The telescope is an invention dating back to Renaissance times.
It is thought that telescopes were originally built by spectacle makers and improved by Galileo, however some argue that a Dutchman called Hans Lippershey was responsible. It is doubtful that either man would recognise the moden lenses.
There are several different types of telescope, and while the design in each is a little different, each of the telescope types does the same basic job of focusing light.
The way most telescopes work is by focusing light that passes into it. When you look at something in the eyepiece of a telescope you see a larger image, this is actually a result of the lens making the light converge; essentially concentrating the light it receives in order to make things appear larger or brighter.
Refractor Telescopes (Refracting)
The first of the telescope types created was the refracting telescope which makes use of precisely ground lenses to focus the light. Refracting telescopes are not usually used in large research telescopes because they are so large that gravity makes the glass lenses sag, creating distortion.
Refracting telescopes use the glass of the lenses to bend light, such as in a prism, in order to bring the light to focus. This basic design is the principle behind binoculars and telephoto lenses for cameras. This design isn’t without problems, though, it can focus different colors of light improperly, resulting in a blurring effect called chromatic aberration. Similarly, spherical aberration, imperfection due to the curved lenses used in telescopes, also creates blurred images.
Refractor telescopes are of the design most would envision when thinking of a telescope today. The long thin tube, a lens at one end, an eyepiece at the other, and an image focused and observed. After almost 400 years, this classic design remains relatively unchanged, and its primary purpose of star-gazing, exploration and discovery – also the same.
Invented by Hans Lippershey in 1608, the refractor telescope was actually developed for military use. Galileo Galilei however thought otherwise and took his knowledge of the invention and refraction and created an instrument of his own: for astronomical purposes. As a result, exploring and better understanding the universe was to take a giant leap forward.
Defined by its long thin tube design, the refracting telescope uses a lens (the objective lens) at the front end, to collect light. At the back of the telescope, there’s another lens – the eyepiece or ocular – that the observer looks through. The collected light is “refracted” or bent using lenses. It is then focused and magnified at the ocular end and through the eyepiece, the image is brought into view. Its best images are typically those from long distances and not surprisingly, the refracting telescope is best suited for observation of the moon, planets, the sky.
Indeed, refractor telescopes are highly regarded for their sharp images of long-distance objects. However, for shorter-length images, there is a tendency for chromatic aberration and/or spherical aberration, which can lead to a halo effect surrounding bright objects. This results in a less-than-clear view, which is a consideration when looking for the right telescope for to purchase.
Historically, there have been variants of the refracting telescope, which are still in use today. These include the Keplerian Telescope, which uses a convex lens rather than a concave one like Galileo’s. The difference is a wider field of view due to the refracting light rays converging. There is consequently, a higher degree of possible aberration. To counter this, a high f-ratio (the diameter of the “entrance pupil,” where the light enters the optical device) is needed.
In addition to the Galilean and Keplerian designs, there’s also the achromatic refractor, which uses two pieces of glass to create the objective lens, and the apochromatic refractor, which focuses three wavelengths in the same plane to help minimize chromatic aberration.
Observatories all over the world use refractors to get closer to the stars. Notable refractors include the Paris Observatory, Nice Observatory, and the Chabot Space and Science Center in Oakland, California, which features two refractor telescopes “Leah” (1883) and “Rachel.” Commissioned in 1914, “Rachel” is regularly open to the public and is the largest such telescope in the western United States.
Reflector Telescopes (Reflecting)
Reflector telescopes gather and shape light differently than refractor scopes, which are built to refract or bend light to form an image. Instead, reflecting telescopes use mirrors to reflect light, which is then focused and observed through an ocular or eyepiece.
Design-wise, a reflecting telescope features a concave mirror (the “primary”) at its bottom, which collects the light. The light is then focused at the “focal plane.” The “focal length” is the distance from the primary to the focal plane. This design eliminates chromatic aberration, though spherical aberration can still be an issue due to misalignment of mirrors.
The first reflecting telescope is believed to have been built by Niccolo Zucchi, an Italian monk and astronomer who used his invention to lay claim to a number of discoveries, including being the first to identify belts of the planet Jupiter in 1630; an examination of spots on Mars a decade later; and in 1652, the idea that phosphors didn’t store light, but rather, generated it. His telescope design would prove the inspiration for James Gregory and Isaac Newton.
Of the reflector designs, the two main ones are Cassegrain and Newtonian. In the Cassegrain model, there is a primary concave mirror (parabolic) which collects light, then a secondary convex mirror (hyperbolic) that reflects that light back through the primary. Both mirrors are aligned about the optical axis (the line which the light path passes). By “folding” the optics, the Cassegrain telescope can be a very compact design, of which there are also variants: Ritchey-Chretien, Dall-Kirkham, Schiefspiegler, and Gregorian (by James Gregory).
As for the Newtonian refractor telescope, Isaac Newton took the design by Zucchi and instead of a secondary mirror as is found in the Cassegrain model, a flat mirror is placed at a diagonal. This reflects the light at an angle into an eyepiece that is located on the side of the tube. The observer is thus positioned at the top of the scope, looking into the ocular which reveals the focused image.
As with refractors, there are advantages and disadvantages to the reflecting telescope, from the (positive) elimination of chromatic aberration to the (negative) potential problems of misalignment (e.g. the Hubble Telescope’searly years). These are some considerations that need to be asked before deciding on which type of telescope is best, as a starter, a secondary, or upgrade. It should be said that the Newtonian refractor is a popular choice for first-time telescope builders.
A reflecting telescope accomplishes the same thing as reflecting telescope, through using curved mirrors to reflect the light rather than lenses to pass through it. While attempts were made to create reflecting telescopes around the time that the first refracting telescopes were being put to use, it was Isaac Newton who managed the first working model.
Reflecting telescope types are favored by designers of large, research-grade telescopes, since big mirrors can be supported on the back, which helps prevent the sagging that that is common with large lenses. Some reflecting telescopes do suffer from spherical aberration as well as coma which causes stars that aren’t in the center of the field of view to appear wedge-shaped rather than as points of light.
The last of the optical telescope types are catadioptric telescopes, which combine refraction and reflection. Although they are given their own category, they are essentially just a combination of the other two telescope types. Because some designs of catadioptric telescopes are easily made and powerful for their size, they are often popular with manufacturers seeking to reach a mass market.
Catadioptric telescopes tend to correct most of the aberration problems of other telescope designs, though as a drawback, they are often designed with a secondary mirror in the center of the tube, which blocks a little of the light that comes into the telescope.</d
It is a testament to the simple, solid design of the telescope that the best improvement that can be made is to combine the best features of the other two telescope types. But no matter which type you choose to use, you will find yourself benefiting from the usefulness of this invention from centuries past.