Crayford Focuser

One of my big complaints about all the entry level telescopes that I have tried (and many that are not entry level in price) is the focuser. So many inexpensive telescopes come with very cheap focusers. That is understandable since the companies are trying to create a product to a price point and they must be thinking that the more you spend on optics, the better the experience will be for the user. But even good optics will be wasted if the user experience of acquiring an in-focus image is hampered by a sloppy, hard to use focuser.

This instructable will show you how to make a very serviceable focuser from some scrap aluminium, some bearings salvaged from dead computer fans, and a metal bar from a DVD/CD drive.

As surprising as it may be, the tools for this build are not that extravagant. I used:

1) Drill Press and drill press vise.

2) Set of numbered drills (or whatever might be the metric equivalent)

3) Counter bore for flat head M4 machine screws

4) 1.5 in and 1 in hole saw.

5) Hack Saw

6) Various files and Sandpaper up to about 600 grit.

7) Flat surface used to true up cuts. (Thick plate glass works and so does granite offcuts from a countertop manufacturer. Ask to raid the scrap bin. You are saving them money on disposal costs.)

8) Taps suitable to the hardware you are going to use (I used M3 bolts and machine screws)

9) centre punch

And optionally a propane torch, deburring tool and a hand held drill/Dremel.

As for materials:

1) 0.25x3 in aluminium bar

2) 1x1x0.25 in aluminium angle

3) 3in aluminium channel

4) 1.25 in Schedule 40 pipe. Pipe is spec'd by ID. Tube, by OD. 1.25 is actually more like 1.33 in but with a set screw it will hold an 1.25in eyepiece nicely.

5) metal rod salvaged from a DVD/CD drive

6) bearings salvaged from discarded cpu fans

7) cap screws and flat head machine screws for assembly ( I used M3 cap screws and M4 flat head machine screws)

8) possibly some 3/8 aluminium rod if you make the knobs that I did the way that I did.

9) and some grub screws to attach the knobs to the axle

My local metal supermarkets has a minimum order size. I had to get twice the needed amount of materials in order to make the $15 minimum order.

You might notice that I do not have any dimensioned drawings. Most of this was done through "qualitative methods" with an eye to proportion and a consideration for the strength of the materials. I give approximate measurements, but use your judgement and your eye. You will get a more personal result that will better suit your purpose.

The Wikipedia entry for the Crayford focuser provides a concise treatment on the history. What makes it elegant is that it is a minimally constrained kinematic design. In a nutshell, unconstrained rigid bodies have 6 degrees of freedom ( translation along each of the three spatial dimensions and rotation about each of the three spatial dimensions). Kinematic design is about constraining motion in a rigid body to a prescribed path using the fewest constraints possible. What that gets you is minimizing internal stress, removing potential deformation due to stress and, most importantly for hand fabrication, reduced or eliminated dependence on tight tolerance fits for your parts to accomplish the desired goal.

The goal of a focuser is twofold:

1) hold the eyepiece centred on the optical axis

2) enable the eyepiece to move only along optical axis of the telescope

That is you want to constrain the motion to a single degree of freedom. You can see in the diagram in the upper left the simplified drawings of the principle. If you have a reasonably smooth and round cylinder (eyepiece tube) then you will have reasonably constrained the motion of the cylinder to its axis by allowing it to roll along a fixed 'vee' bearing (the first order error in this fabrication will be the undulations on the tube surface with the second order error being any non-circularity of the bearings themselves). That will be constructed by placing the bearings on a small length of angle bracket. Then we just need to mount that angle bracket on a base, and then support the rod that will drive the tube.

Easy. Let's make one.

Cut a square of 0.25 aluminium plate about 3 or 3.5 inches square. Mark the centre of the plate. Put a large bit in your drill press and tram the table. Chuck the hole saw in your drill and using plenty of oil, cut a hole in your aluminium plate. Take your time, and clear chips often, but even a bi-metal hole saw will cut aluminium fairly well as long as you don't overheat the tool. Once you are done with the hole you can take some sandpaper and pace around while listening to music de-burring the hole polishing the plate, and contemplating your next move. The 'Vee' bearing.

You need four bearings for the 'Vee'. As I mentioned above, I harvested mine from from old computer cpu fans but they are not expensive if ordered from china. In older fans they used bearings on at least one end of the fan shaft. If you a raiding a pile of junk CPU fans some of the bearings will be shot, but I only had to tear apart 5 to get enough good bearings. To get them out you might need to grind off the head of the shaft since the bearings are held in with a retaining ring that was not designed to be removable (not a snap ring but the same idea without the maintainability requirement). The bearings from the computer fans turned out to be on 3mm shafts so that is what I used for attachment hardware; M3 cap screws. Lo and behold, the M3 cap screw head engages only with the inner race leaving the outer race to run free, and the outer race is slightly smaller in depth than the inner race so it runs free when the inner race is affixed on the M3 cap screw! Win Win!

Cut a section of angle about 1 inch high. Use sandpaper, files and your machining square to get close to perpendicular between the cut face of the angle and the spine.

Then you need to mark the angle bracket edges using your callipers. Aim for a trade-off between bearing overhang and material in which to cut the threads. Then you just need to drill the tap clearance, and tap holes for the mounting hardware.

Finally, locate where you want to place the 'Vee". Have it such that the bearings hold the focuser tube in the centre of the hole in the base plate. Mark the outline of the 'Vee' on the base and drill mounting holes for the base. Transfer the holes to the 'Vee' and drill and tap the 'Vee'. Counterbore the opposite side of the base from the 'Vee' and attach the 'Vee' to the base plate with flat head machine screws.

This was a bit harder to solve since the scrap I had access to was a 3" channel. It was too wide as you want to have the bearing points on the axle as close together as practical. What I ended up doing was taking the 3" channel and cutting it into two angle brackets and then mated them together by overlapping edges. On channel the inside corner has a radius. I filed it square to get a nice mating fit between the two angles. Then mark, drill, tap and counter bore for the flat head hardware.

After that part is done you can drill the holes for the axle. What I did was measure the thickness of the back face of the channel assembly and then add 2x the diameter of the axle. That way there is some clearance between the bottom of the channel and the axle. I plan to mount a Delrin/Teflon/UHMW pad into the back of the channel and enable variable pressure on it by mounting a thumbscrew. That is for another project.

In the Crayford focuser there is no rack and pinion gear assembly. The driving force for moving the tube is governed just by the friction between the axle and the tube. To give the axle a bit more surface area to contact the tube on I hand milled a flat onto the tube. Starting with a file to take away bulk material, and then moving on to successive grits of sandpaper mounted on the flat. It is surprisingly easy to keep it aligned just by eye and hand especially as the flat develops a bit of width. Gentle pressure and patience are your friend here. That and some meditative music to listen to.

Not figure out where it is (but if you are like me then there might have been a moment when you had to go looking for it) but figure out where to drill holes and mount the channel/axle assembly. What I did was to place the tube into the base + "vee" assembly and mark out where the channel assembly sat, Then I removed the tube and moved the marking for the edge of the channel towards the focuser tube hole about 0.5mm. That will allow a a pressure exerted between the axle which will flex and the tube/Vee combination. Then mark the baseplate where you will drill holes to mount the channel assembly. Try to hit the verticals of the channel as you avoid the join (and the material is thicker on the ends). Drill, tap and counterbore.

There are many ways to solve this part of the puzzle (Crazy Brits! This guy just used a spare screwdriver!). What I did was to use a hole saw to cut a disk out of a plate of scrap aluminium. What you get after that is an annular disk of aluminium. You have the centre hole from the hole saw and the rest of the disk. That left me with the problem of attaching the knob with a hole that was about 4mm to the approximately 2mm axle. To solve that one I chucked a short piece of aluminium rod in the drill press and sanded/filed down the rod slowly to get to an estimated friction fit between the hole left by the hole saw. I heated the knob with a propane torch to expand the aluminium and quickly pressed in the rod while the knob was still hot. The second knob worked better than the first. Aluminium likes to glom onto itself (for those not familiar with the term "glom", think an almost magnetic attraction and sticking between the parts). Practice; practice on scrap. Then I drilled and tapped the shaft of my adapter for grub screws.

Last, you need to mount a retaining screw for the eyepiece. Just an M4 cap screw preferably with knurling on the cap. Look at your eyepieces and see if any have a machined depression for an eyepiece screw lock. Mount the retaining screw so that it will fall in that location.

And you are done. Mount it on your telescope and enjoy the backlash free, consistent axis of motion and buttery smooth action of your hand crafted Crayford focuser!

Thanks for reading, and if you like this i'ble, please vote for me in the Space Contest! And if you build one, please post a picture in the comments. I would love to see it.