it apart, see how it works, and try to improve it.

The focuser itself consists of 3 parts. One part is attached to the scope and contains the focus threads and the linear slide mechanism

The second part is the actual focus ring. When focusing inwards, the lip inside the focus ring rides on the tip of the focus tube. Strangely, it doesn’t ride on the convenient bearing surface that appears to have been cut behind it

The third part is the outer flange. This screws into the focuser and together they provide a small channel in which the lip on the focus ring rides. If you look carefully, you can see a shoulder on the flange just below the threads. The height of this shoulder determines the size of the channel in which the lip will travel.

The backlash problem is caused entirely by the difference in dimensions between the thickness of this lip and the channel in which it rides. So to improve backlash we have to tighten those tolerances. There’s several ways to do this. We could pack out the focus ring from behind or we can pack it out from the front. I chose the latter solution and I’ve managed to almost eliminate the backlash whilst still being easy to turn. To do this, I used a 0.1mm M37 shim. Unfortunately, the inner dimensions of an M37 shim don’t quite fit over the shoulder on the outer flange so I had to very carefully trim the inner dimension. This is obviously not an off-the-shelf solution but does prove that an improvement can be made in this area.