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Telescopes are designed to work like the human eye, but with the capacity to collect much more light than a human eye can collect.  This is why the “size” of a telescope is so important. The size refers to the telescope’s aperture, or diameter. Think of a telescope as a light collecting bucket.   The diameter determines how much light can enter. The more light that can enter, the brighter the image. I am fond of referring to my telescope as my light bucket!

Rhiannon's 10
10" Reflecting Telescope on Dobsonian Mount

The 2 basic types of optical telescopes are REFRACTING TELESCOPES and REFLECTING TELESCOPES:

REFRACTING TELESCOPE (AKA Refractor):   A refracting telescope uses lenses to bend and focus light.  Of the different types of telescopes, a refracting telescope operates most similarly to a human eye.  The objective lens in a “refractor” is like the lens of our eye.  It is convex, so it bends light to a point.  The distance at which the bent light comes into focus is called the focal length.  In our eyes, this point is on our retina.  In a refracting telescope, this is where the eyepiece catches the light and magnifies it.  This is the type of telescope that Galileo used. The biggest obstacle with this type of telescope is that it tends to get very heavy and very long as the diameter of the objective lens increases to let in more light.  George Hale is famous for building the largest refracting telescope in the world in 1890 and housing it at theYerkes Observatory in Wisconsin. 

Denver has its own giant refracting telescope at Observatory Park in the Chamberlin Observatory.  

Concave vs. Convex

 yerkes 40 in refractor

Inside the Chamberlin Observatory in Denver
Inside the Chamberlin Observatory in Denver

 REFLECTING TELESCOPE: (AKA Relflector):  A reflecting telescope uses a large parabolic mirror as its objective lens.  This component is convex in shape, and is referred to as the primary mirror .  The diameter of this mirror determines the telescope’s ultimate light collecting capability; the aperture.  The primary mirror is situated inside the truss tube. If we continue to think of the telescope as a “light bucket”, the primary mirror is situated at the bottom of the bucket, waiting to catch and focus the incoming light pouring in.  The focal length of the primary mirror determines how long the truss tube must be.  It must be long enough for the incoming light to hit the parabolic mirror and refocus at a focal point, where the secondary mirror bounces the crisply focused image toward the eyepiece to be magnified.

DiagramReflecting Telescope; inside view of vintage Astroscan
Vintage Astroscan

Reflecting Telescope
Dobsonian Reflector at SBO
Reflecting Telescope; inside view
Peering inside of a Reflecting Telescope


Here are a few important values you may want to know how to find for the telescope that you are working with!

Focal Ratio:This is also called f-number, and it is written as a ratio:  f/4.5

Focal Ratio= (focal length)/(diameter of primary mirror)

So if I have a telescope with a 10in primary mirror, and a 45in focal length, its f-number is f /4.5!  

4.5 = 45 / 10

Magnification:This is the the power of magnification possible for a specific telescope when using different eyepieces.  This value is found by dividing the telescope’s focal length by the focal length of the eyepiece.

Magnification = (focal length of primary mirror) / (focal length of eyepiece)

**hint: the focal length of the eyepiece is always printed on the eyepiece in mm.  Remember to convert to like terms before dividing!!

Some other types of Telescopes include......

RADIO TELESCOPES: Radio telescopes 

also collect light waves from the electromagnetic spectrum, but not in the range of the visible spectrum.  Radio waves have a longer wavelength and a lower frequency than visible light. A radio telescope is a large “dish” antennae.  The size of the dish is like the aperture of an optical telescope. Since the radio waves being intercepted and studied are coming from distant locations, even beyond our galaxy sometimes, a huge amount of radio waves need to be collected in order to provide effective date.  Often, a group of these antennas are linked together in an array, using interferometry, in order to collect more data over a larger area. A famous example is the Very Large Array in New Mexico.  It is possible to build a small array to use in amateur astronomy and as a learning tool.  Here is a student paper f a student Radio Telescope Project:

Radio Telescope antenna; VLA
Very Large Array

INFRARED TELESCOPES:   Infrared telescopes


These cutting edge instruments use lasers to detect slight ripples in space time caused by gravitational waves from distant cosmic events.  Check out the links on Wiki to see current projects and how they work!  The most famous is LIGO

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