Most entry level and inexpensive refractors fall into the category of achromatic refractors. These instruments behave much like their more expensive counterparts at low magnification and on objects fainter than Saturn. On brighter objects they will exhibit some false color, or a violet fringe around the object. They also will not reach the same levels of magnification as their more expensive apochromatic cousins. These limitations may not even be noticeable on f/10 instruments, but become more pronounced at faster (lower) focal ratios.
Refractor Telescopes & Apochromatic Refractor Telescopes
Refractors are the optical design that most people associate with a telescope. Like any other telescope, their primary function is to gather as much light as possible, rendering distant objects brighter and easier to see when magnified. Refractors do this by using an objective lens group at the front of the telescope, which uses two or more pieces of glass to focus light at the viewing end of the telescope. The bigger this objective lens, the more light the telescope can gather, and the more the viewer can see. Refractors equipped with a 90 degree star diagonal produce images that are upright but right-left reversed, making them useful for both terrestrial viewing and stargazing purposes.
There are three primary types of refractors: achromats, ED Achromats, and apochromats. Functionally they differ in the level of correction of chromatic aberration, or false color. Chromatic aberration is most commonly seen as a violet halo around bright objects, but there are other more subtle visual impacts. False color occurs because glass lenses alter the path of light in proportion to its wavelength; red and blue light exit a lens at different angle. All refractos have at least two lenses, and some have three or more, in order to get different wavelengths to come into focus in the same plane. Refractors are categorized according to their construction, which also corresponds with the level of color correction:
- Achromatic: An achromatic lens uses two-lenses in the objective, called a doublet, to correct for chromatic aberration. The two lenses are designed to focus both red and blue light at the same point, while green is focused to a slightly different point. Since the human eye reacts more to green light in the dark, the green light will appear brighter and more in focus compared to blue and red light. The result is a slight chromatic aberration, which often appears as purple halos around bright objects. Achromatic refractors are typically sold as inexpensive beginner telescopes.
- ED Achromatic: ED achromats utilize exotic high refractive index glasses to minimize chromatic aberration in doublet designs. Well executed examples of this type are often represented as apochromatic telescopes and can be nearly indistinguishable visually from the more expesive apochromatic refractors. Some false color may appear around the brightest stars in photographs, but it may not be objectionable to most users.
- Apochromatic: Apochromatic means virtually free of false color. Achromatic refractors often use three lenses in an objective, called a triplet, to correct chromatic aberration completely bringing red, blue and green light all to the same point of focus. Other more exotic examples use four or more lenses in multiple groups. The result is an image devoid of halos, producing pinpoint stars against a black background. This means apochromatic refractors are capable of higher magnifications and produce excellent images for astrophotography. The primary limiting factor for an apochromatic refractor, however, is its relatively higher cost compared to an achromatic or ED achromatic model.
Refractors have a wider variation in quality level than any other type of telescope, and the better ones are certainly the most expensive in dollars per unit aperture. That’s why choosing the right type of refractor for your needs is so essential — and Skies Unlimited can help. We’re committed to providing you with the type of refractor that will fit your needs exactly. We offer refractor telescopes for sale from all major suppliers, including Takahashi, TeleVue, Sky-Watcher, Stellarvue, Explore Scientific, and more.