Introduction: 1.25" Crayford Focuser With Eccentric Tensioner
Almost a year ago I decided to make a Dobsonian style reflector telescope. I finished it late in the spring and was able to enjoy it all summer, fall and even a little during the winter. I've observed Saturn, Jupitor, star clusters, nebula and spiral galaxies. It truly was a rewarding project and I still use it on a regular basis.
When I built my telescope I used a 1.25" rack and pinion style focuser. It has worked OK, but I knew there had to be something better out there. The motion on the rack and pinion focuser is not very smooth and it seemed to make large jumps in focus for small movements at the knob.
After some research I discovered that many amateur telescope makers use a Crayford style focuser due to its simple design and smooth motion. Many focusers also make use of a planetary reduction knob for fine focus adjustments. With these design goals in mind I set out to make my own.
Please be sure to read my summary at the end if you want to follow my Instructable to make your own focuser. I give a review on how well it performs and what changes I plan to make on my next revision.
Step 1: Features
There are a few features I want to point out that may not be obvious at first glance. They are what makes this focuser a step above my previous one. These features are on various focusers that I came across while researching how to make mine, but I don't think I found one with all of these features (except for the expensive ones).
- Aiming set screws - There are 4 set screws on the mounting plate that can be used to adjust the alignment of the focuser. This is very useful for collimating the optics.
- Eccentric bushing - This is used to apply pressure to the draw tube. By rotating the eccentric bushing the shaft that is attached to the focus knob is moved closer to or farther away from the draw tube.
- Planetary reduction knob - This is helpful when trying to obtain a nice crisp image. It provides a 6:1 reduction for fine focus. 6 rotations of the focus knob results in 1 rotation of the shaft pressing against the draw tube.
Step 2: Materials
- 1/4" Acrylic
- Bearings, spacers, nylon lock nuts, bolts - OpenBuilds
- 1-1/4" Schedule 80 PVC - I bought an adapter that was threaded on both ends and cut off the threaded sections
- Planetary Reduction Drive - 6:1 reduction ratio
- 1/4" rod - I used Stainless steel
- Various hardware - nuts, bolts, etc.
- Epoxy and/or super glue
- Flat black spray paint
Step 3: Laser Cut Parts
Cut out the required parts from 1/4" acrylic using a laser cutter. I've attached a dxf file that contains all of the parts. I cut the blue parts in the picture out of 1/8" acrylic, but either thickness will work.
Step 4: Assemble and Paint
- Assemble the base plate using super glue.
- Assemble the bearing mount using super glue.
- Assemble the eccentric adjusting ring and reduction knob mount using super glue.
- Assemble parts onto the mounting plate. For this step I tacked the parts in place using hot glue and then filled in the gaps using epoxy.
- Assemble all of the parts together to make sure everything fits and works as expected.
- Paint all of the parts. I chose flat black to reduce any glare from stray light.
- Mount the base plate on the telescope tube. I used superglue to hold it in place while I drilled the thru holes for the bolts.
- Re-collimate the optics of your telescope. With the new focuser you will need to do a full collimation of the optics. Here is a link to a good tutorial.
- In order to create some extra friction between the draw tube and focus knob shaft I roughed the surface of the pvc tube with 80 grit sand paper. I also put a piece of shrink tube on the focus knob shaft.
- The planetary reduction knob took a lot of effort to turn when I first received it. So, I took it apart, cleaned off the grease (which was extremely thick and sticky) and replaced it with some white lithium grease.
- Allow the epoxy to cure for a day or more. If you assemble everything after a couple hours it will still be slightly flexible.
Step 5: Summary / Conclusion
I was fortunate enough to have a nice clear night when I finished construction on my new focuser. It worked very well and produced nice crisp images, however there are a few revisions I will have to make before it's ready for serious observing. Here is a list of pro's and con's that summarize its performance:
- The alignment set screws in the mounting plate worked great for collimation
- The planetary reduction knob was very helpful in making fine adjustments in focus
- The movement of the draw tube is much smoother than with the rack and pinion focuser
- The use of epoxy for assembly did not work well. Although all of the parts are still together and functioning, I've had to reattach some pieces. I would like to redesign the parts so that they can be fastened together with bolts/screws.
- The acrylic flexes too much. When rotating the eccentric bushing to press the draw tube against the bearings, the whole system flexes. As a result the draw tube is not held as tight as I would like it to be. I'm going to look into different materials to use. Another solution may be to design a way to connect the bearing mounting block to the focuser shaft to keep them from flexing apart.
Although there is still some progress to be made, this project has given me a very nice (and economical) Crayford style focuser that out-performs my old rack and pinion style focuser by leaps and bounds.
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