Friday, April 4, 2008

Refractor and Reflector

Most astronomers agree that a good refractor is the instrument of choice for viewing the moon and the planets. Typically, the refractor’s field is narrow, which enhances the contrast offered by good optics and brings out the details of such things as the lunar surface and planetary detail.
Refractors, however, are not the best choice for deep-sky work—looking at dim galaxies, for example. They are great for bright objects, but a refracting telescope with the same light-collecting ability of a decent reflecting telescope would be prohibitively expensive.
Some of the cheapest, mass-market telescopes are refractors, but most of these will perform poorly. Most good refractors are long, heavy, and expensive—although the recently introduced Meade ETX-60AT and ETX-70AT are compact yet high-quality entry-level instruments. The disadvantage of expense is obvious, as is that of weight: You’ll be discouraged from taking the telescope with you on trips to the dark skies of the country. Length poses a less obvious problem. The longer the tube, the less inherently steady the telescope. A large refractor requires a very firm mount and tripod.

Traditionally, the Newtonian reflector has been the most popular telescope with experienced amateurs, although, in recent years, affordable Schmidt-Cassegrain and Maksutov-Cassegrain instruments have found increasing favor. Generally, a reflector gives you more aperture—and thus more light—for your dollar than a refractor, and the reflector’s mirror is not subject to chromatic aberration (the differences in the ways various colors, especially red and blue, are focused), which all but the most expensive refractor lenses suffer from. Although reflectors may be large, they are generally lighter than refractors; however, they tend not to be as robust, and unlike a good refracting telescope, they do require at least some minimal maintenance to realign optical components occasionally.

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