A. GENERAL & HISTORY

Note: microscope invented 1654, also in Holland. Responsible for opening up a second kind of "invisible world."

1. Collect more light: detect fainter objects---most important function;
   • Light gathering power depends on diameter²

   • Thus, a 10-in diameter telescope collects (10/5)² = 2² = 4 times as much light as a 5-in telescope.

   • An 8-in telescope (widely used by amateur astronomers) collects 1600x more light than the human eye. Can detect over 2000x as many stars (10 million compared to 5000).

2. Resolve sources better: see more detail; depends on both diameter of telescope and optical quality

3. Magnify sources: make image larger for easier study

B. DESIGNS

• An objective or primary optical element forms an image (i.e. an accurate representation of original scene) at a usable focus, where it can be studied by eye, recorded by film or other detectors (as in a camera), or fed into yet other instruments

• Refracting: the objective is a lens (shaped, transparent glass) which refracts (or bends) light rays to a common focus. The image at the left below shows how a flat glass surface bends light rays (in this case, two flat surfaces at a angle combine to make a prism). The shorter the wavelength, the stronger the bending.
  The image at the right shows how a glass surface can be continuously curved to bring all the light rays passing through it from a distant object to a common focal point.
  Galileo's telescopes were refracting. Largest refractor: 40-in diameter (built 1896).

  Refraction of Light By a Prism (click for descriptive animation)	Shaped Convex Lens

  
  

• Reflecting: the objective is shaped mirror, which reflects rays off its front surface to a common focus. See picture below. Invented by Gregory; improved by Newton.
  The mirror is easy to support from behind, unlike a lens which must be supported from its edges and tends to sag. Reflectors have many other advantages. (Click here for more information on these.)
  All large telescopes are therefore reflectors. Largest reflector: 400" (10-m) diameter (built 1993).

  Reflection of Light by a Figured Mirror
• Applet. Here is a Java applet illustrating the differences between refraction, reflection, and diffraction.

• For distant objects (including all astronomical objects), the incoming rays are parallel to one another. Such rays are focussed in a plane which is one focal length from the objective. This is called the "focal plane." Click on the button below for a Java applet illustrating image formation for objects at different distances.
  Applet

• In the focal plane, the light rays from a distant object form a one-to-one representation of the distant scene which is called an image.

• Ordinarily, a camera or other instrument in placed at the focal plane. For "visual" use of a telescope, an eyepiece can be used to magnify the focal plane image so it can be viewed by the eye. See the illustration above.

C. IMAGE QUALITY

• Optical Figuring:
  • Light is a wave. In order to produce a good image, telescope optics must be figured to a minimum tolerance of about 1/4 of the wavelength (distance between crests) of the light they are intended to focus. For optical telescopes, this is about 10^-5 cm.

  • Scale comparison: if a 320" (8-m) diameter telescope mirror were scaled up to the size of the continental United States, i.e. about 3000 miles diameter, then the maximum ripple allowed in its polishing would be only about 2 inches!

  Diffraction

• Diffraction:
  • A fundamental limit on resolution is set by the physics of light. Since it is a wave phenomenon, light spreads out or diffracts when it passes through an aperture. This smears out images.

  • Diffraction is worse the longer the wavelength of light and the smaller the telescope aperture. Click here for illustrations of how telescope size improves resolution.

  • A 10-in diameter telescope with perfect optics can resolve 1 arc-sec.

  • Note that all stars are so distant that they are smaller in angular size than 1 arc-sec and therefore appear as point sources in such a telescope. Only a handful of stars can be resolved by even the largest telescopes.

  
  

  
  "Seeing" Produced by Earth's Atmosphere

• Seeing:
  • The Earth's atmosphere also refracts light, and because it is constantly moving, there is always a blurring and jittering of images in a telescope. Astronomers call this "seeing." Seeing actually dominates diffraction in most cases and usually limits resolution in practice to 0.5-2 arc-seconds.

  • Above is an enlarged image of the bright star Betelgeuse seen though a large telescope. It is a large blob, broken up into smaller near point-like units. Click on the image for a video of the seeing effects:

  • To partly overcome seeing effects, special equipment such as adaptive optics can be used. Or, telescopes can be placed in space (where there's no atmosphere).

D. CURRENT MILESTONES:

HST has produced the highest resolution images yet obtained at visible wavelengths, with blur sizes of only about 0.05 arc-seconds. This is because of its high quality optics AND the fact that it is above the Earth's atmosphere, so it does not have to contend with seeing. Its high resolution also allows it to detect very faint sources.

Because the Earth's atmosphere screens out many parts of the the EM spectrum (see Study Guide 10), telescopes for the gamma-ray, X-ray, ultraviolet, and parts of the infrared and microwave spectrum must be placed on spacecraft outside the atmosphere.

E. DETECTORS

• Detects only 1-2% of incident photons but allows long integrations (hours)
  
• Provides permanent storage of info, though not digital
  
• Large formats (up to 20" square for astronomy)
  
• Was the main astromonical detector used between 1900 and 1980.

• Solid state electronics; widely used now in video cameras & TV
  
• Astronomical applications pioneered during development of Hubble Space Telescope (1974-80).
  
• Works well at both very short (TV) and very long (astronomy) exposure times
  
• 50-100x more sensitive than film
  
• Digital image storage for immediate computer processing
  
• Small formats (2-in typical) but can "mosaic" to create large areas
  
• Now are the standard detectors used in astronomy

Sunset over the William Herschel
Telescope (La Palma, Spain)

Study Guide 14
Optional: Seeds textbook Chapter 6

Seeds textbook, Sec. 21-2, 22-1
Study Guide 15

Early history of telescopes, with references.
More detailed notes on telescopes (Nick Strobel).
Hubble Space Telescope

HST Photo Gallery

Large Binocular Telescope
Keck Observatory
Very Large Telescope (VLT)
National Radio Astronomy Observatory, Charlottesville