Refractor Telescope
Written by Tammy Plotner
Are you interested in learning more about the refractor telescope? How does it work, what's its history and how did it evolve? What types of refractor telescope designs are there and what can you expect to see when you look through one? What is achromatic and apochromatic? If you have these kinds of questions and want more answers, then follow along as we take a closer look at the refractor telescope…
How Does A Refractor Telescope Work?
A simple refractor, or refracting telescope is a hollow tube which uses a primary lens at its opening to diffract, or change the path of incoming light waves. This primary lens is called the "objective lens" and is used to collect more light than the human eye. When light passes through the objective lens, it is bent – or refracted. Light waves that enter on a parallel path converge, or meet together at a focal point. Light waves which enter at an angle converge on the focal plane. It is the combination of both which form an image that is further refracted and magnified by a secondary lens called the eyepiece.
History of the Refractor Telescope
The very first type of refractor telescope was a very low magnifying power "spy glass" that many consider first introduced by Hans Lippershey, but there were also other opticians in the Netherlands at that time selling the same basic idea. The first refractors were a novelty, meant more for amusement than scientific research. It didn't take long before military implications were added and the race to improve the design began. Rather than buy outside their own country, Italy persuaded their own optical genius – Galileo Galilei – to try his own hand at the design. While many believe Galileo originated the refractor telescope, he greatly improved the design through trial and error with a convex objective lens and a concave eyepiece. However, Galileo and was the first to use it in a scientific manner to study the heavens.
One of Galileo's friends, Johannes Kepler, also improved upon the design. This time using a convex lens at both ends his refractor telescope. Now, instead of the restricted field of view of the galiean model, the light rays were able to converge as they passed through the eyepiece – instead of in front of it. This simple improvement gave a much wider field of view and improved the comfort distance the eye must be away from the eyepiece to achieve focus – eye relief. Yet, there was still a flaw. This arrangement caused the image to be inverted. Sure, the design was great because high magnification could be achieved with this style but the focal length of the telescope quickly became an issue. Imagine a telescope 150 feet long just to get it into focus! You can't fault Hevelius for trying…
The Evolution of the Refractor Telescope
Unfortunately, other design flaws quickly plagued the beginning refractors. By changing lens configurations to make shorter focal lengths, chromatic and spherical aberration was introduced. Since each color in the spectrum has its own wavelength, it gets focused in a different position when it is refracted and focusing all optical wavelengths of light into a single point by refraction is impossible. However, the longer the telescope – the less the chromatic aberration and unwanted color smears.
Another initial problem with early refractor telescopes is called spherical aberration. Lens grinding methods were crude compared to modern standards, and the edges suffered the most. Not only would the light be bent too much or not enough for good focus, but the act of holding the objective lens by the edges causes the glass itself to sag! So many things could go wrong as the lens size and grinding proportions changed… Even the glass itself was extremely difficult to get pure enough for high quality lenses.
Close to a century passed before a man named Chester Moore Hall came up with a solution – use two objective lenses that had different dispersion properties! "Crown" glass made from alkali-lime silicates has low refractive index and low dispersion factor. "Flint" glass is high in lead (used in lead crystal), but extremely pure with high refractive index and low dispersion. By polishing the surface and joining them together, the red and blue spectrum could now come to focus on the same plane.
Types of Refractor Telescopes
The process of combining two objective lens styles together in the crown and flint fashion was later independently discovered and patented by John Dollond. This design quickly improved and the "achromatic refractor" telescope was born. The design became immediately popular and the joined two objective lenses were referred to as a doublet. Over the course of the years, many doublets were created and known by such names as Littrow, Fraunhoffer, and Clark after their engineers.
Design improvements have changed very little since that time, but the quality of the glass has dramatically increased. Today's "apochromatic refractors" have objectives built with special, extra-low dispersion materials referred to as ED glass. The introduction of flourite has also helped to overcome chromatic aberration as well. However, lens sag may never be overcome and giant refractors will never happen because of gravity.
Looking Through A Refractor Telescope
As with any telescope, the size of the primary or objective light gathering source is the key. The larger the primary, the more light can be gathered revealing ever fainter objects in greater detail. However, since a refractor telescope lens can never be large – why has the design endured? Unlike a reflector telescope, the refractor tube is sealed. Like looking through glass, looking through air also causes a certain amount of diffraction, as well as turbulence and heat wave issues. Because the interior of a refractor is never exposed to the outside air, the view is considered to be far more crisp and steady – making the refractor ideal for studying planetary details or resolving close double stars. Since both the primary and secondary light gathering source are locked into place, this also means the refractor telescope requires less maintenance to keep its optical parts aligned.
Another consideration on behalf of the refractor telescope is its ability to be used for both terrestrial and celestial applications. Special additions to the viewing end of the telescope called a "star diagonal" will invert the image again so that it is correctly oriented and give the viewer a more comfortable position when aimed at the zenith. The small size and portability of the refractor also makes it an excellent choice for travellers and modern binoculars are nothing more than a pair of twin refractors!
Filed under: Astronomy
