Why don’t you just buy one?
That’s a question I’ve heard countless times, and when it comes to building a telescope, you really have to think about it twice. After all, buying a decent telescope is quite easy and relatively cheap. You can get a good view at the moon, the planets and some galaxies for a few hundred dollars including shipping to Whitehorse. On the other hand, building a telescope is an arduous task; you have to find the materials, build your own tools and spend hundreds of hours around a polishing stand. it also involves a lot of patience and precision. At first sight, anyone sensible would take 15 minutes to place an order on-line and wait for the telescope to arrive. Well, call me crazy… I chose the DIY option.
As a maker, I always favor building over buying, even if it costs more money for the final product. You simply can’t buy the pleasure and knowledge that you gain by building something yourself.
Making the mirror of the telescope is the most time consuming part but also the most interesting. First, you need to order a circular piece of glass with a low coefficient of expansion (pyrex, zerodur, borosilicate, etc). Then you need to build a circular tool out of waterproof plaster, cover it with tiles and rub it against the glass with silicon carbide in between. Using finer grit will slowly make the concave surface smoother. Once you’ve reached the desired sagitta, you can polish the surface using a different tool; this one is covered with pitch and the polishing agent is cerium oxide. When you have a nice polished surface, you need to transform that spherical surface into a paraboloid. This is where the time consuming part begins. During that step you will remove a minute amount of glass to approach the perfect theoretical shape. If you complete that step successfully, your mirror will have a surface so regular that if you were to stretch it to the size of a football field, the highest default would only be a thousand of an inch high. Of course, to control the surface with such precision, you need to build a special instrument which takes even more time and material.
When you’ve reach that step, it’s already been a few month since you began the project. However, if you managed to reach the desired precision, you most likely have a better mirror than most commercial mirrors.
Once the mirror is complete, it is sent for aluminizing. During this operation, a thin coat of aluminium is evaporated onto the surface of the mirror; this requires a vacuum pump and a high voltage source. As much as I would like to do it myself, I reckon it is not really worth building an entire vacuum chamber for a single mirror.
The focal length is measured on the finished mirror. We will use this measurement to design the tube of the telescope. I wanted to use something nicer than plywood so I went for red cedar trims that I resawed to get 1/4″ boards. These boards where assembled together using bird’s mouth joinery to form an hexadecagonal tube (16 sides). Some baffles were laser cut on YuKonstruct’s Epilog laser cutter; these will prevent internal light reflection. The tube was then painted black inside and coated with several coats of spar varnish to make it dew proof.
The last step consists of putting everything together. A hole is drilled on the side with a hole saw to accommodate the focuser; a cell is built for the primary mirror and a support is made to hold the elliptical mirror in place. I went with a curved vane for the ease of build and for the fact that it will limit diffraction spikes around bright stars.
As I was busy building other things, it took about a year to complete the instrument. On the first afternoon after completion, I tried to locate Jupiter in the evening sky; it took about 5 minutes to spot it with the naked eye because the sun was still shining bright above the horizon. Once I found it, I aligned the scope and focused on the planet. As a first observation, I didn’t know what to expect. Well…turns out I saw details on Jupiter that I never saw before on other instruments.
I am now really eager to try it on deep sky objects on a dark winter sky. Next project: make a proper stand for the instrument.
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