While many people understand that telescopes are viewing devices that magnify faraway objects, fewer of us are aware of how exactly telescopes enhance our vision. Our eyes alone are unable to see faraway objects due to the fact that these objects don’t take up enough space on the retina of your eye. A telescope essentially uses lenses, mirrors or a combination of both to collect more light from a distant object to make it appear brighter to your eye, allowing you to see it.
While a refractor telescope gathers light through a series of lenses, reflector telescopes use mirrors to magnify images. However, regardless of how a particular type of telescope gathers light, all telescopes effectively magnify images through the following steps:
- Once a telescope takes in enough light, it brings that light to a single concentrated point called the focus.
- The eyepiece magnifies the light from that focus to take up a large space on the viewer’s retina, allowing a person to see a close-up image of a faraway object.
If an image is blurry or not magnified enough, the viewer can manipulate the telescope’s aperture, the diameter of the telescope’s objective lens or primary mirror. The larger the aperture, the more light the telescope can gather and, therefore, the brighter the image will be.
Keep in mind that you can use different eyepieces with the same telescope to get different degrees of clarity and magnification. In fact, many astronomers typically own several eyepieces to use for different circumstances.
Refractor Telescopes: It’s All About the Lens
While Galileo was the first astronomer to use a refractor telescope centuries ago, technological advancements have allowed this type of telescope to remain popular in modern times. Refractor telescopes usually consist of the following basic elements:
- an eyepiece lens
- an objective lens, the largest lens that gathers the most light in a refractor telescope
- the tube that holds these two lenses apart at the correct distance for optimal viewing.
In general, refractor telescopes are best for viewing the details on planets or looking at binary stars, rather than for viewing galaxies, nebulae and other faint deep-sky objects. This is primarily due to the fact that refractor telescopes can’t get a large enough aperture to effectively view larger celestial bodies.
Depending on the brand you choose and the extra features you want, refractor telescopes can cost anywhere from $100 to upwards of $3,000.
Reflector Telescopes: Mirrors for More
Created by Isaac Newton, reflector telescopes use mirrors to gather light instead of lenses. This type of telescope uses a primary mirror, which functions like the objective lens of the refractor telescope, along with a secondary mirror that focuses and transmits light to the eyepiece.
The focal ratio, also called the f/number, is how you measure the brightness of the image. Mathematically, the focal ratio is the numerical result of dividing the focal length (the distance from the mirror to the focus point) by the aperture length. Rich-field (also known as wide-field) reflector telescopes have short focal ratios with wide fields of view and are great for viewing larger celestial entities, such as comets, star clusters and galaxies.
While reflectors are extremely popular telescopes today, they do have some drawbacks, such as coma, a phenomenon in which an object at the edge of the viewing field appears elongated and blurry. Another downside to reflector telescopes is that they can require more maintenance than their refractor counterparts in that you do need to occasionally clean and align the mirrors. However, reflector telescopes are usually less expensive than refractor telescopes, which can make them a good choice for those who are just starting to practice astronomy.
Like refractor telescopes, reflector varieties usually cost anywhere from $100 to upwards of a $3,000.
A catadioptric telescope, also referred to as a compound telescope, combines the features of both refractor and reflector telescopes by using a combination of lenses and mirrors to magnify objects. The most popular type of catadioptric telescope is called the Schmidt-Cassegrain telescope (SCT).
In this type of telescope, the primary mirror has a much deeper curve than those in reflector telescopes to bring the focal plane closer (and, therefore, reduce the focal length). The secondary mirror is characterized by a positive curve (meaning it is convex, sticking out rather than curving inwards). The positive curve bounces the light back through a hole in the primary mirror so that it reflects to a final diagonal mirror that bounces the light to the eyepiece.
By folding light rays back on themselves three times, catadioptric telescopes significantly reduce the viewing defects associated with both refractor and reflector telescopes. As a result, catadioptric telescopes tend to be more expensive than other types of telescopes.
How to Build a Telescope
If you really want to save money and understand the principles of telescopes, then there’s no better way to learn than by building your own. Surprisingly, many amateur astronomers choose this path for their first telescope.
If you are considering building your own telescope, you may want to buy a kit that contains all of necessary pieces, as well as instructions on how to put them together correctly. Alternately, you can choose to grind the mirrors or lenses yourself and then find information online, in books and/or in astronomy magazines on how to build telescopes. Just be sure to choose a design that meets your needs and follow the instructions. Once you get the hang of it, you may even find yourself able to adjust the design accordingly.
Check out our Affordable Telescopes page for tips on amateur astronomy on a budget.