A telescope that uses only lenses is called a refracting telescope, or simply a refractor. It gathers and focuses light entirely through glass lenses rather than mirrors, bending (refracting) incoming light to produce an image. Refractors are the oldest telescope design, the type most people picture when they hear the word “telescope,” and they remain popular today for both casual stargazing and serious astrophotography.
How a Refractor Works
A refracting telescope has two main optical elements: a large objective lens at the front and a smaller eyepiece lens at the back. Light from a distant object enters the tube and hits the objective lens, which bends the rays inward so they converge at a single point called the focal point. The eyepiece then magnifies that focused image so your eye can see it in detail.
The focal length of the objective lens, the distance between the lens and the point where it forms a sharp image, determines how much magnification the telescope can achieve when paired with a given eyepiece. Longer focal lengths produce higher magnification but require a physically longer tube. This is why many refractors are noticeably long and narrow compared to mirror-based designs of similar power.
The Color Fringing Problem
Glass lenses bend different colors of light by slightly different amounts. Blue light bends more than red, so the colors don’t all land at exactly the same focal point. The result is a defect called chromatic aberration: faint rainbow fringes around bright objects like the Moon or Jupiter. This was the single biggest limitation of early refractors, including the telescopes Galileo built in 1609, which could only magnify about 20 to 21 times and suffered noticeably from blurred, color-fringed images.
Telescope makers eventually solved this problem by combining multiple lens elements made from different types of glass. Each glass type bends light differently, so pairing them can cancel out the color separation.
Achromatic vs. Apochromatic Lenses
Modern refractors come in two main optical grades, and the difference matters if you’re shopping for one.
An achromatic (or “achromat”) refractor uses two lens elements to bring two wavelengths of light, typically red and blue, to the same focal point. This eliminates most visible color fringing and works well for casual observing. Achromats are the more affordable option and make up the majority of entry-level and mid-range refractors on the market.
An apochromatic (or “apochromat”) refractor uses three lens elements and corrects all three primary wavelengths of light, red, blue, and green, so they converge at the same point. This virtually eliminates chromatic aberration, producing images with pinpoint color accuracy and high contrast. Apochromats cost significantly more, but they’re the preferred choice for astrophotographers who need clean, color-accurate images.
Many modern apochromatic refractors use extra-low dispersion (ED) glass, which contains rare-earth compounds that dramatically reduce color separation beyond what standard optical glass can achieve. The result is sharper images with better contrast and true-to-life color rendition.
Why Refractors Stopped Getting Bigger
The largest refracting telescope ever built sits at Yerkes Observatory in Williams Bay, Wisconsin. Its objective lenses are each 40 inches (about one meter) in diameter, weigh 500 pounds, and require a tube 62 feet long to reach the proper focal length. It was completed in 1897, and no one has built a larger refractor since.
The reason is physics. A lens can only be supported around its edges, since light needs to pass through it. As lenses get larger, they sag under their own weight, distorting the image. Mirrors, by contrast, can be supported from behind, which is why every major observatory telescope built after Yerkes switched to mirror-based (reflecting) designs. Today’s largest research telescopes use mirrors 25 to 39 feet across, a size that would be impossible with lenses.
Refractors vs. Reflectors in Practice
For backyard astronomy, refractors have several practical advantages over mirror-based reflectors. Their sealed tube design keeps dust, humidity, and debris away from the optics, so they need very little maintenance beyond an occasional wipe of the front lens. The lenses are permanently fixed in alignment, which means you never have to collimate (realign) them, a regular chore with reflector telescopes whose mirrors can shift during transport or temperature changes.
Refractors also tend to deliver higher contrast on bright targets like the Moon, planets, and double stars. Reflectors place a secondary mirror in the center of the light path, creating a small obstruction that slightly reduces contrast. Refractors have no such obstruction, giving them an edge for detailed planetary viewing.
The trade-off is cost per inch of aperture. Because quality glass lenses are expensive to manufacture, a 5-inch refractor can easily cost as much as an 8- or 10-inch reflector. Since a telescope’s light-gathering ability depends directly on the size of its main optic, reflectors offer more raw power for the same budget. This makes reflectors the better choice for observing faint, deep-sky objects like galaxies and nebulae, where sheer aperture matters most.
Who Should Use a Refractor
Refractors are ideal if you want a low-maintenance telescope you can grab and go without fussing over alignment. They excel at lunar and planetary observing, where contrast and sharpness matter more than raw light-gathering power. A quality 3- to 5-inch apochromatic refractor is also one of the best tools for wide-field astrophotography, producing clean, color-accurate images across a wide field of view.
If your main goal is seeing faint deep-sky objects on a budget, a reflector will give you more aperture for your money. But if you value portability, ease of use, and crisp high-contrast views, a lens-only refractor is hard to beat.