Building an 8" f/8 Open Truss Tube Newtonian Telescope
This type of telescope, the long-focus open truss, has always been a favorite of mine. It presents an attractive and impressive appearance and embodies several features that many observers consider desirable: the large focal ratio, considered preferable by many planetary and double star observers; the open tube, which eliminates the deleterious effects of warm air currents without the use of fans; and the light weight, thus reducing the need for larger and heavier mounts. When completed, the OTA, exclusive of mount, will weigh about 20lbs. The open tube structure also allows the wind to blow through and reduce the effects of buffeting motion on a long tube. The true Surrier double truss design can conveniently accommodate longer focal ratios of even f/10 and longer without adding significant weight by simply recalculating the length of the truss tubes and slightly extending the length of the central section to increase stability. The telescope may easily be re-scaled to accommodate a 10" or a 12.5" mirror. The telescope also breaks down into two half-length pieces for easy car transportation but does not require time-consuming re-assembly and re-collimation; four wing nuts does it all. I have also done some new work on the aesthetics of the design, particularly the central section where the boxiness has been reduced by the use the carefully contrived curved outer surfaces. External rings have also been eliminated for a smoother and sleeker appearance. The back end has been streamlined and simplified and made to match the front end in appearance. Collimation bolts have been recessed into the back plate much as you would see in a Cassegrain and are of the Allen head type with 20 pitch fine threads for better control over collimation. The spider has been improved and the suggested model is a curved vane type I have developed that appears to work very well and show no diffraction spikes on the brightest of stars.
I hope that this design will inspire the use of new construction techniques and other new ideas, but mostly I hope it will enable people to make a telescope of high quality and utility without spending a fortune. My techniques are fairly simple and my design simplified to the greatest extent possible, the result of years of thought and practical experience. I am not a machinist or a cabinet maker, but I am a thinker and careful thinking can accomplish a lot and overcome almost anything. There are no more parts in this telescope than are absolutely necessary. The construction is straight-forward and the goal utilitarian. This is not a furniture making project adapted to astronomy or a machine shop job - I'll leave that to others. There is nothing superfluous, yet everything you need for the best observing possible. I have purposely used simple and inexpensive tools; a basic table saw, a band saw, a small drill press and a power drill is about it. And one could replace the band saw with a hand held saber saw and the drill press with a hand drill and drill guide. You don't need a machine shop and lathe or a super equipped woodworking shop, just some basic skills, patience and the desire to have a great telescope.
Using things I have built is important to me and a source of great joy and satisfaction. I hope that these few ideas may help others build a telescope of their own that they can be proud of and give years of excellent observing.
I have divided this project into several sections as follows:
- General instructions
- Photographs of actual construction with detailed technique explanations - Tube and Struts
- Photographs of actual construction with detailed technique explanations - Saddle and Cell
- Building and installing the spider and secondary mirror
- Calculating strut length
- Parts List - Coming soon
- Accessories you'll need - Coming soon
- Set-up for observing - Coming soon