A homemade Crayford Focuser for a 20-inch Dobsonian
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Last touched 2011 October 28
When TJ was built into the obsession clone version in 1999, I had a vision of making a nice Crayford Focuser for it. These focusers are not only renown for their smooth action but also because it within the range of many amateur telescope makers to construct. Indeed, in the 1980's I had made a simple one for a 10-inch telescope.
In the rush to make TJ operational, though, I sandwiched a thick piece of balsawood between two pieces of 1/8th-inch model aircraft grade plywood and used a hole saw to create an opening just large enough for a piece of 2-inch PVC tubing. The tubing makes a fine focuser tube; easy to get, cheap, cut to length, coat the inside with glue, fill up with sawdust and paint black when dry. Use acetone on the outside of the tube to finish to a pretty, smooth glossy finish. Drill a hole at one end and tap for a machine screw of your choice to use as a set screw to hold the eyepiece/eyepiece adapter. This push-pull focuser worked and worked quite well. In fact, I used this "temporary" focuser for more than 10 years!
With rare foresight, I mounted the balsawood focuser on a piece of 1/4-inch plywood and used four push and four pull screws to attach to the upper tube assembly. This allows me to first to colliminate the focuser as needed to the opticial path. It also means I can swap out focusers by loosing four allen head screws. Since the "push" screws are on the focus board, the collimination for any one focuser is held as long as I don't mess with the push screws.
I liked the push pull focuser. I think everyone else hated it. Or at least though it was odd. While every other focuser on the field cost at least $60 and many into the hundreds, mine was about four bucks worth of hardware. Hundreds of times, as other amateurs looked through TJ, I had to explain how to focus it. It would have likely remained this way, but...
At the 2010 Delmarva Star Party, 11-year old Maggie won a door prize. A pair of very, very pretty "sport" focuser knobs apparently meant to be installed on one of those multi-hundred dollar focusers that are out there. Even as we drove home from the party, I realized the time had come. It would be a waste not to use those pretty knobs built to look like sport car wheel rims.
My early Crayford focusers had used telfon against the PVC pipe as the bearing surface. I knew from experience I could do better. After much thought and some fooling around at the hardware store, I came home with a joint fitting for 2-inch PVC electronical conduit. By slicing out a quarter of the fitting, I had the base on which the focus tube could ride against. A trip to a local hobby store found a selection of small machine screws, including 4-40 and 2-56 screws, and taps for them. After some measurements and testing, an ebay hunt found ten tiny ball bearings that had an inside diameter similar to the 2-56 screws and enough height to act as the bearing surface for the focuser tube.
The rest is mostly told in the photos below. Post assembly and testing, I realized ... something I suspected would happen ... that the bearing support was bending and no longer square with the focus plate. This I fixed with a piece of 1/4-inch aluminum plate.
All told, I have about $50 tied up in the focuser. Time will tell just how much of a success it is. I certain am not throwing away the balsawood focuser! After all, it is only a five minute job to swap them out.
Click on most images for a larger/high resolution version of the image
Most of the parts, waiting to be assembled. The piece of teflon at the bottom becomes the bearing surface for the knob shaft. In this version, it was just wedged between the shaft and what I call the pressure plate (the thing with the springs). Later, this arrangement was changed. The ball bearings screw into holes that are tapped on the grey plastic; the electrical conduit that supports the tube. The clear piece of round tubing is heat shrink. It goes on the shaft and provides friction against the tube.
The bearings and their set screws. These bearings were meant to be used for computer fans. They came in a tube of ten for about $20 on ebay in 2011.
Close up of the fancy knobs that started it all. The shaft is 1/4-inch aluminum from Lowes. I carefully drilled the ends and tapped for the screws that hold the knobs on.
The focuser base with some of the hardware attached. A section of plastic electical pipe is the support for the bearings. The long screws are the posts for the tension springs. Note the wood blocks that will support the bottom hinge of the pressure plate.
An early version of the pressure plate. The brass shaft is the base and attaches the plate to the focuser base via the wood blocks on the base. The plate is made from sheet aluminium flashing from a hardware store. Cut to size, holes drilled, while flat, then bent to shape using straight edges. The middle holes are for the focuser shaft that is pushed against the focus tube. Early versions were like this. Later versions put two small pieces of teflon bolted to the plate that act as a bearing surface for the focuser shaft. This greatly smoothed up the focusing action. The tension springs attach to the screws on the base. By sliding the springs up and down the long screws, you can adjust the tension. This version was missing the teflon bearing, but also was too short. The focuser shaft should contact the focus tube just about half way between the ball bearings.
The tube support, showing the tapped holes for the bearings, and the screws that the tension springs attach too. The holes are tapped for 2-56 screws. These were the largest threads I could tap into this material. I then used the decimal diameter of a 2-56 screw to determine the inside diameter for the bearings. An ebay search for mini bearings with this ID in millimeters found me what I needed.
The bottom of the focuser plate, showing the various holes and hardware. This was made from a piece of 1/4-inch luan plywood.
Here is the focuser, painted and installed as seen from the "top." A couple of points of interest: you can see the teflon bearings for the focuser shaft nicely in this photo. The teflon is attached to the pressure plate with 4/40 screws. A piece of rubber heat shrink insolation was slipped over the shaft and shrunk down to provide a rubber grip for smooth movement. I considered painting everything black. That would be ideal, at least from an observer's point of view. However, I didn't paint aluminum parts that would be subject to wear as the paint will only chip off.
Assembled, "front" view. Once assembled, the tube support would not stay square. The answer was a piece of 1/4 alumium plate, drilled and tapped onto the tube support as seen. In hindsight, a hard plastic might have been just as good and lighter. You can see where I played with adjusting the tension by moving the springs up and down, and in the process, chipping the pretty black paint off the posts.
bbunge@ladyandtramp.com
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