Above: Optical layout of a typical refractor
When most people think “telescope” they think of a refractor. The very first telescopes, including Galileo’s, were refractors. In fact, all telescopes were refractors until Isaac Newton invented the first reflecting telescope in 1669, six decades after Galileo first pointed his telescope to the heavens.
How Refractors Work
The main purpose of all telescopes is to gather light. This is contrary to the common belief among first-time stargazers that the most important function of a telescope is to magnify objects. While telescopes do magnify objects, the most important thing they do for astronomical observing is to gather much more light than the observer’s eye alone could. Refractors gather light by using an objective lens located at the front (skyward) end of the telescope. The objective lens consists of two or more pieces of glass which focus the light to a point at the back of the telescope where an eyepiece is placed for viewing (or a camera for photographing). The larger the objective lens, the more light the telescope can gather and thus the more detail can be seen by the observer.
Refractors provide a correct image, meaning everything appears right-side-up. This makes refractors well-suited as multi-purpose telescopes for both terrestrial observing and stargazing, unlike Newtonian reflectors which produce inverted images and are only suited for astronomical observing.
Types of Refractors
Refractors come in two basic types, achromatic and apochromatic. The difference lies in the quality of the objective lens in terms of correcting an effect called chromatic aberration. Chromatic aberration occurs because of the familiar prism effect of glass. When light passes through the objective glass to be focused, each wavelength (or color) of light is bent by a different amount. In a simple lens consisting of a single lens, this results in red light focusing farther from the objective than blue light. In other words, the focal length of the objective is longer in red than in blue.
Above: A simple lens focuses red light farther from the lens than blue light, yielding chromatic aberration
To correct for chromatic aberration, two lenses can be used in the objective. A two-lens objective is called a doublet. Each piece of glass has different optical properties, allowing chromatic aberration to be minimized (but not eliminated). This type of objective is termed achromatic. In an achromatic objective, the design of the optic is chosen so that red light and blue light focus to the same point. This leaves green light focused to a slightly different point. When dark adapted, the human eye is most sensitive to green light and much less sensitive to red and blue. The green light is considered the primary wavelength in a refractor as this is what the observer’s eye will focus on when observing. This means the red and blue light will end up slightly out of focus because these colors are focusing slightly farther from the objective than the green light.
Above: An achromatic objective focuses different wavelengths of light to different points
The diagram above greatly exaggerates the focus difference between green and red/blue light for clarity. In actuality the difference is typically around 1/2000th the focal length of the telescope, or around 0.5mm on a typical small refractor. Still, this amount or chromatic aberration, or false color, can be seen as a purple halo around bright objects such as planets and bright stars. In order to bring red, green, and blue light to the same focus point, three lenses must be used in the objective. This type of color correction is known as apochromatic, and a three lens objective is called a triplet. An apochromatic objective can minimize chromatic aberration to the point where colored halos cannot be seen around bright objects.
Above: An apochromatic objective focuses different wavelengths of light closer to the same point than does an achromat
Advantages and Disadvantages of Refractor Types
The obvious disadvantage of an achromatic refractor is that it suffers from some chromatic aberration which can detract from the quality of the image, especially on certain objects such as the planets. However, the drawback to an apochromatic refractor is that it is more expensive, due to the need for extra glass (and for another more important reason that will be seen below). Achromatic refractors are typically inexpensive telescopes (although more expensive than a Newtonian reflector of the same size). They are well-suited to general observing, including both stargazing and terrestrial observing since they provide a correct image. They are not, however, very good for photography because cameras are much more sensitive to chromatic aberration than the human eye, so any aberration will become much more significant in a photograph than it is visually. For the most critical observing, an apochromatic refractor can provide some of the best possible views. Apochromatic refractors are also very well-suited to astrophotography.
Achromatic Refractor Pros & Cons
- Terrestrial/celestial observing
- Not very good for photography
- Usually less compact than a similar diameter apochromatic refractor
- Relatively inexpensive
Apochromatic Refractor Pros & Cons
- Terrestrial/celestial observing
- Excellent for photography
- Usually more compact than a similar diameter achromatic refractor
Types of Glass in Refractors
Part of the reason apochromatic refractors are more expensive than achromatic refractors is simply because they have more glass in them. But another reason is that they use different types of glass which are much more expensive to manufacture. While three lenses can bring threewavelengths of light (red, green, and blue) to a single point of focus, there are other wavelengths of light (violet, for example) that are still focusing to a different point. This means there is some remnant chromatic aberration. This leftover false color can be further minimized by using exotic types of glass.
Fancier glass types such as extra-low dispersion (ED) glass and fluorite spread out the different colors of light less than regular glass types. This means the remnant false color is less than it would be in an apochromatic objective using standard glasses. Such an apochromatic refractor is ideal for photography and high-resolution observing. Unfortunately, these types of telescopes are also among the most expensive.
One other application of these glass types is in higher quality achromatic refractors. By using ED glass or fluorite in a doublet objective, chromatic aberration can be significantly minimized without the need for three lenses. While not as well-corrected as an apochromatic refractor, these telescope offer a considerable improvement over a standard achromatic scope for a price below that of a typical apochromat. These high-quality doublet refractors are almost always called “apo” refractors, although by the strict definition of an apochromatic objective (bringing three colors to the same focus) they are technically only very good achromats. The quality varies among these types of telescopes, but even the inexpensive ones are quite a bit better than standard achromats, and the best are almost every bit as good as a true apochromat.
Refracting telescopes probably cover the widest price range of any telescope type. A good quality refractor will start around $200-300. This will get you a small (70-90mm aperture) achromatic refractor on a small equatorial mount. A good goto achromatic refractor will start around $300-400. High-quality doublet “apo” refractors are dropping considerably in price of late. With an equatorial mount, such scopes in the 80-100mm aperture range start at under $1000. Better “apo” doublets can cost $2000 or more for a 100mm aperture. True apochromatic triplets in the 80-100mm range can start under $1000 if they do not incorporate exotic glasses. The best apochromatic triplets will cost almost $1000 per inch of aperture for the telescope only (not including a mount). A high-quality 10″ refractor with a suitable mount might cost as much as a new BMW M3 sports car. Although, as cool as an M3 is, a scope like that will take you a lot farther a lot faster! But it can be seen why refractors are only common in smaller sizes and why less-expensive reflectors are much more popular in larger apertures.
Above: When the item on the left is the obvious choice, you’re truly an addicted astronomer. 0 to 60 million light-years in 4.8 seconds.
Is a Refractor Best for Me?
For beginners who want a telescope capable of both terrestrial and astronomical observing, a refractor can be a good choice. Above about 5″ inaperture, refractors become very large and fairly expensive, so 6″ and larger achromatic refractors are not very common. For advanced observers who are looking for a high-quality portable telescope or a great wide-field photographic instrument, an apochromatic refractor can often be the best choice.
Beginning observers who are not looking to do any terrestrial observing will usually be better off with a Newtonian reflector which will provide much more light-gathering power for the money. Schmidt-Cassegrain telescopes provide the ability to do both terrestrial and astronomical observing with a much more compact package, so intermediate observers often choose these telescopes over a smaller-aperture, similarly-priced refractor.