Telescope Basics

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The Mount is as important as the telescope

Image courtesy of Astronomics
  1. Least expensive mount, most bang for the buck; a variation of the altazimuth mount.
  2. Home assembly possible: made of wood, plastic, Teflon.
  3. The telescope tube becomes part of the mount assembly, so that the center of gravity of the large reflector tube is always over the center of rotation on both axes. This eliminates the need for heavy counterweights, and is extremely stable.
  4. Used with large reflectors (10 inch aperture or greater).
  5. Incompatible with astrophotography, unless you're very serious. Yet Dobsonian telescopes, with their large apertures, are very compatible with real-time observation. Remember that the astrophotographs taken with smaller scopes are long-exposures, with detail and color never seen by the owner's eye. The alternative to astrophotography is actually seeing details with your eyes in real-time (as opposed to letting the camera "see" it for you).
  6. An 8-inch Dobsonian makes a great first telescope for the beginner (see the review in the January 2000 issue of Sky and Telescope).

General features of telescopes

  1. Aperture
    • Diameter of objective lens or mirror.
    • Most important feature of any telescope.
    • How wide an aperture should I get? As wide as you can afford! It varies as to which type of telescope you choose, as explained below (a 4-inch refractor costs as much as a 10-inch reflector with comparable performance). Yet no matter how wide your telescope is, most objects will remain "barely visible." To enjoy a telescope requires contentment with visual subtleties and the patient control of one's "aperture fever."
  2. Objective lens or mirror
    • Largest diameter lens at the star-end of a refractor.
    • Largest diameter mirror at the bottom or base of a reflector.
    • Although there are many variations, the basic idea is that the objective creates a cone of light that comes to a focus at the focal point. The focus is then magnified for viewing by an eyepiece.
  3. Light Gathering power
    • Depends on aperture.
    • Allows clear, detailed viewing of faint objects.
    • Telescopes are light buckets, or light funnels, more than image magnifiers.
  4. Resolution
    • Refers to crisp, distinct images. Poor resolution means fuzzy images.
    • Greater the larger the aperture.
  5. Magnification
    • Usually expressed with a number and an "x" for "times." For example, a magnification of 20x means that an image appears twenty times larger than actual size.
    • A rule of thumb for practical performance is to expect a maximum of 50x of useful magnification per inch of telescope aperture.
    • Equals focal length of objective lens/mirror divided by focal length of the eyepiece.
      • Sample calculation: A refracting telescope has an objective lens with a focal length of 50 cm. The eyepiece used has a focal length of 10 cm. What is the magnification?
    • A higher magnification:
      • Dims images. Twice the power, one-fourth the brightness; half the power means four times brighter.
      • Decreases field of view.
      • Requires better viewing conditions (air stability).
      • Larger apertures compensate for these effects.
    • When shopping for a telescope, ignore claims about magnification. Buy aperture, not magnification.
    • Answer to sample calculation above: 5x.
  6. Eyepiece
    • Use eyepieces to obtain the desired total magnification as suggested in the table below. Calculate total magnification by dividing the focal length of the objective by the focal length of the eyepiece, as noted above.
    • Don't buy a telescope without reserving some money for good eyepieces!
    • Magnification


      Wide field of view (the upper end of this range will be about half a degree, or about as wide as the apparent diameter of the moon). Useful for sweeping Milky Way; greatest ease in identifying star fields.
      General observing; rich field observing of nebulas, open clusters, galaxies.
      Globular clusters and smaller nebulae
      Narrowest field of view.
      Detail of moon, planets, close double stars. Requires better than usual viewing conditions.



Catadioptrics (Schmidt-Cassegrain)

Image courtesy of Celestron

Image courtesy of Celestron

Coda: The Joy of Amateur Astronomy

Spend half of my money on the mount? You must be kidding!!

Nope. As an example, here's how Paul Baughman prepared his backyard to receive a 5-inch refractor.

Above: Note the solid pillar in the center. That's Joey Baughman and their cat standing beside it.

Below: Joey Baughman and his grandmother Carolyn Baughman survey the 600-E German Equatorial mount fastened to the top of the pillar.


Above: The 5-inch refracting telescope from Astro-Physics has found a home in Strawberry Point, Iowa. Compare its length to Joey.

Paul says: "Since I'm too old and too fat to become an astronaut this is the closest I'll get to outer space." Thanks, cousin, for leading the way to the stars!

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