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Telescope – Beginners Guide: Reflector vs. Refractor December 30, 2009

Posted by aquillam in Astronomy, telescope beginners guide.
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There are two basic types of amateur optical telescopes, reflectors and refractors. The  difference is in how they collect the light:  reflectors use a mirror, and refractors use a lens.

Even if you can’t look inside to see how the light is collected, you can usually tell them apart. In refractors, the light must travel all the way through the tube, so refractors are long for their width.  Since reflectors use a mirror, the light travels back and forth in the tub, so most reflectors are short for their width.

Basic Structure

Refractors

Refractors are generally long and narrow, with the wide objective at the front and small eyepiece at the back

Refractors always have a large lens at the front. You point this lens at the object you want to view, so it is called the objective lens. The light travels through the tube to the eyepiece, where you put your eye to view the object. If you look the wrong way through a refractor, you can still see the object you’re looking at, but minimized instead of magnified.

The refractor is the original and stereotypical telescope.  If someone wants to include a telescope in a movie (or other popular media), it is usually a refractor. Galileo and latter Kepler used refractors.  Spyglasses, prevalent in pirate movies, are refractors. However, they are usually not the best astronomical telescopes because they are limited in size and expensive to build compared to reflectors.

For more on refractors, see the entry on refractors.

Reflectors

Reflectors have a big mirror at the back, so they remain wide. The eyepiece can be located in several different places., so it is not shown here.

Issac Newton realized the limitations of refractors, and developed the first reflector.  Reflectors all have a primary mirror at the bottom of the tube. However, there are many different types of reflectors with eyepieces in back, or the side, or pure imaging ‘scope with a camera mount in the middle of the tube. Some reflectors even have lenses on the front. No matter what the configuration, if you look in the wrong end you see yourself rather than the object you’re trying to look at.

Reflectors are the more common type of telescope, and all large professional telescopes are reflectors, because they are cheaper and easier to build. There are many different types, and each have their own benefits and drawbacks.

For more on reflectors, see the entry on reflectors.

Differences

Large mirrors are much lighter than large lenses, and are much easier and cheaper to make. Mirrors only require one perfect surface,  they don’t need perfect glass or supports, and they don’t need to get thicker as they get larger. Lenses must be perfect all the way through and have two perfect surfaces. Wider lenses should also be thicker, or they’ll have ridiculously long focal lengths. Mirrors can be supported all across the back surface, but lenses can only be supported around the edges so large lenses may actually sag.

There are of course plenty of other differences, and each type of ‘scope has its own set of problems.  See the individual pages for each ‘scope.

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Comments»

1. Astrophotography Refractor Vs Reflector | Pafos Photos - March 20, 2011

[…] For more on refractors, aquillam.wordpress.com […]


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