Reflector Telescope

Would you like to learn more about the reflector telescope? Exactly what is a reflector telescope, who invented it and how does it work? What's the history of the reflector telescope and its modern day uses? What can you see with it and how do you choose this type of telescope for yourself? If you have questions like these, or you'd just like to learn more, then follow along as we take a closer look….

What Is A Reflector Telescope?

Reflector Telescope Works

A reflector, or reflecting telescope uses an arrangement of one or more curved mirrors to gather light and return it along an optical path to a point of focus. The most critical element of this type of telescope is the major light gathering source – the primary mirror. Light strikes the parabolic, reflective surface of the primary and returns to a point of focus called the focal plane. Because each spherical or parabolic shaped primary mirror is slightly different, the distance the light needs to travel to achieve focus is called the focal length. At its focus point, the image (in a simple reflector telescope) is collected on another mirror surface called the secondary. The secondary mirror is then aimed towards the viewer who uses a series of lenses called an eyepiece to magnify the image and send it to the eye.

The History Of The Reflector Telescope

$Sir Isaac Newton\'s Reflector Telescope$

Sir Isaac Newton's Reflector Telescope

In the early seventeenth century, perhaps one of the very first reflector telescopes was built by a man named NiccolÃ² Zucchi. He had the right idea, but the mirror grinding process was primitive and it was difficult to place the secondary in the optical path without blocking the incoming light. Several design methods were tried, but it wasn't until 1668 that a diagonal secondary mirror was added to reflect the image at 90Â° angle. This engineering advantage belonged to Sir Isaac Newton and suddenly the the reflector telescope became not only practical – but superior to the other telescopes of the day. His simple concept has lasted to the present day and the design is still referred to as the "Newtonian Reflector Telescope".

Modern Reflector Telescope Designs

Since Sir Isaac Newton's day, the reflector telescope has continued to remain popular because it is the least expensive way to gather light over large surfaces. The size of the primary light gathering source is very important, because the more light that can be collected, the fainter the object of study can be and the better it can be resolved. While the Newtonian design has been adopted for some of the world's largest telescopes, it has undergone many design changes to create hybrids. Why is this important? Because the longer the focal length is (the light path between the primary and final point of focus) the more useful magnification can be applied. This is expressed in a simple term called the focal ratio and will be defined in terms like f/4 for a short focal length and f/10 for longer.

In order to achieve this long focal length without making the telescope tube longer, a reflector design was introduced called the cassegrain. Light enters the telescope, is collected on a parabolic primary mirror at the rear and is reflected to a hyperbolic secondary mirror mounted on a clear glass plate at the front of the scope. From there, the light is reflected and refocused once again back through a hole in the primary mirror where it is then magnified with an eyepiece. This process is what is known as "folding the optical path". The result is a short telescope body that's long on focal length.

Other types of reflector telescopes also used a folded optical path and their designs make them application specific – such as astrophotography. Reflector telescope hybrids are usually named after their designer, so you will encounter styles such as Schmidt Cassegrain, Dall-Kirkham, Ritchey-ChrÃ©tien, Dobsonian, Gregorian, Herschelian, Schiefspiegler and Yolo. Almost all major observatory telescopes in the world, such as Palomar, Mt. Wilson, Lick, and even the Hubble Space telescope are a type of reflector!

The Reflector Telescope and You

Before you buy a telescope of any type, remember this simple fact: The larger the light gathering source (aperture), the better the object is resolved and the fainter the object may be. This is why Observatories use very large reflector telescopes – to reveal very faint objects in greater detail. Do not forget that even a huge telescope will not make something larger just because the telescope is bigger! For example, the size of Jupiter magnified 50X will be the size of Jupiter magnified 50X in all telescopes… but a large aperture scope will allow you to see small details on the planet where a smaller aperture telescope cannot. That's resolving power! The smaller the numbers are when it comes to resolving power, the finer the detail.

If you choose a reflector telescope for yourself, please remember the more simple Newtonian designs will sometimes need a minor adjustment to the mirror positioning to align everything perfectly for the best possible image. This process is called collimation and is not much different than tuning a guitar. There are even tools to help!

What can you expect to see in a reflector telescope? For the average person, a reflector telescope with an aperture of 4.5" (114mm) will provide a lifetime of fascination with the Moon, planets, Sun (with special filter), double stars, star clusters and bright nebulae and galaxies. If you feel you may be interested in studying deeper, it is always best to go with the largest aperture you can afford. All reflector telescopes also come on an arrangement called a mount, so take the time to study a little bit and choose what you feel will work best for you. Some reflector styles only work for astronomical or photographic applications, but many designs – such as the cassegrain – can be multi-purpose. The very best part about a reflector is a 12" aperture can cost less than a 6" aperture refractor!

You can thank Sir Isaac Newton for that…

Filed under: Astronomy

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