Focuser Mod

A significant problem with the LX90, and possibly many SCTs is the diameter of the focus knob. The knob on the LX90 is only 22mm (7/8th of an inch) and this diameter does not allow fine control over focusing that is warranted for imaging or careful visual observation. Somehow I had to increase the diameter. I know of at least one commercial focus dial that can be bought to fit the LX90 and I suspect many LX90 users have forked out their hard earned money for this device. The very first version of a home-made device, cleverly constructed by Bill Arnett, was called a Jiffy focuser (named after the American Jif brand peanut butter lid used). You can find Bill's design here.

The principle is simple. You need something that will grip the focus knob and enable a ring of greater diameter to fit around it. The Jiffy mod uses a grommet that stretches over the knob to it is attached a Jiffy lid with a hole of equivalent diameter. My mod is a variant of the Jiffy design which results in a more solid and weighty mechanism.

Components used

A yellow lid from a 235g bottle of Vegemite is the smaller lid you need. This lid allows the washer to be inserted approximately 1/3 of the way in without force. The other larger lid was floating around in our pantry so I cannot say precisely what it's from other than to say it's blue and that it has an outside diameter of 66 mm and a width of 19mm. As far as lids go, it's a very attractive lid with raised lines on the side, 72 as a matter of fact. I recommend going to the supermarket and looking specifically for this lid if you want one that will give you a method of measuring the angle of each turn, for each raised line represents 360 / 72 = 5 deg. Hey that's pretty neat!

	Rubber toilet cistern washer (old-fashioned type) measuring 9mm x 59mm: I found this in the plumbing section of Bunnings Hardware store. This is a solid firm rubber plug with a small central hole
	Two plastic jar lids: You need one that will just fit around the cistern washer and another a little wider that will fit firmly around the first lid.

Method

Of course you may not have a drill bit of exact size - that's OK, use preferably a spade bit of 19mm (3/4 inch) or so and ream the rest of the way with a circular file or sandpaper wrapped around your finger. Keep checking to ensure you stop sanding at the point when the lid fits firmly.

Cut a hole in the rubber of a size that will fit your focuser knob. you need to allow for a little stretching so cut it smaller than you need. It is the stretching that creates a firm fit and locks in place the two lids. The diameter I needed was exactly 18 mm so I used a 19 mm (3/4 inch) spade bit and when finished this measured 18 mm due to slight stretching during the drilling process. It's best to get this hole as close as possible to the correct diameter with a drill - rubber is harder to ream than is timber or plastic. If you don't have the correct sized drill bit, err on the side of the hole being too small, for it can always be stretched a little further to fit over the focus knob.

With the two holes being drilled and correctly resized, firstly fit the yellow plastic lid to the focuser (top of lid first). Next stretch the washer over the focuser knob and keeping it flush with the top of the knob slide the yellow lid back and over the washer as far as it will go without using force. Finally, place the blue plastic lid over the assembly and click into place. if you've got your sizes right, it should lock into place.

Cut a hole in the centre of the smaller lid, slightly smaller than the diameter of the focus knob. It is a fact that the LX90 has been produced with knobs of lightly different diameters, so you will have to measure the diameter of your focus knob and cut a hole a mm or so less in diameter. The hole I needed was exactly 22 mm (~27/32 inch). You want this lid to fit the focuser knob VERY firmly.

Projects - Focuser Mod