A simple wooden Crayford-style focuser

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.

Wooden helical Crayford focuser

I've done this project a while back, but in case anybody is interested, I am documenting it.  My Coulter 8" scope came with a friction focuser made of pipe fittings that Coulter described as "helical", I think because you aren't going to do very well just pushing the focuser tube in and out unless you give it a twist as well.  With practice, the focuser was usable for light eyepieces, but it really didn't work for something as heavy as a Hyperion 13mm (and there are eyepieces heavier than that).  So I decided to make a helical Crayford focuser, because I was too cheap to buy a KineOptics one.

The idea behind a helical Crayford is that you have ball bearings against which the draw tube rotates.  However, the ball bearings are slightly twisted so that their axes make a small angle with the angle of the tube.  Consequently, as you rotate the draw tube, it moves in and out.  If the ball bearing angle is larger, the movement is faster, and if it's smaller, the movement is slower.

I mounted the ball bearings on two T-shaped supports made by taking a #12 flat-head screw for the stem of the T, and JB Weld'ing a segment of 1/4" square rod to the head of the screw for the line.  I then JB Weld'ed a  skateboard ball bearing (I expect from VXB's ebay store) at each end of the T's arm, trying as best I could that everything would be square and the arms of equal length (not that I am very good at that sort of thing).  I then took a piece of 3/8" particle board (Baltic Birch plywood would have been better, but I was being cheap) and mounted three posts on it.  Two of the posts would sport the ball bearings, with the stem of the T going through the post, and with various nuts and spacers to hold the bearings in place.  The third post had a little flap adjustable with a thumbscrew.  The flap was made of a piece of bondable PTFE JB Weld'ed to a piece of metal, with the thumbscrew bearing against the metal, all as in my push-pull Crayford.  For the draw tube, I used the aluminum draw tube from the Coulter.  In the picture, the hole for the focuser tube wasn't cut yet.

Here's another picture, with a better view of the T-shaped ball bearing supports.  You can see the gray JB Weld all over the  place.  Two things to be careful of when gluing are (a) don't get any JB Weld on the bearing surface, because then movement will be bumpy and (b) ensure that the JB Weld stays on the inner ring of the ball bearings, so it doesn't stop the ball bearing from moving!  Note also that the posts are located in such a way that the focuser tube would fit snugly in.  At this point in the process, the posts are, I think, screwed in place.

Eventually, I unscrewed the posts, cut a hole of the focuser with one of my trusty cheap Harbor Freight hole saws, sanded the faux veneer off the particle board, glued the posts with JB Weld to the particle board, with screws for added strength.  Or something like that--I don't remember all the details.  (It's possible that at some point the posts were weakly affixed with craft glue to help with layout and then pushed off.)

I did have one worry, and that was that the posts might bow outward with time, thereby decreasing tension, and generally making the focuser not work too well.  So I JB Weld'ed some strips of hardwood that I had lying around in my scrap pile, joining the posts as if with a fence between fence posts.  I was generous with the JB Weld.

Finally, I cut the particle board down to size, removed the old focuser (cutting the sonotube around it, ripping it out, etc.--it was a messy procedure), and attached the particle board with screws and some hardwood rails to fit to the curvature of the tube.  I also painted the wood and JB Weld black.  It's pretty heavy, but somehow fits the home-made styling of the Coulter scope.

The focuser has had a bit of image shift, which has increased with age.  Lately, it wasn't doing too well, so I had a closer look at it.  It turned out that the T-shaped supports, under pressure, had rotated further, so that the angle between the axis of the ball bearings and the draw tube was too big for good movement.  I made the angle smaller, added washers under the nuts (did I forget? or maybe I just ran out of washers?), and tightened it up.  I am guessing they'll rotate away again.  If they do that, and maybe before that, I may replace the nuts with locknuts, and/or add some toothed washers.

The focuser weight is an issue--I've had to increase the counterweight--but that's part of the price of making stuff out of wood.  And the price is hard to beat: a couple of bucks for the bearings, and then some screws, washers, nuts, square rod, and various pieces of scrap.

6" F/5 Dobsonian: Wooden Crayford focuser

Update: I now prefer the much simpler design of this focuser.

Like I said, I hate making focusers.  If I were making this for myself, I'd probably stick with a push-pull Crayford like on my travel scope.  But I'm making this for a friend, so I made a more normal Crayford.  The draw tube is aluminum with 1.5" outer and 1.25" inner diameter.  Actually, my eyepieces and laser collimator didn't fit when I got it, so I had to sand the inside of the tube.  The method was to take a screwdriver, wrap some sandpaper and foam around the handle, affix it at one end with duct tape, and spin the screwdriver with a drill.  Took a while, but eventually I got the bore large enough.

The main part of the focuser is a particl board plate, with two 5/16" dowels along the sides to fit the curvature of the tube, and three posts.  The two of the posts on the right are, I think, oak, and the one on the left is cut from 1"x1" poplar square rod, and they all have little plywood thingies behind them to keep them from falling over.  They are both glued and screwed to the plate.  The two posts on the right get ball bearings (the cheap ones for skateboards that one can buy on ebay, with wood screws JB Weld'ed inside them), and the one on the left has a hole  cut out in it for a holder for the focusing shaft.  There are two screws on the holder for adjusting tension.

The non-fixed parts consist of a little H-shaped poplar thingy that fits in the slit in the left post.  It has some V-shaped holes cut in it and lined with bondable PTFE.  Between the H-shaped thingy and its post there is a folded rubber thing to add some spring, cut from a bicycle inner tube.  The adjustment screws on the post bear against the rubber.  The shaft is 1/4" stainless steel.  The knobs are cut from pine boards.  They're not as straight as they could be, but they're light.  I cut the knobs by first drilling about 1/3" deep into a pine board with a 1.25" paddle bit.  Then I cut the pieces out with a larger (1.5" or 1.75") hole saw.  Result: cylinders with a recessed hole that lightens it and makes it look better.  I then put a bolt through each, and spun it against sandpaper on a drill.  The shaft then fits in the holes that the hole saw mandrel made.  I put some PTFE rings on the shaft, and I filed the ends of the shaft to make it rougher and less round, and glued the knobs in placed with, of course, JB Weld.  (I use JB Weld for most bonds other than wood-wood.  I use Titebond II for most wood-wood joins.  Except that the dowels at the bottom of the focuser, to make the focuser hug the tube, were attached with Gorilla Glue, because I wanted a glue that would (a) dry quickly and (b) make little gussets.)

Motion is delightfully smooth.  The next major step will be mounting the secondary.

6" F/5 Dobsonian: Drilling a focuser hole

I was really nervous about making the focuser hole--drilling in a cardboard tube can be messy.  On my Coulter 8" when I replaced the focuser, I had to enlarge the focuser hole, and it was a quite big mess.  I thought of trying to use a rotary tool, but I couldn't figure out which bit was appropriate, and had no scrap to try it out.

But I decided that the glue-impregnated cardboard just might take a hole saw, and risked drilling with a hole saw.  Success!  There was just one little bit of tearout on the inside, but I glued that down while impregnating the hole with water-diluted Titebond II.

The focuser is going to be a Crayford.  It's almost finished--it just needs some 5/16" dowel glued on the underside for mounting on a curved tube, and I'll buy that tomorrow after dropping the kids off at school.  I'll post on it when it's finished.  This morning I got to do some trigonometry to try to figure out where to place the dowels so the center of the focuser board would be tangent to the telescope tube, and the dowels would be tangent to the focuser board and the tube.  Somehow the calculation didn't work out.  Then I found a simpler method with Pythagoras' Theorem, which did work out.  Geometry is fun.  I don't know how some people manage to do serious woodworking without using trigonometry.  I guess they might be really good at scale drawings, or just able to eyeball things in ways I can't.

My 8" F/4 travel scope: Focuser

I hate making focusers and am too cheap to buy. This is a push-pull Crayford: the draw-tube moves against PTFE runners. The draw tube is made out of super-cheap PVC conduit (something like two or three dollars for 12 feet). Unfortunately, the conduit's inner diameter is significantly more than 1.25", so I lined it with PTFE strips, which I sanded down to fit eyepieces.


The focuser's tension is adjusted by having a piece of metal glued on its bottom part (with JB Weld), and with a thumbscrew bearing against the top part, and with PTFE on it, on one of the three posts.


Here is a picture of the focuser board at an early stage in the production. The elongated hole is for the secondary stalk.