How To Make A Telescope                    

Commercial telescopes are designed to meet customers' expectations. They demand light and transportable instruments, as cheap as possible. To meet these criteria, manufacturers reduce the quality of the mount by making it light and flimsy. Very good mounts do exist, they are strong and stable, but their cost is so high that you may want to invest this money in better optical parts and make the mount yourself.

Grind a mirror yourself is an adventure that requires a lot of time and skills. The financial advantage is not very obvious when you start.

However a simple mount can be much stronger and a lot more stable than what you could buy for the same price. You only need basic woodworking skills.

1. Why Making A Telescope?
2. Make Mount or Optics?
3. Technological Choices
       3.1 Start with a simple instrument
       3.2 Do not copy commercial instruments
       3.3 Do not hesitate making it too strong
       3.4 Avoid useless settings
       3.5 Materials
4. Examples
       4.1 Concrete Stand
       4.2 Friction of Axles
       4.3 Spider
       4.4 Barillet
       4.5 Hydraulic Eyepiece
5. Personal Experience

The technologies used by manufacturers are not available to amateurs. By trying to copy their solutions, you are taking the risk of making something too complicated, too expensive and too flimsy. And also the risk of not succeeding.

Always think about oversizing your telescope. Small parts can be broken during transportation or when manipulated in the dark. Do not hesitate to make things too strong or too heavy, that may be the most important piece of advice. The heavier an instrument is, the less sensitive to vibrations it is.

An adjustable part is weaker than a fixed part. Most adjustable tripods you find in stores are the example of what not to do.

Wood absorbs vibrations better than steel. It is also easier to work with. For the stand, concrete is particularly effective to make a stable instrument.

Vibrations

Touching the instrument, hitting your eyebrow against the eyepiece, kicking something because you have not seen where you were walking or a little wind: anything can make your telescope vibrate. Optical amplification only making things worse, the image starts oscillating to a certain frequency during a certain time. This defect is all the more annoying that the frequency is high and damping is long. A good mount damps vibrations in 1 or 2 seconds and its frequency must be around 1 oscillation per second  or lower.





The concrete block weighs 45 kg, the column, a concrete filled PVC pipe weighs 25 kg. The whole scope reaches 75 kg.

A column stand is mechanically less stable that a tripod but it is easier to move around, especially in the dark when you do not see where you step.

I did not have the appropriate tools to make a steel stand, therefore I cast a column of concrete in a PVC pipe. This heavy solution is simple and efficient.

Dobson telescope amateurs use teflon pads. This is justified by the significant length of their optical tubes that requires the azimutal axle to be as short as possible.

The most important is to minimize static friction and to keep a dynamic friction that will help keeping the telescope in position  in case of minor vibration.

You can use friction of steel against wood without using expensive teflon pads that are sometimes difficult to find. You can also use sealed ball bearings and add some friction.

The spider is the part holding the secondary mirror in the middle of the tube. It provides precise positionning of the secondary mirror on 3 rotation axis and 3 translation directions. It is the most complex part of the telescope.

The barillet orientates the primary mirror on 2 rotation axis. It is more simple than the spider but must be carefully designed because it supports the primary mirror and must not damage it.

Try the hydraulic focussing system. When you start with a small budget, it is the cheapest solution and its performance exceeds a lot of capstan and gear systems.

I bought a 150 mm in diameter mirror, focal length 750 mm, lambda/12 according to the manufacturer, a secondary mirror sold for it, a 7 x 50 viewfinder and a 15 mm orthoscopic eyepiece (50 times magnification with this focal length). It is a compromise that allowed me to start with a powerful enough telescope to avoid being desappointed.

From this equipment, I made 4 telescopes by reusing the optical parts of the previous one every time. The two first telescopes were not very stable, I took them apart to make the last one.