Very simple focuser hack for SW 150i

[Zermelo]

The focuser on my SkyWatcher 150i is a basic rack-and-pinion, unsurprising for the price point, but sometimes a bit of a pain to control finely enough. I’m not looking to spend any serious money upgrading it, but I did want to see what I could tweak.

The first thing I did was to slacken off (slightly) the screws holding the plate against the spindle, as the operation was very tight when new – that helped a bit (and I think that without doing this first, the “friction fit” approach described below wouldn’t have worked). I will eventually get around to taking it all off as per AstroBaby's tune-up.

Improving the fine control without a major change means doing something with the focusing knobs – they’re quite small, so the effective “gearing ratio” when you operate them is on the harsh side. Some folk have described fitting larger diameter replacements, either bought or made, and even using ones with a planetary-style mechanism to achieve a reduction in the ratio. I didn’t fancy this, as I couldn’t see how the existing knobs were attached to the spindle without trying to prise them apart (possibly terminally). The other option is to increase the effective diameter of the existing knobs, for which purpose a clothes peg is apparently quite popular, but I’ve also come across descriptions of chop sticks inserted into holes drilled at intervals into the circumference, and punctured lids from peanut butter jars.

I wanted something that was cheap, relatively tidy and non-destructive. The answer seemed to be some sort of thick sleeve that I could fit over the knob. It would need to be a tight fit so as not to slip in use, to be not so large as to foul against either the focuser tube or the main OTA, and to be thick enough that it didn’t flex sideways when grasped. I thought I might find some larger rubber washers that would do the job, but none were thick enough to be rigid in use. However, a bit of searching found these spacers that are apparently used in vehicle shock absorbers.

My calipers said the diameter of the focuser knobs was around 29.5mm, and the nearest spacers that were available had an internal hole 30mm and outside diameter 60mm. I ordered one that was 10mm thick, not quite as deep as the knobs, but which allowed a bit more space on the inside edge for free operation. I’d hoped the internal hole might be a but undersized when it arrived but it was spot on, so I wound five or six turns of masking tape around the knob first. To avoid taking the tape off when fitting the spacer, I positioned one side first and stretched it across the face as I pushed. When it’s flush with the knob’s outer face, it’s just clear of the focuser body and OTA. There might be enough room to stick some kind of friction surface around the outside to improve the grip, but I don’t think it’s going to be necessary.

I decided to do only the one knob, so I now have a very Noddy “dual speed” affair.  Because the clearances around the fitted spacer are quite tight, it’s worth checking the positioning of the spindle in the focuser body first – mine was fractionally off centre, so there was more room one side than the other (assuming you have no preference).