|The focuser fitted with a 25mm Celestron X-Cel LX eyepiece|
The Crayford telescope focuser design – developed in 1971 by John Wall and named after the Crayford Manor House astronomical society – has a number of important advantages over rack-and-pinion and can be home built out of wood by almost anybody with a bit of patience and the right tools at hand.
Although some prefer to build the design in metal, with the associated advantages, that approach requires specialized metal working tools, generally out of scope for most home builders.
The 2” woodshop design presented here is scalable, has proven to be sturdy, reliable and, most importantly, when built to tight enough tolerances, allows for very precise focusing, even under heavy loads.
I designed this focuser specifically to carry the weight of a Nikon D7100 camera (± 800 gr) on prime focus, which is considerably heavier than even the largest eyepieces. So far, it is holding up admirably.
Even though building this design is probably possible utilizing hand tools only, for tight tolerances the following specialty tools are recommended:
Electrical table saw and/or
Miter saw: electrical table top (enhanced precision wood cutting) and/or manual (PVC cutting)
Belt sander (80 grit belt)
Vertical drill stand
Drill press vice
Hole saw, 54mm (2 1/8”)
Wood drill bits: 1mm, 3mm, 4/4.5mm, 5/5.5mm and 8mm
Thread tapping set
|Nikon D7100, prime focus; 6" f/8 telescope, 1.200mm; f/8, 1/500 s.. Click, then click on "Open in new window" for full size|
Plywood: 15mm (9/16)
Oregon pine or any other type of solid hard wood, 2x3”
25mm Sliding door wheels with ball bearings (4)
6mm Slotted machine screws (4)
6mm Axel shaft
Nylon, rimless 6mm wall plug (2)
6mm Acorn nut, flat washers (6), lock washer
6mm Nylon bolt (focus retention, optional)
Pressure PVC tube (blue): 50mm (2”)
Pressure PVC coupling: 50mm
Reduction bushing: 50-32mm (2”-1 1/4”)
4mm Retention screw with jig-knob
Sanitary PVC end cap (grey), 50mm (collimation cap)
Sand paper: rough, medium, fine
Clear, weatherproof varnish
Spray-paint, black matte
Matte black wood paint
Note that, although I spent only about U$ 25 on PVC, wheels, spray paint and some hardware, most materials used were scraps, bits and pieces lying around in the workshop. Buying everything new may result in slightly higher costs.
Still, a branded 2” Crayford will set you back U$ 250 or more, which makes the effort well worthwhile.
For the same reason, don’t rule yourself by the given measures, but adapt according to what you can get your hands on. A 2x3” piece of wood may measure slightly different from the 42x72mm I used, while my focuser axel was made out of a surplus 6mm tent peg, for example. You may have an 8mm shaft lying around; just put it to good use, scaling the corresponding hardware to 8mm as well.
With the proper tools, this design can be built in approximately 12 hours, including sanding but without considering the drying time of the wood glue (which depends on the type used) and three hands of varnish, the drying of which typically would take at least 72 hours.
Moreover, applying an optional two hands of black matte paint on the mirror facing side (as done here) would add another 24-48 hours, depending on the type of paint used.
Thus, from start to finish, this project would typically take a week of your life...
My original v1 design considered a solid block of 2x3” Oregon pine, the tube track cut with a 54 mm cup saw and the roller tracks cut out with a jig saw.
This proved inconvenient, due to the imprecision and flex of the jig saw, which resulted in the roller tracks mounting askew.
The latter, in combination which small roller wheels, resulted in the focusing tube running off axis, which made for unreliable, imprecise focusing.
The v2 design – presented here – pretends to solve the v1 problems, specifically aiming at properly aligning and/or enlarging the moving parts (wheels and focuser axis) plus building to much tighter tolerances.
Still, in the process, some mistakes were made. Most importantly, building the focuser body out of plywood rather than solid wood. The latter resulted in splicing during the M6 bolt threading process for the rollers, which had to be patched with a piece of 18mm Oregon pine glued on.
However, I have now included this patch as part of the design, because it is far easier to glue a separate piece onto the wheel block than cutting its bottom part to the proper size for tight insertion into the base plate.
Still, it is highly recommended building the focuser body parts out of compact, fine grain wood. Oregon pine will do, but I suspect that oak or some other type of hard wood will do even better, especially when it comes to threading.
That said, the harder the wood, the more difficult its processing. The trick is to find a material that is resistant, but not so hard that cup-cutting, for example, becomes a pain.
This is not a wood working class. I’m starting out from the supposition that you know how to properly work different materials, that you have the right tools at hand and that you are able to make virtually perfect 90º cuts or perforations.
If not, do not waste your time trying to do something that cannot be done, or at least not done right.
Moreover, allow for mistakes, even when you do know what you’re doing. We all make them, and correcting or starting over while having the right back-up materials at hand is a lot less frustrating than having to interrupt the process and running out to acquire them.
Finally: when using electrical-mechanical tools, protect yourself at all times. Use the protective measures required, such as goggles, gloves, masks, etc., and do not deactivate or remove any protective measures implemented on your tools.
I almost cut off the top of my left hand index finger, a few years ago, and still live to lament the consequences.
For 1/1 scale PDF building plans, click here -- 7 pages, 390Kb.
Step 1: cutting to size and drilling holes
Cut the base plate and the wheel block mounting plate roughly to size, but leave room for error. You can always shave off a couple of mm later.
Leave the focuser body and the wheel block add-on at least 5cm over length on the top end, or as long as your miter saw allows to still be able to cut to a 45º angle.
Using the vertical drill stand and drill press vice, apply the PDF templates to cut a vertical 54mm hole in the base plate, the focuser body, and the wheel block add-on (page 5, bottom), respectively.
Since a cup cutter has a limited cutting depth, first drill a centered 3mm pilot hole throughout the focuser body, to guide coinciding cup cutting from both sides of the wood block.
Always add some sacrificial material below to avoid splicing or a rough exit hole whenever drilling or cutting throughout.
Use a table saw to cut the 64mm wide opening at the top of the base plate (make sure the add-on fits tightly).
Do not yet cut out the spaces for the bottom guiding wheels or drill the holes for the wheel block mounting plate, we will do this once the wheel block is ready for mounting.
When using a 6mm focuser axel, drill an 8mm hole from the right to the left end of the focuser body. Make sure this hole overlaps with the position of the focuser tube, i.e.: runs ± 4mm inside the 54mm focuser tube hole.
Drill a second 8mm hole to the left of the first one, with a 4 mm overlap; you should end up with an 8x12mm oval.
It may be necessary to drill pilot holes first, to assure the second 8mm hole is properly centered and aligned with the first. Eliminate excess material between the two with a small, flat file or chisel.
Next, drill a 5 or 5,5mm hole (depending on your material) for the axel shaft pressure screw, then thread to 6mm. Make sure it is centered on the axel.
Even though most designs, including the original John Wall model, include two pressure screws, recent research suggests that one screw is better, because it more efficiently curves the axel towards the center of the focuser tube.
Finally, drill a similar hole on the left hand side of the focuser block for the focus retention screw, then thread to the corresponding screw size. It ought be dead center on the left side center of the focuser tube (see page 4).
For threading, drilling holes ought to be 0.5 to 1 mm smaller than the corresponding screw, depending on material. Experiment with scraps to determine proper size. In the case of wood, a too tight pre-drilled hole may result in splicing (PVC is much more forgiving), while in both wood and PVC too wide will inevitably result in a lack of grip for the corresponding screw and/or early wear-out on the thread.
In all cases, make sure sufficient material is available for threading, especially when dealing with soft materials. The current design accounts for this, but if you make changes, it is important to have this present. In materials like wood and PVC, shallow, wide threads deteriorate quickly under load.
Step 2: cutting up the focuser body
Once all the holes are drilled, you need to cut the wood block into two parts: the focuser block and the wheel block, respectively. All cuts need to be perfectly vertical, especially on the wheel block. If not, the focusing tube will run askew, causing imperfect focusing.
Once cut in two, you must cut two 45º angles on the wheel block as well as on the add-on, to mount the guiding wheels. This is the most critical and dangerous operation of all, and must be as precise as humanely possible.
Make absolutely sure the wood is properly seated and tightly affixed under a 90º angle on the miter saw table before proceeding. Double check your cutting line, which must be left standing; i.e.: must still be visible once the cuts are made.
Next, drill and tap the guiding wheel screw holes. The reason we left the wheel block and add-on over length initially, is that the pieces can now be seated with sufficient support into the drill press vice, which would be virtually impossible with them cut to size.
You can also use the over length to cut two additional 45º angles opposite to the ones made previously, which allows to angle the piece perfectly perpendicular relative to the drill bit. Alternatively, you can use a small block of wood cut to 45º.
Once the holes are drilled, you can proceed to cut the parts into the correct lengths. Finally, fine sand all the focuser body parts and the top and bottom of the base-plate until smooth. However, do not sand the areas which are to be glued, nor the edges of the base plate, as yet.
Step 3: putting the pieces together
First, glue the wheel block body on top of the add-on; make sure they are properly centered and aligned on their circular cut-outs; let dry. Next, glue the mounting plate onto the add-on and the body of the wheel block.
When using the right glue, glue clamps and an adequate drying time, it ought not to be necessary to use screws; if you still want to screw the pieces together, do so from the bottom.
Once the glue has dried completely, mount the finished wheel block on top of the base plate, and check for fit. It ought to be tight, but not so tight that you need excessive force to join both pieces.
The circular hole in the body ought to line up perfectly with what is left of the hole in the base plate (use the template or a jig to confirm this); if they overlap, leave the excess on top, i.e.: on the mounting plate. If it were necessary to make adjustments, do so on the base plate, not the wheel block.
Once the base-plate and the wheel block are properly aligned, drill the holes for the carriage bolts. Make them fit as tight as possible; a little friction on initial insertion is perfect.
To make sure the parts do not move during this operation, it may be necessary to temporarily clamp or screw them together. Also, take advantage of this temporary union to cut off excess material – if any – on the base-plate or wheel block mounting plate, using the table- and/or miter saw.
With the carriage bolt holes drilled, unmount the wheel block and proceed to make the cut-outs for the bottom guide wheels in the base plate with a jig saw.
We did not do this before to facilitate the alignment of the wheel block on the base plate. Once finished, you may glue the focuser body onto the base plate; make sure the circular cut-outs are perfectly aligned.
Once dried, you can sand down the edges of the base plate and any other parts that still need sanding, after which you need to apply at least 3 layers of clear, weatherproof varnish. Sand lightly between each layer.
If you wish to apply black matte paint on the mirror facing side, do this after varnishing; it will avoid staining the edges of the base plate, because excess paint can now be easily wiped off.
Step 4: making the focuser tube and collimation cap
Cut a piece of 50mm pressure PVC tube (2mm wall) to the required length and use a flat sander with an 80 grit sanding belt to flatten a part of the tube (blue in the image above). The sanding direction must be perpendicular to the tube's length.
Using the manual miter saw, cut a 50mm coupling to slightly less than half height, assuring that the part does not include the internal stopper rim.
Temporarily insert a short piece of tube and a coupling, for example, to make sure the pieces are aligned horizontally and the cut is to 90º.
Sand all rough edges with fine sandpaper.
Glue the 50 to 32mm reduction bushing into the coupling, leaving about 10mm salient. Once dry, glue the focuser tube into the coupling, using the reduction bushing as a stopper; this will virtually guarantee that the tube is glued in straight.
Temporarily apply the left-over part of the coupling to the end of the tube to help assure that the piece is seated at a 90º angle on the drill press vice, then drill and thread a hole in the salient part of the bushing for the eyepiece retention screw, exactly opposite to the flattened part on the tube.
Lightly sand all parts with fine sandpaper and cover the flat area on the tube with masking tape. Use black matte spray to paint the tube inside and out. Apply at least 3 layers (5 is better on the outside), allowing every layer to dry completely before applying the next. Once completely dry, remove the masking tape and use a soft cloth with a little solvent to remove any trace of leftover glue.
The collimation cap is made out of a 50mm sanitary PVC end cap. A drawing for the retention screw cutout and a template for drilling the 1mm hole are included on page 7 of the building plans. Print the template, cut out the circle and place it on the interior of the cap. Then drill the hole and sand eventual rough edges.
Painting black is optional. If you want to do this, apply at least three layers of black matte spray paint, but ONLY on the outside. Collimation cap insides need to be bright or even reflective to facilitate proper optical alignment.
Step 5: mounting the hardware
If you have not already done so, cut the axel shaft to the correct length, and thread both ends. Sand the center of the shaft with 80 grit sandpaper, parallel to its length.
Cut two smooth buses out of the top of the 6mm nylon plugs, insert the shaft into the focuser body, apply a bus to each end and push them into the focuser body; they ought to fit snugly into the 8mm shaft hole, if done properly.
Apply two flat washers, then affix the jig knob to one end of the shaft and a flat washer, a lock washer and an acorn nut to the other end. Hand tightening suffices; do not over tighten the nut, because it will make the axel turning very stiff.
This design is right-handed, but you can obviously change the position of the jig knob to the left-hand side; keep in mind that you may want to change the position of the focus retention screw to the opposite position, as well.
Mount the guiding wheels on the wheel block with 6mm slotted machine screws, or with any other size that leaves as little play as possible on the axis. Next, mount the wheel block onto the base plate with its two carriage bolts, using a flat washer under the nut.
Insert the focus retention- and axel shaft pressure screws about half way, then insert the focus tube.
If everything has gone according to plan, the tube will need to slightly push back the axel shaft, which means a bit of jiggling and/or exerting a tiny bit of force.
If the tube enters too easily, there is no need to panic; just tighten the axel pressure screw until the axel acquires the required grip for the tube to smoothly move up and down in its seating.
It is important to emphasize that all parts only need to be hand-tightened; excessive tightening will make movement overly stiff.
If the focuser tube slips or does not move as expected, sanding may not be rough enough. Re-sand both the axel shaft and the tube until the latter moves regularly and smoothly without excessive stiffness or force.
If you painted the mirror facing side of your focuser black, now is the time to paint or spray paint any metal hardware left unpainted. If you use spray, make sure to mask all parts that you want to be left untouched.
Finally, the focuser must be mounted in a 90º angle relative to the main mirror. Many people use top cages to house their secondary plus spider plus focuser; if this is your case, you’d obviously want to dimension the base plate to fit the cage and you may need to adjust accordingly.
I, on the other hand, have a three-tube truss telescope, and made a “halfring” to accommodate the focuser, attaching it in a 90º angle with three 1,5” dry-wall screws. This reduces top weight and allows me to move it up and down the truss for now, until its definitive position has been determined; same for the diagonal.
I may end up affixing the focuser to the top ring eventually, for additional precision and stiffness.
But not yet...
I hope you enjoyed this tutorial. If you decide to build my contraption, please do not hesitate to ask in case of doubt, and/or leave a comment on how it went.
Thanks for visiting, and: clear skies!
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