How Does a Telescope Work?

It is a common misconception that the primary purpose of a telescope is to magnify the stars, making them larger and thus easier to see. While a telescope does magnify, the more important part of the telescope is that it brightens the image by bringing in more light, allowing you to better see what is being magnified.

There are many telescopes out there that advertise themselves as very powerful, offering as much as 500x magnification for little money. However, these telescopes are not very useful because they have small apertures that bring in little light. Images that you see in the telescope will always appear dark and indistinct.

There are two primary forms of telescopes, which brighten images in different ways.

Refracting Telescopes

A refracting telescope offers the sharpest images for the astronomer. It consists of a long tube with the aperture at one end and the eye piece at the opposite end. The tube narrows as it moves towards the eyepiece. This is the type of telescope that most people think of when they first think of a telescope.

The refractor works by gathering light through the aperture. (The higher the aperture, the more light can enter into the telescope). This light is sent through what is called the objective lens and focused as it travels through the tube. As the light is focused produces a sharp, clear image for the astronomer looking through the eyepiece. The image produced is upside down, however for the astronomer this is not an important matter.

Reflecting Telescopes

The reflecting telescope, like the refracting telescope, gathers light through the aperture. However, unlike the refractor it does not focus the light through a narrowing tube. The reflecting telescope is essentially straight, although there is some narrowing, and is thus larger and bulkier than the refracting telescope.

Unlike the refractor, a reflector does not have an objective lens. Light travels through the tube then is reflected off what is called the objective mirror at the end of the tube. This mirror is curved. The light then travels back up the tube, where it hits a second mirror. The second mirror reflects the light into the eyepiece, which sits near the top of the telescope.

The image produced in a reflecting telescope is not as great as that produced by a refracting telescope. However, for the same money a reflecting telescope can be purchased with greater aperture than one would get with a refracting telescope.

Reflecting telescopes are also called Newtonian telescopes, after Isaac Newton.

Catadioptric Telescopes

Catadioptric telescopes work on the same primary principle as the reflecting telescope. However, before light travels to the objective mirror it is first filtered through a correcting lens. This allows images to be produced in a shorter telescope than would be required in a reflector, so that you can increase aperture while not getting too large of a telescope.

Choosing the Telescope That is Right for You

In order to know what telescope is the right purchase for you, you should ask yourself a few questions. First and perhaps most importantly: how interested are you in astronomy? A telescope is a major purchase, although one can be purchased for about a 100 or 150 dollars, most telescopes that are going to prove worthwhile will run in at least the 300 dollar range, and only go up from there.

If you are interested in astronomy, but perhaps not that interested, there are other options. Even viewing the stars with the naked eye can be enjoyable. One can also purchase excellent binoculars for less than the cost of a telescope that will allow increased viewing of the stars without requiring a major monetary commitment.

How much do you want to see? Your ability to see with your telescope will depend greatly on the size of the aperture of your telescope. Refracting telescopes, for example, offer the sharpest image for the amount of aperture they provide, however they do not (and cannot) offer very large apertures, because they would be far too large and bulky past a certain point. However, a small refracting telescope can be great for casual viewing of the moon, the planets and double stars, but will not offer much in the way of deep space viewing.

Reflecting and catadioptric telescopes offer much larger apertures, however these larger apertures are also going to prove to be more expensive.

GOTO Telescopes

Something else that has hit the amateur astronomy market in recent years are GOTO telescopes. These are telescopes that offer computerized control with built in capability to direct your telescope towards celestial bodies without you having to do it yourself. This can make it much easier to find objects of interest without a lot of guesswork: the computer does it all for you.

While there are GOTO telescopes in the range of the average low-end telescope (300-500 dollars), these telescopes are not going to offer the same amount of aperture for the same amount of money as a refracting or a reflecting telescope. On the other hand, it will make your viewing of the stars all the more easy once you get going. The GOTO telescope has somewhat increased interest in astronomy, because of this ease of finding bodies in the skies.

Unless you are planning to purchase a GOTO telescope, the best advice is to find the largest amount of aperture that you can afford. This is going to provide you with the most viewing pleasure.

Telescope Mounts

Telescopes and telescope mounts are generally sold separately, although sometimes they can be sold as sets. Telescope mounts are very important, and something you should be thinking about when purchasing your telescope. For example, you might think that buying an expensive catadioptric telescope will allow you to have a large amount of aperture in a smaller scope, but once you include the telescope mount they can prove to still be rather bulky.

Most telescopes are too large to be held up by themselves, and even those that can a mount is better. Your hand is far from steady holding something of this size, and all you will find is frustration trying to keep the telescope focused in order to view the stars.

There are two primary types of telescope mounts: altazimuth and equatorial mounts.

An altazimuth mount moves both horizontally and vertically in relation to the horizon. It must be adjusted manually in order to keep celestial bodies in sight. This is the most affordable option for the amateur astronomer.

An equatorial mount moves both horizontally and vertically in relation to the celestial equator (hence the name equatorial). It can be moved manually with only one hand, and can also be hooked up to motor control so that you can move the telescope with the push of a button. While more expensive this can often be more convenient, especially for extended viewing.

One third type of mount is the Dobsonian mount, which is a box shaped mount used to hold reflecting telescopes with an aperture wider than 6 inches.

Telescope Eyepieces

Every telescope of course comes with an eyepiece, and many come with several. While the telescope itself provides the brightening and sharpening of the image, it is in the eyepiece that magnification occurs. Eyepieces can also provide other image adjustments as well.

Barlow lenses are commonly used for increased magnification. It is fitted between the eyepiece and the telescope.

Wide-angle eyepieces offer a wider field of vision than that offered by a normal eyepiece. If you wish to view a star cluster or some other larger object, these eyepieces will assist you.

Diagonal prisms are used to adjust the alignment of the image. All telescopes invert images when focusing them for the astronomer. A 45� prism will invert the image again so that it is right side up. A 90� prism will also invert the image but will flip it from right to left as well.

Using Your Telescope

Once you have chosen and purchased your first telescope, you are ready to go out there and start exploring the stars! Astronomy has been a passion of mankind since earliest times, and the study of the celestial bodies is a fascinating hobby for anyone.

It is always best when stargazing to find a place away from lights and distractions, offering you the cleanest view of the stars. Find a field, or a park, or drive out into the country if you can. Find a clean, empty spot where you have room to set up your telescope and start exploring.

If you can’t go out to a special spot, even your own backyard can work. Set up your new telescope on its mount, and you are ready to begin. It is as simple as that.