Instruments of Astronomy
'Telescopes'



Until the Italian scientist Galileo Galilei aimed his crude "spyglass" skyward in 1609, all stargazing had been done naked-eye. Over the years, many telescope designs have been created, but they are all only modifications of the two basic types: refractor and reflector. The word "telescope" was coined in 1648 from the Greek word teleskopos meaning "farseeing."A refractor passes light through a glass objective lens or combination of lenses at the front opening (aperture) of the telescope tube. The light then passes through the focusing lens called an eyepiece (also called an ocular lens). Refractors excel at lunar, solar, and planetary views. Small refractors (under 4-inch aperture) are easy to transport and will give you decent views of nebulae. The larger the aperture, the longer the refractor has to be to work properly. Refractors with apertures larger than 100mm (four inches) require very long tubes which in turn makes them very expensive and bulky. The bending of the light through the objective also causes the various light wavelengths to come into focus at different distances from the objective. This can be overcome to a certain extent by the use of different configurations of lenses, but not entirely. Except for the most expensive models, small refractors generally suffer from some amount of achromatism, which results in colored haloes and cross-like patterns around bright stars. For deep sky work, you would need a large high-quality refractor, but their size and bulk make them difficult to transport to a dark site. Focusing is accomplished by moving the eyepiece closer to or farther away from the objective lens. Refractors require very little maintenance ast the lenses are the only optical components exposed to dust and moisture. Good refractors with an aperture over four inches are very expensive. You can get more telescope power for your money by purchasing a telescope with mirrors.A reflector bounces and focuses the light off a curved mirror and passes the light through an eyepiece. Reflectors are best at deep-sky work, are generally easy to transport, and are the cheapest per inch of aperture to manufacture. Reflectors are better for deep-sky observing than refractors, because they give sharper star images and are more compact thana similar power refractor. The size of a reflector is determined by the diameter of its primary mirror. The most popular reflector style is based on the design created by Scottish mathematician James Gregory (1638-75) in 1663, then improved five years later by Sir Isaac Newton. The Newtonian design employs a secondary mirror, angled at 45 degrees to the primary, mounted in the light path to direct the image to the eyepiece. Focusing is accomplished by moving the eyepiece drawtube in and out of the telescope until the sharpest image is obtained. To align (collimate) the optics, the primary mirror is adjusted to be in alignment with the secondary mirror. One problem with refleftors is that their mirrors are exposed to dust and moisture and are hard to keep clean. Their reflectiveness slowlyh degrades as the mirrors tarnish.A popular telescope design is the Schmidt-Cassegrain. In this catadioptric type of instrument, the light passes through a glass corrector plate, bounces off a primary and secondary convex mirror mounted on the corrector plate, passes through a hole in the primary, then through the eyepiece. This design allows for a long focal length telescope in a compact package that is easy to transport and set up. These telescopes are good overall astronomical units. They give excellent views of solar system and deep sky objects. The closed design also protects the mirrors from dust and tarnishing moisture. The design for this type of reflector was conceived, in 1672, by the French doctor Jacques Cassegrain (also known as Guillaume Cassegrain and N. Cassegrain) (1652-1712). Little is known of him and it is believed that he never constructed a telescope of this design. James Short (1710-68) was the first person known to build a Cassegrain-type telescope. Estonian astronomer Berhnard Voldemar Schmidt (1879-1935) developed the concept of passing the light beam through a curved corrector plate as it entered a reflecting camera. (Robert Garfinkle, Star Hopping Your Visa to Viewing the Universe, pp. 25-28).Dobsonians are reflecting telescopes constructed from cheap materials. They were invented by John Dobson, a populist proselytizer who championed the view that the worth of the telescope should be measured by the number of people who get to look through them. Dobson is credited for saying, "To me, it's not so much how big your telescope is, or how accurately your optics are figured, or how beautiful the pictures you can take with it; it's how many people in this vast world less priviliged than you have had a chance to see through your telescope and understand this universe. That is the one thing that drives me!"Dobsonians employed the same simple design that Isaac Newton dreamed up when he ewanted to study the great comet of 1680- a tube with a concave mirror at the bottom to gather starlight, and a small, flat secondary mirror near the top to bounce the light out to an eyepiece on the side- but they were made from such inexpensive materials that you could build or buy a big Dobsonian for the cost of a small traditional reflector. You couldn't buy a Dobsonian from John Dobson, though; he refused to profit from his innovation. Poverty was his habit, and the mother of his invention. Dobsonians made it possible for stargazers of average means to own large telescopes. Observers armed with big Dobsonians didn't have to content themselves with looking at planets and nearby nebulae: They could explore thousands of galaxies, invading deep-0space precincts previously reserved for professionals. (Timothy Ferris, Seeing in the Dark, pp. 37-39).