Reflectors - Refractors - Binoculars - Schmidt Cas- Mountings - Eyepieces - Filters - Goto Telescopes
Lenses and mirrors invert and reverse the images we view. Different combinations give different results:
Binoculars: Correct view, as seen without optical aid.
Zero or two mirrors, as in straight through a refractor and a reflector: The image is upside down and backward.
One or three mirrors, as in a refractor with a diagonal mirror, or Schmidt-Cassegrain: The image is rightside up, but backwards.
Refracting Telescopes
Refractors look like the classic
telescope, and are in fact directly related to the first telescope to
be pointed toward the heavens by Galilio in 1609. They really have changed
very little and operate by the same principle. Light enters through a
curved lens at the front and is refracted to a point at the back where
another lens, the eyep
iece,
magnifies the gathered light.
From the very beginning, refractors suffered from a problem caused by refraction of light. Not all wavelengths, or colors, meet at the same point. The result is that the object being viewed is surrounded by a halo of color. The original single lens objective was soon replaced by one made with two lenses, correcting the problem to a great degree. Unfortunately, focal lengths must be fairly long for all wavelengths to converge close to each other. For this reason, a refractor of 6 inches diameter is very big and heavy compared to its counterpart, the reflector.
Two-element lenses, called achromats, must be figured on 4 surfaces, as opposed to one for a reflector. Refractors cost a great deal more, inch for inch, than a reflector. But there is a desirable side to refractors. Unlike telescopes that use mirrors, refractors have no central obstruction. The image in a refractor tends to be much sharper than in reflectors. A 3inch refractor has greater resolution than a 4 inch reflector
Most of the color problem is solved by refractors made with 3 objective lenses, called apochromats. The greater expense, however, again keeps the size down. A 6 inch apochromat will cost several thousand dollars, compared to an 8 inch reflector starting around $600.
Many new, inexpensive refractors made in China come with short focal lengths and are quite manageable and affordable. However, they suffer more from color distortion. Some of this can be corrected with the use of a filter called a V-Block that eliminates the blue fringe seen around bright objects such as the moon and planets. A 4.7 inch telescope of this type, offered by all the major telescope companies, runs around $600 and makes a good choice for a first time telescope buyer.