Deciding which type of telescope to buy is no easy decision. There are many factors to consider but the first is likely what type of telescope to buy.
There are three main types of optical telescopes to consider. The types are defined by how the light is processed by the device.
- Refractor telescopes use lenses (dioptrics).
- Reflector telescopes use mirrors (catoptrics).
- Catadioptric telescopes use both lenses and mirrors.
Refractor Telescopes
A refractor telescope uses lenses to collect and focus light. These type of telescopes are known as dioptrics. The very first telescopes were refractor telescopes. Galileo Galilei, based only on descriptions of the first practical telescope created by Hans Lippershey, made a telescope in 1609 with about 3x magnification, though he eventually improved it to bout 30x.
Though the image was warped and blurry, Galileo was still able to see Venus, the Moon, and the moons around Jupiter.
How Does a Refracting Telescope Work?
A refractor telescope use two lenses to bend the light into focus. One lens, the objective lens, is fixed to the end of the tube and the other lens is in the eyepiece and is responsible for magnification.
The objective lens gathers the light and brings it into the tube. It creates the real image in the tube. at the focal point the real image become a virtual image that the eyepiece can then magnify. The bigger the objective lens is, the more light it can collect, which in turn makes it able to see thing farther away and with greater detail.
What Makes a Refracting Telescope Good?
When it comes to judging the quality of a refracting telescope, the main factors to consider are the quality of the lens, the diameter of the lens and the magnification of the eyepiece.
Quality of the Lens
The quality of the lens dictates the quality of the image. It is really that simple. A bad quality lens will give you a warped and distorted image. Imperfections in the lens from the manufacturing process are the culprit here. If you wear glasses, imagine a scratch on the lens. Besides driving you crazy, the image that comes through the scratch will interrupt the real image, distorting the quality.
Diameter of the Lens
Size does matter, at least when it come to the size of your objective lens. The more light an objective lens can gather the more detail it can gather. A 12cm objective lens will result in a 9x increase in quality over a 4cm lens. The diameter of the objective lens is called the aperture.
Magnification
The eyepiece magnifies the virtual image to the eye. Magnification has limits though and that manifests itself in the quality of the image. The amount of magnification that can be used is directly related to the aperture of the objective lens.
Aperture and magnification are the two more important consideration when choosing a refractor telescope. These two ideas will be the deciding factors on what you can see in the night sky as well as how well you can see them.
Although, they are important, there are other considerations for refractor telescopes. This brings us to the idea of focal length.
Focal Length
The focal length is the distance the light collected through the aperture travels between the center of the lens and the focal point of the real image. Focal length is measured in millimeters, but the measurement is not of the actual physical length of the lens, but rather the magnification properties of the lens and you’ll often see it printed or engraved on the front or back of the telescope, usually between about 400 and 3,000 millimeters. When looking at a telescope, it is usually safe to assume that a longer tube means a longer focal length and a shorter tube means a shorter focal length.
You would be forgiven if you thought the longer focal lengths are always better. It is a natural assumption, but the reality is that it depends on your application. If you are into astrophotography, for instance, short focal lengths are favorable.
We will dive deeper into these ideas in another post of Star School.
Technical Issues with Refractor Telescopes
One last thing we need to discuss is the inherent issue of a refractor telescope. One of these technical issues is size. If you have a small lens, you do not have the ability to just add magnification and have a clear image. A small lens hust can not let in enough light to show you clear images of far away objects. Think of a spyglass, they are not bad for closer distances, say across the street, but you will not see object that are very far away.
Large lenses have issues as well. Since they are held by their edges, a large lens may experience lens sag, due to gravity, if not properly held in place. This will lead to distortion of the image, called lens distortion.
Chromatic aberration is also a problem to be aware of in refractor telescopes. Light is made up of many different wavelengths that we see as color. Specifically, in telescopes, the red and blue light do not focus at the exact same distance, resulting in colored halos. This can be corrected using compound lenses made of different types of glass.
Variations of Refractor Telescopes
Achromatic Doublet
An achromatic lens is designed to limit the effects of chromatic aberrations. The most common type is the achromatic doublet, essentially two lenses designed to focus the red and blue light to a common focus. Many types of achromat have been devised over the years, using different materials, lens shapes, and optical properties, but it comes down to focusing away the chromatic aberrations.
Apochromatic Doublet
An apochromatic lens (or APO) is more of the same. As we came into the digital age we needed even better correction of the chromatic aberrations. In fact astrophotography must have an APO, especially when trying to photograph the ultraviolet to the near infrared wavelengths. They still use two lenses (doublet) but the quality and the precision are much higher.
Apochromatic Triplet
You guessed it, an apochromatic triplet uses three lenses. Still the same game though of correcting chromatic aberration. As our digital cameras get higher and higher quality our objective lenses must keep up.
Petzval / Quadruplet
A Petzval lens is essential two doublets with an aperture stop in between. It was developed by the German-Hungarian mathematics professor Joseph Petzval in 1840s Vienna. These are considered to be the highest end of refracting telescopes in terms of clarity. They are a favorite for many astrophotographers.
Reflector Telescope
- Newtonian
- Dobsonian
- Ritchey-Chrétien
- Dall-Kirkham & Other Cassegrains
Catadioptric Telescope
- Schmidt-Cassegrain
- Maksutov-Cassegrain
- Rowe-Ackerman Schmidt Astrograph
- Maksutov-Newtonian
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