ARCHIVED - Picking a Telescope

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January 06, 2006— Ottawa, Ontario

Telescope

A telescope is a marvelous tool for exploring the night sky, providing you pick the right one. The trouble is that there are lots of "wrong ones" out there, so one needs to be careful. The right choice can lead to years of enjoyment; the wrong choice can end up as a bulky dust collector, taking up valuable storage space.

A telescope does two things: it makes things look closer and it gathers enough light to provide a useful image. For looking at wildlife or ships in daylight, the magnification is probably most important because even small telescopes can collect enough light to form a good image. However, this is not true in astronomy.

In astronomy collecting as much light as possible is of paramount importance and magnification is a much more secondary issue. The final element is a secure, steady tripod for the telescope. Nothing is more annoying than getting fleeting glimpses of what you are trying to look at as the tripod wobbles. Magnification doesn't only make things look bigger; it also makes the wobbling worse.

Telescopes come in two main types:

  1. Refractor telescopes: the light is collected by a large lens – called the objective – at one end of the telescope and formed into an image that is magnified by another lens – the eyepiece – at the other end. The ordinary sea captain's sort of telescope is of the refractor kind.
  2. Reflector telescopes: this kind uses a mirror as an objective. Since light gathering power is crucial in astronomy, and it is easier to make and support a big mirror than a big lens, most astronomical telescopes are of this type. Light from most things in nature is made up of many colours. Unless lenses are well designed, the colours making up the starlight won't be in focus at the same point, so the images may have coloured fringes. Mirrors don't have this problem
The moon

When buying a telescope for the family astronomer, there are some nasty problems lying in wait. Although some of the astronomical telescopes sold in department stores are perfectly good, there are many that aren't. Usually these look horrendously hi-tech and offer incredible magnifications, such as 400 times. Since there is no value in having a telescope magnification exceeding two times for each millimetre of objective diameter, claims like this are not meaningful. To avoid problems, buy your telescope from a speciality store, such as your local science store. IF YOU ARE NOT AN EXPERT, DEAL WITH SOMEONE WHO IS. If you are buying a refractor, don't go for an objective smaller than 75 mm. For a reflector, a mirror 150 mm in diameter is a reasonable minimum size. As we indicated last week, a good plan would be to buy a book such as Terence Dickinson's Nightwatch, which contains important information on choosing telescopes or binoculars, read those chapters before going shopping, and then make the book part of the present. It's worth the trouble. Make that first look at the sky through a telescope something the family astronomer won't ever forget.


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National Research Council of Canada
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