An explanation of Refractors and Reflectors

Refractor or Reflector telescope what’s the difference
Refractor telescope and reflector telescope are both types of telescopes that have a primary mirror to refract light from its point of entry into the eyepieces of the instrument. Still, there is a fundamental difference with respect to their design. If you look at a typical refractor – it has a single piece of glass; usually quartz or crown glass, mounted in an equally well-polished metal mount. The front element (height) is concave and acts as a lens for the incoming light. This can be anything up to 20 inches in diameter and 10 feet long. There are no lenses on a refractor, so only the central area of the front element (where the light enters) is used for focusing. Most refractors also include a collimation system that makes sure rays entering the eyepiece are parallel.
Image by Will Kalif

 

In contrast, a reflector telescope has two mirrors separated by a spacer. Light passes through its primary lens and strikes one mirror. It reflects off this mirror and hits another mirror. Sometimes these second mirrors are just small flat surfaces, and sometimes they may be illustrative for extra precision. The light reflected off the second mirror travels back towards the first mirror and strikes another mirror. Then finally, the light emerges out of the tube where it reaches your eyes. While the optical design may seem similar, the way the image is formed differs significantly. The secondary mirror is almost always convex in a reflecting telescope, not concave like the primary mirror in a refractor. A convex mirror can focus more light than a concave mirror. Therefore, a reflector telescope uses two high-quality mirrors instead of one small, low-quality one. This allows them to collect significant energy from even very dim objects while simultaneously improving resolution. This technology is called “light gathering power” because the same amount of light will go farther when traveling through space.
Image by Will Kalif

A reflector telescope also needs a way to direct the beam of light coming out of the eyepiece. For example, if you’re using an equatorial mount, the mirrors typically lie on axes offset from each other and perpendicular to the axis of the polar axis.

The focal length is determined by how far apart the two mirrors are placed and the size of the entrance hole in the primary mirror. When designing a new reflector telescope, we must balance the characteristics of both our primary mirror and secondaries – the larger the aperture, the greater the focal length, thus making it easier to use smaller-sized pupils (smaller holes).
Cost Differences:
Reflectors are cheaper than refractors and come available in many different sizes. You get what you pay for, though – a cheap refractor is likely to suffer from poor optics, whereas a high-end refractor will probably cost several thousand dollars and provide better performance. Likewise, while modern refractors can be found for around $1000, less expensive models can’t compete in terms of performance.
Physical Differences:
Refractors tend to be shorter in height since the primary mirror forms a magnified version of the object rather than entirely inverted. They also have much higher f/ratios (focal ratios), meaning that they generally require longer focal lengths to achieve the same magnification (and vice versa). While there are many exceptions, you can mount a 100mm refractor at the bottom of a stand and still see things without having to move the whole thing. You’ll eventually lose brightness as you increase magnification due to diffraction effects known as vignetting. The tradeoff here is in control through – a typical 6-inch refractor does not allow you any movement of the mount. On the flip side, though, you can keep your eye glued to the scope with little need to re-center yourself.
Since most people buy a telescope for observing rather than photographing, most telescopes sold fall under the category of “refractors,” not reflectors. The reason is simple – they are easy to build and transport due to a lack of moving parts. They also offer affordable prices, often starting under $300. However, the disadvantages are obvious – they are limited to visual observation. These days, the best optical observatories are equipped with excellent CCD cameras that allow us to see millions upon millions of stars at once. If you wanted to view planets, comets, or nebulae, you would have to look into a reflector.