Refractors Virtues and Vices

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. See Chapter 5 "The Art of Collecting Life (with a Telescope)" for an explanation of the limitations of refractors.

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:

What is the resolution of your telescope? Here's a good rule of thumb: To find the minimum separation (in arcseconds) of a double star (see Chapter 17, "Of Giants and Dwarfs: Stepping Out into the Stars") that can be resolved with a telescope, divide 12 by the aperture (in cm). Thus a telescope with a 4-inch (about 10 cm) diameter mirror could resolve stars that are separated by a mere %, or 1.2 arcseconds, depending on the seeing that night.

Astronomer's Notebook

What is the resolution of your telescope? Here's a good rule of thumb: To find the minimum separation (in arcseconds) of a double star (see Chapter 17, "Of Giants and Dwarfs: Stepping Out into the Stars") that can be resolved with a telescope, divide 12 by the aperture (in cm). Thus a telescope with a 4-inch (about 10 cm) diameter mirror could resolve stars that are separated by a mere %, or 1.2 arcseconds, depending on the seeing that night.

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.

Astro Byte

Price comparison: A high-quality 4-inch refractor from a leading manufacturer sells for $995. The same manufacturer sells a high-quality 10-inch reflector for about $900.

Astro Byte

Price comparison: A high-quality 4-inch refractor from a leading manufacturer sells for $995. The same manufacturer sells a high-quality 10-inch reflector for about $900.

Astronomer's Notebook

Rich-field telescopes are an exception to the under-$400-is-junk rule. A rich-field instrument in the $300 range can give very good value for the money. Just don't expect a high degree of magnification or stability.

Astronomer's Notebook

Rich-field telescopes are an exception to the under-$400-is-junk rule. A rich-field instrument in the $300 range can give very good value for the money. Just don't expect a high degree of magnification or stability.

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