Focuser Improvements

Improving small telescope focusers

There are some simple improvements that can be made to small telescopes to increase their performance.  This page is about improving the focuser mechanism of small telescopes.  Try each one, starting with the first, to see if your focuser only needs a minor adjustment, or a bit more complicated and involved rebuild (which is not too terribly hard to do either).

Tighten up!

Look for the focuser knob holder plate underneath the focuser of your telescope.  This plate is usually held in place by a few small screws, and there are notches in it which keep the axle of the focuser in place.  It also allows for variable tension on the axle, which adjusts the tension of the entire focuser mechanism.  So the first thing to do to improve focuser performance is ensure that the screws on this plate are tightened with equal pressure, giving the focuser knobs as smooth a feel as possible.

A sticky situation

However, that may not make the focuser work any better.  Ask any amateur astronomer who has had an older telescope (sometimes even newer ones) and many are aware of the "clear sticky goo" that many low-cost telescope manufacturers call "grease" which is placed on the focuser rack gear.  This is stuff is sticky, and usually leads to focusers that are either A) hard to move or B) don't stay where they should because this "grease" seems to have a memory which makes the focuser move back slightly after focusing.

The solution?  GET RID OF THE STUFF!

Older focusers were made of metal, usually chromed brass, and can withstand most any solvent like kerosene, denatured alcohol, acetone, etc.  Many inexpensive telescopes made from the 1980's through to today have plastic focusers.  Better to use a plastic-safe solvent - try testing "Goo-Gone" or "Goof-Off"  before using them on a large surface.

So instead of tightening up the focuser plate, this time remove it and place the plate and screws in a small bowl to keep them safe.  Now slide the focuser out of the focuser housing (obviously remove any eyepieces or protective caps).  Using an old toothbrush or other small brush, use a solvent to remove all of the old grease from both the focuser rack (the flat gear on the bottom of the focuser tube), as well as the pinion gear (the small round gear on the focuser knob axle).  

Let  them dry.  

Using a small amount of automotive or white lithium grease, dab a small amount along the length of the rack, and a tiny bit between the teeth of the pinion gear.  Then replace the focuser tube in the housing, the focuser knobs/axle, then the focuser plate and screws.  Retighten them.

How does it work?  If it moves more smoothly and doesn't flop around in the housing, congratulations!  You'll find it much easier to focus the telescope now.

Rebuild it

If the focuser is loose, it will change the collimation (ability of light to focus properly) of the telescope when different eyepieces are used.  Heavier eyepieces may cause the tube to sag, which - given the increasing amounts of glass and metal used in better quality short focal length eyepieces -results in poor images at the eyepiece.

To test if the focuser is loose, first rack the focuser all the way in.  Grab the end where an eyepiece would be inserted, and see if the focuser wiggles.  Now rack the focuser all the way out, and do that again.  How much "play" is there in the focuser?  If there is a lot, there may be missing or worn pieces inside the focuser housing.  Here is how to fix that.

Look inside

Take a look at the inner diameter of the focuser housing.  In older telescopes, there may be felt or a cardboard-like material.  In newer focusers, it is likely a plastic-looking material.  Or... there may be nothing there at all!  If there's nothing, it is possible there are some parts missing.   What most focusers need is a surface on which they can slide, so when the tension is adjusted on that focuser plate, the focuser tube slides along that surface.  If that surface is gone, worn down, or just not very good, it may need replacing.  How?  Read on!

Get the slippery stuff

PTFE is a material most people know as Teflon.  No, not the thin white plumbing tape - this is actually thicker and has adhesive on one side.  Look for "skived" PTFE tape, preferably in a 3 mil thickness.  In the United States, it is available through McMaster.com, here.  It really shouldn't make too much difference whether it is acrylic or silicone adhesive, but personally I prefer acrylic.  In any case, just get a 1/2" wide roll, of 5 yards length, which will be WAY more than you need, but it isn't terribly expensive, and comes in surprisingly handy for other projects (like Dobsonian bearing surfaces).

Remove the current plastic or felt surfaces, or if none exist, start to layer the tape in two spots 120 degrees from where the focuser rack goes.  The pressure of the focuser will then be equal on the focuser tube that way.  Layer up the tape so that when the focuser tube is slid into the focuser housing, it is centered within it.  Then replace the focuser plate and axle/knobs, set the tension and "Voila!"  The motion should be buttery-smooth.

That should provide a much better-functioning focuser, that slides like it should.