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Refractors / Refracting Telescopes
Refractors or refracting telescopes are a design that use only lenses to focus the light; every other type of telescope relies on one or more mirrors to do that job. The long, slender optical tube of a refractor is the image that most people associate with a telescope. Refractor telescopes have the advantage of having an unobstructed light path, which produces higher visual contrast than other optical designs. Stars appear as pinpoints against an inky black background when viewed in a refractor.
The disadvantage to refractors is the cost per unit aperture; lenses are simply more expensive to produce than mirrors. Eight-inch Newtontian reflectors often cost less than 80mm refractors, however, the visual experience is quite different. Apertures larger than what is commonly available in a refractor are highly desirable for observing deep space objects. Having said that, however, we think that anyone who is truly serious about amateur astronomy should own a good quality refractor of 80-105mm aperture. See more information about refractors below.
More about Refractors
Not all refractors are created equal. Refractors may be categorized into three groups that effectively equate to "good," "better," and "best." These groups are achromats, ED achromats, and apochromats. The difference comes down to the level of color correction. The level of color correction can in turn determine the highest useful magnification of the telescope. Allow us a moment to explain chromatic aberration and color correction.
Light passing through a lens bends in proportion to both its wavelength and the refractive index of the lens material. What that means is that red and blue light will end up in different places after passing through a lens. The technical term for this is chromatic aberration. All refractors use at least two lenses in an effort to reassemble the visible spectrum back into one place. Achromatic refractors will exhibit some chromatic aberration, or false color on bright objects, and will exhibit some limitations in maximum magnification. False color is normally seen as a violet fringe or haze near the edge of bright objects such as the moon, Venus, Jupiter, or bright stars.
Chromatic aberration can be reduced by using lenses made of low refractive index glasses. ED achromats are visibly superior to simple achromats, especially at higher magnifications. ED achromats will exhibit some residual false color on the brightest objects, such as the moon, Venus and Jupiter. These instruments are typically capable of at lease 2X magnification per millimeter of aperture, meaning that an 80mm instrument should be cable of exceeding 160X magnification.
Apocrhromatic refractors are advertised as being free of chromatic aberration, or false color within the visible spectrum. A true apochromatic refractor should be capable of bringing three widely separated wavelengths of light into focus at the same point. When viewing a bright object such as the moon through an apochromatic refractor, the white lunar limb will transition directly to the inky black of space without the violet fringe seen in achromats. Apochromatic refractors are also able to exceed the rule of thumb 2X of magnification for every millimeter of aperture.
The dividing lines between ED achromats and apochromatic refractors are sometimes blurred by manufacturers claims. Unfortunately some manufactures advertise their ED achromats as apochromats. At least one of our vendors, namely Stellarvue, clearly differentiates its ED achromats from its apochromatic triplets. On the other side of the coin, Takahshi uses the "ED" nomenclature to describe what are arguably some of the finest apochromaic refractors available today.
