This set of notes by Nick Strobel covers: the basics of telescopes and effects of the atmosphere on images. Vocabulary terms are italicized.


Types of Telescopes


Telescopes--instruments used to gather and focus light. Visible and large parts of radio spectrum can be observed anywhere on ground; small parts of infrared (IR) must be observed above water layer in atmosphere (elevations of 2750 m (9000 ft) or above); gamma-rays, X-rays, UV, most IR, and rest of radio must be observed in space. The part collecting the light is called the objective.

    Refracting Telescopes


  1. Refractor--use lens to bend light. First ones built. Disadvantages: a) Chromatic Aberration--redder colors bent less than blue colors so see rainbow of colors around the image. Use multiple compensating lenses to counteract this OR have really long objective focal length to minimize effect (see drawing). This is why the early refracting telescopes were made very long. b) Support for lens only at ends of lens. Tends to sag under own weight. 40 inches is maximum size of refracting objective built.


    Reflecting Telescopes


  2. Reflector--use mirror to reflect light. All modern research ones are this type. Advantages: a) No chromatic aberration. b) Support for objective all along one side so they can be BIG! Parabollic-shaped mirror focuses all parallel light rays to single point (remember that celestial objects are so far away that all the light rays from an object hit the Earth as parallel rays). Focus is before the eyepiece, so the image in astronomical telescopes (even with the refractor above) is upside down. Terrestrial-viewing telescopes use other lenses to re-invert the image right-side up. Spherical Aberration--mirror not curved enough (shaped like part of a sphere) so all of the light is not focussed to a single point. Hubble Telescope objective suffers from this so it uses corrective optics to compensate.


Powers of a telecope



    Light Gathering Power

  1. Light Gathering Power--telescope acts as a light bucket so the bigger the objective, the more light is collected and the image is brighter. Faint objects only seen with BIG objective telescopes.

    Resolving Power


  2. Resolving Power--ability of telescope to see really small details so objects that are close together in the sky are easily seen as separate. Absolute minimum resolvable angle in arcseconds = 1.22 * 206,265 * (observation wavelength) / (objective diameter). The wavelength and diameter must be measured in the same length units. Radio wavelengths are LARGE so the radio telescope objective must be LARGE to get decent resolving power. The atmosphere will usually smear images so this theoretical resolving power not reached with telescopes on ground. Speckle interferometry can get rid of atmospheric distortion. Adaptive optics is another way to remove atmospheric distortion.



  3. Magnification--ability of telescope to make image bigger. Least important power because it magnifies telescope distortions and atmospheric distortions so a small fuzzy faint blob only becomes a big fuzzy fainter blob. Magnifying power = (focal length of objective) / (focal length of eyepiece); both focal lengths in same units.

Atmospheric Distortion



  1. The air is in turbulent motion and light from celestial objects is bent randomly in many ways over time periods of tens of milliseconds. Images dance about (twinkle) and images are blurred. This atmospheric effect is called seeing. Good seeing is when the air is stable and the twinkling is small.

    Reddening and Extinction


  2. The air also absorbs and scatters different wavelengths in different amounts. Redder light is scattered less by atmosphere molecules and dust than bluer light. Since blue light scattered more, we have a blue sky and objects appear reddened when viewed through lots of air. Some forms of light scattered at every wavelength and do not make it to the surface--- extinction of that radiation at every wavelength. Get on high mountaintops to look through less atmosphere and less distortion. Even better, observe from space!

    Atmospheric lines

  3. Gases in atmosphere can create absorption lines light from celestial object. Need to remove atmospheric spectral lines from spectroscopy data, otherwise you'll find a hot star with molecular nitrogen, oxygen and water lines!


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last updated 29 Aug 95

Nick Strobel -- Email:

(206) 543-1979
University of Washington
Box 351580
Seattle, WA 98195-1580