This plastic focusser looks like most others...
...but with a twist. At the end of the draw-tube, there's a cell containing a cemented doublet-lens...
- doublet 'corrector'3.jpg (24.52 KiB) Viewed 4609 times
That's what extends the focal-length of the telescope, from an
f/4 to an
f/8; a barlow if you will. It also, arguably, corrects errors associated with the spherical primary-mirror. More on that later, and during the collimation phase.
Right; let's now concentrate on how to enable the draw-tube to rack in and out, straight and true, along its entire length, with no binding nor slop whatsoever. You want the precision of the collimation achieved to enable sharp, pleasing images at the low, medium and high powers; all the way up to a theoretical 250x, and beyond even, particularly when observing the Moon.
You want to center the draw-tube nigh perfectly within the body of the focusser. That is accomplished by a triad of bearings...
- drawtube integration2c.jpg (15.09 KiB) Viewed 4609 times
The bearings run the entire length on the inside of the focusser's housing...
- focusser7e.jpg (16.22 KiB) Viewed 4609 times
...top to bottom.
There are several solutions for DIY bearings for a draw-tube. The best solution, in my not-so-humble opinion, is the use of super-slippery PTFE(Teflon) sheets, cut into the three narrow strips required...
- PTFE2b.jpg (9.5 KiB) Viewed 4609 times
I got my own sheets from this vendor...
...and economically.
For draw-tube bearings, I generally use the .015" and .020" in thickness, along with two other items...
I also use scissors, single-edge blades, a #11 X-Acto knife, and 91% rubbing-alcohol to clean and de-grease the surfaces to be joined(very important).
You cannot glue PTFE to anything, or to itself, unless you first etch the surface of the PTFE with a dangerous acid. Instead of glues, I use Scotch-brand, double-sided, clear tape. The roll of aluminum-foil tape shown there is available from your local HVAC dealer. It's used to seal and secure joints for lengths of ducting, for heating and air. For our purposes, the aluminum tape, about .003" in thickness, is used to build up and adjust the thickness of the substrate for the PTFE-bearings...
After each bearing is installed, you then trim off the excess with a blade. Install however many layers of the aluminum tape required first, trim it, then place a longer length of double-sided tape onto that, then a longer length of the PTFE onto the tape, press them down hard all along the bearing's length, then trim.
Trial-and-error is the name of the game. Focussers and their draw-tubes can differ from telescope to telescope, hence this highly-adjustable method of installing bearings for the draw-tube. You may not need to build up the surface of the housing with the aluminum tape, or two per bearing may be required, or four. In any event, all the components are removable, and with the PTFE reusable, over and over until you get the thicknesses of each all round, just right. You do want all three bearings to be at the same thickness as much as possible. There have been times when I've had to make just one bearing slightly thicker, or thinner; again, trail-and-error. Also, the draw-tube's rack must come into close contact with the pinion-gear between the knob-assembly, but not too tightly meshed together. The thicknesses of the bearings will have an effect on that. The first thing I like to do is to insert the draw-tube loosely into the housing, without the pinion-gear installed, then insert the PTFE strips in between, loosely, and to get an idea of the thickness for each that will be required.
The clear, double-sided tape, once pressed onto the PTFE, will lift off straight-up, perpendicular to the strip of PTFE, easily. However, when pulling the two components, forcefully, and parallel to a surface once pressed down, it will not budge in the slightest...
...and that suits our purposes admirably. In other words, the bearings will not slip out when racking. I suppose, over time, the adhesive of the double-sided tape may or may not fail. It may last for years, or decades even. If and when it does fail, simply replace the double-sided tape.