Reflectors - Refractors - Binoculars - Schmidt Cas- Mountings - Eyepieces - Filters - Goto Telescopes
The Dobsonian Telescope:
Named for the inventor of this instrument, John Dobson, the design started a revolution in amateur telescope making circles. In its purest form, it is made from recyclable materials such as discarded sink counter top, plastic pads for bearings, tubes from construction sites, and glass found in ship yards.
Reflecting Telescopes
Reflecting telescopes have been around since the middle of the 17th century. The reflector was invented by Sir Isaac Newton who discovered mathematically that light reflecting off of a parabolic surface, unlike light refracted through a lens, focused all wavelengths, or colors, of light together at the same point. The loss of color distortion, chromatic aberration, greatly improved resolution.
The reflectors main, or primary, mirror is supported on the edges and back by a mirror cell. A smaller, secondary mirror sends the light out the side of the tube, so the image can be viewed without blocking the incoming light. This mirror blocks and scatters the incoming light, degrading the image to a degree. Short focal length telescopes require a larger secondary mirror so suffer more from this effect. They are used for low power, wide field views. The best reflector for planetary and lunar viewing has a long focal length and small secondary.
Since a reflector has only one surface that needs to be figured, unlike 4 to 6 for a refractor, reflectors cost much less, inch for inch, than refractors.
Telescopes with mirrors can be built much larger than refractors, and it is not uncommon to see amateur built instruments with mirrors of 16, 24, and even 40 inches in diameter. That is a lot of light gathering power.
There is a price to be paid with very large aperture telescopes. They become very heavy and Cumbersome to set up and use and of course are very expensive. Reflecting telescopes with 10 inches or less aperture are much easier and more practical to use, more affordable, and are taken out more often, so over time actually gather more light.
Considering the potential power and light gathering offered by large telescopes, "aperture fever" overcomes many amateurs who build their own.
Mirror alignment in reflectors requires frequent adjustment, called "collimation". The light bouncing off the primary mirror has to be aimed directly at the secondary mirror, and the secondary mirror must direct the light to the center of the eyepiece. Sound tricky?
It is not very difficult, but it takes a little practice before you can accomplish it quickly.
This can be done in the daytime. Aim the telescope at a bright object. Now look down the eyepiece tube with the eyepiece removed. To center your eye, punch a small hole in a film can and insert the can in the focuser. You should see a reflection of the primary mirror and on that a reflection of the secondary. If everything is properly collimated, all will be centered and it will look like a bulls eye.
If you can't see all of the primary centered in the tube, adjust the screws on the secondary holder. Now check to see if the secondary is in the center of the primary. If not, adjust the screws on the back of the mirror.