Refractors
The more expensive refactors are known as ED achromats and apochromats, with the differences being the level of color correction and cost. The differences between these two categories might be considered to be at the connoisseur level; it will take a greater level of experience or knowledge to quickly discern the differences between them. In very simplistic terms, achromats tend to cost a few hundred dollars, ED achromats often land in the neighborhood of a thousand dollars, and the best apochromats often cost several thousand dollars. Which type is right for you depends heavily upon how the instrument is to be used. More information on the differences is presented at the bottom of this page.
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. The impact of chromatic aberration is small at lower levels of magnification.
- 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. These are excellent instruments for visual observers.
- 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. They are higly prized by astrophotographers for their high fidelity color rendition, pinpoint stars, and black background..
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.