Telescopes . . . BIG Eyes


Now that we have an understanding of what light is and why it is important to astronomy, how do astronomers study the light from stars and other celestial bodies? The telescope is the principal tool of the astronomer, and its primary function is to
 
 

Although, telescopes used at different wavelengths are very different, the information that we want from them are the same, and they all have some common characteristics.


What Information Do We Want Telescopes to Provide?

. . . in other words, what are we going to do with the light that the telescope gathers?
 
    1.
     
     

    2.
     
     
     
     
     
     


Performance Characteristics of a Telescope

Whether you are concerned with the design and performance of a gamma-ray, optical, or radio telescope, two characteristics determine the telescope quality.

Light Gathering Power (LGP)

 

 
 
 
 

What factors influence a telescope's LGP?

  1. Telescope Size:

  2.  

     
     
     
     
     
     
     
     
     
     

    For example, we usually talk about relative Light Gathering Power:
     
     
     
     
     
     

    So, we have the First Rule of Telescope Design:
     
     
     

  3. Exposure Time

  4.  

     
     
     
     
     
     
     
     

    For example, the exposure time of the Human Eye
     
     
     
     

    Note, this is why it can be somewhat disappointing when you actually look through a telescope. The images in books and other publications are almost always time exposure images of faint objects, sometimes several hours.

Resolving Power

 

 
 
 
 
 
 
 
 

Minimum Angle of Resolution =
 
 
 
 
 
 

For Example, the 5 meter Hale Telescope:
 
 
 
 
 
 

So, the Second Rule of Telescope Design:
 
 

What other factor limits the resolution of ground-based telescopes?
 
 
 
 
 
 
 

Optical Telescopes

  1. Refracting Optical Telescopes

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    Problems with Refractors:
     

    As a result, the largest refractor ever built is the one meter Yerkes Telescope (1890); however, excellent quality small refractors (3 to 5 inches) are still built and used by many amateur astronomers.
     
  3. Reflecting Telescopes

  4.  

     
     
     

    What shape must the mirror be?
     

    Reflector Designs:
     

    Advantages of Reflecting Telescopes:
      How big do Reflecting Telescopes get?

New Optical Telescope Designs

In an effort to construct bigger and bigger optical telescope mirrors, astronomers have devised some new techniques that will allow them to build some very large telescopes.

The Hubble Space Telescope

In an effort to get an optical telescope above the Earth's atmosphere and eliminate the problem of atmospheric turbulence and bad seeing, NASA designed the Hubble Space Telescope.
 

Radio Telescopes

After World War II, scientist pointed their newly developed radar detectors to the heavens and realized that there was an enormous amount of electromagnetic radiation in the radio part of the spectrum that was being emitted from astronomical objects . . . hence the birth of Radio Astronomy.

Radio Telescopes are big metallic dishes that focus radio waves to a common focus, but the big problem with Radio Telescopes is
 


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