My son wanted to build a telescope out of a mailing tube we had, so we bought a 70mm lens (300mm FL). The part of telescope making that I like the least is focuser-making. But I think I now have a Crayford-style design that is very easy to make. It is easier to make than my previous wooden
Crayford and
helical Crayford, and has about the complexity of my
push-pull Crayford but works an order of magnitude better.
It's a block of hardwood (cherry, from an old crib), 2.75"x2.75"x1" in dimensions. I used my drill press to drill out a 1.5" hole (A) in the middle for the 1.5" outer diameter aluminum focuser tube. I then drilled a second hole, 3/8" in diameter, all the way through from one side to the other (B), so that hole A and hole B would meet and overlap by 1/16". This second hole is for the focusing rod.
Finally, I drilled two more holes from one side to meet up with hole B (C and D) for adjustment screws that press on the focusing rod through PTFE pads, and tapped them to fit the adjustment screws simply by forcing a screw into the wood. These PTFE pads (E) are small strips of
1/16" PTFE
, bent into a C-shape, with the back of the C facing the adjustment screws.
I purchased an 8" x 1" x 0.0045" strip of self-adhesive PTFE tape (F) on ebay from
mousetape.com for about $3 shipped. I stuck a strip along one side of the focusing tube.
I then enlarged the 1.5" hole in the middle slightly. My method of doing this was to use a
sanding drum mandrel
in my drill press with a somewhat oversized
sanding drum
(which would stick out on both sides past the work piece), with the work piece resting on another piece of wood with a hole in it, which was on the drill press table. As a result, I could keep the work piece aligned at right angles to the sanding drum while moving it about so as to enlarge the hole. I enlarged the hole until it was about 1.56" in diameter. I then finished the exposed raw wood with Titebond II diluted with water and sanded. I stuck another strip of PTFE tape (G) inside the focuser hole, opposite where the focusing rod will go. Then I put it all together, and it worked just fine.
Update 1: I added knobs. I had some oak circles from using a hole saw on some piece of oak some time ago, and after sanding them (by putting on a bolt and spinning with a drill against a sanding block), I press-fitted them on the focusing rod. If it starts slipping, I'll drill a hole through the rod and knob and put in a screw.
Thanks: I am grateful to
John Wall for the idea of putting PTFE on the focuser tube.
Update 2: I just had a bit of trouble with the wooden thread for one of the adjustment screws getting stripped. Treating the hole with CA glue helped, though.
And I added a screw on one end of the focuser tube as a stop, and I added a screw to hold the eyepiece. Because the focuser tube is only 2" long, so as not to compromise the length of movement, we drilled little depressions in the main block for the screws.
Update 3: And here is a not-to-scale diagram of what the main holes in the block of wood look like.
And I added a screw on one end of the focuser tube as a stop, and I added a screw to hold the eyepiece. Because the focuser tube is only 2" long, so as not to compromise the length of movement, we drilled little depressions in the main block for the screws.
Fantastic Crayford !
ReplyDeleteHow do you think this would work for Newtonian (Dobson) ?
Should be fine. The only issue is that you will need to fit it to the telescope's curve.
ReplyDeleteAn easy way is just to attach two wooden strips (e.g., dowel with one side sanded flat?), one on each side of the underside of the focuser block, in such a way that the tube makes contact with the strips and with the center of the block.
An elegant method would be to cut or grind a matching curve into the underside of the focuser block. This is probably hard to do while keeping the focuser square to the tube. If you've got a lot of patience, you could stick some sandpaper to the telescope tube, cover up the mirrors, and then rub the focuser block against the telescope until it takes on its curve. (If it's a smaller tube, maybe some roughing out would be good first?)
If you have a router, you could make the focuser from a block that's curved on one side. Take a piece of 3/4" hardwood or hardwood ply. Cut out a square that's about 2" wider on each side than the outside diameter of your telescope tube. Using the router, cut out a hole in the square, exactly centered in the square, that's equal in diameter to the outside of your telescope tube. Now cut out four curved segments from the sides of the square, stack them together, clamp well and glue, and you have a block that's 3" wide (4 x 3/4"), with a nice curve in it, and 1" minimum thickness. Then sand the straight face smooth, and drill the focuser hole starting on the curved side. As a bonus, you get a big circle of wood, which you can cut in half and use as altitude bearings for this or another project. :-)
If you want to make a 2" version, I suggest a thicker block, maybe 2" thick.
ReplyDeleteIt occurs to me that one could cut costs by using milk jug plastic in place of PTFE pads for part E. It would not affect performance much because friction at that location doesn't matter much.
ReplyDelete