Building an 8" f/6 Closed Tube Newtonian Telescope
This page is under construction at this time. Changes may occur.
This closed tube 8" f/6 Newtonian reflector represents my latest efforts to produce a Newtonian of moderate aperture that will produce images of the highest quality, as good as an apochromatic refractor. Yes, as good. And you can built this telescope at home for a relatively low cost and a little effort. It is important not to omit any the of the critical elements of construction. If you eliminate something you will get degraded performance. These critical, or as I call them, essential elements, include the following: a conical mirror, high-quality optics, curved vane spider and, if you have a closed tube, forced ventilation. You will get performance like an apo but at a fraction of the price. And you get real aperture. Last summer and friend and I tested one out and was amazed at its performance. But, once again, to emphasise, you can not selectively omit any of the essential elements.
One of the most important things in this telescope is the use of the conical cross-section mirror as opposed to the older constant cross-section mirror. It may not seem that important at first glance but the use of the conical mirror and how it is mounted is the basis for producing a truly different kind of Newtonian telescope. To begin with, a conical mirror has only about 60% of the mass of a standard full-thickness mirror, so it has much less internal heat to dissipate. But there are other subtleties in play. A conventional mirror has three other defects: 1. It needs to be held in a cell; 2. It has mirror clips; and 3. it can not be held firmly. This last defect is a concern for astro-imagers who complain about mirror shift. Conical mirrors attach firmly and do not shift. Mirror clips are far more destructive than might be imagined. The defining of the entrance pupil of a telescope is critical to the formation of the outer diffraction ring. Ever notice how carefully refractor makers make those cell retaining rings? But take a look at a lot of Newtonians; the mirror has three, huge, clunky mirror clips - clamps that extend over the surface, clamps with sharp, diffraction producing corners. What a way to begin the process of forming an image. The wavefront is wounded just as the battle has begun. With a conical mirror the edge is clean and clear - like a refractor. And then there is the business of removing heat from the mirror. With a conventional mirror the cell and the close proximity of the back of the mirror to the tip-tilt plate acts like an insulator and impedes the free flow of air and the process of radiation. With a conical mirror the back side is largely exposed to free-flowing air and internal heat easily radiates out and pulled away from the surface. The front coated surface of a mirror is reluctant to let heat out. Remember, aluminum reflects inward as well as it reflects outward, so the thermal equilibration we desire happens largely through the back and sides. The rear fan sucks air through the front of the tube. The air flows freely around the mirror and out the back. You don't need five fans to do the job - one is sufficient.
The curved vane spider is the other half of the new design concept. The new curved vane type I have developed appears to work very well and shows no diffraction spikes on the brightest of stars. After all, when was the last time you looked through an apo and saw spikes on stars? You see, we have just gotten used to and have come to expect and accept defective, low-grade performance from reflectors.
External tube rings have been eliminated for a smoother and sleeker appearance and 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 24 pitch fine threads for better control over collimation and coil springs have been replaced with belleville washer springs for increased lateral stability. Constant collimation is no longer required.
The f/6 focal ratio is the shortest that will give ultimate performance. I used to think that an f/8 was necessary but recent experiments have shown that f/6 instruments work just as well.
I hope that this design will inspire the use of new construction techniques and other new ideas and refinements, 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. 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.
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 cell
- Parts List
- Accessories you'll need - Coming
- Set-up for observing - Coming