So the overall length of the scope is quite long and can be unwieldy for some. The focal length of the objective lens or mirror of a telescope will influence to some degree the overall length of a telescope. This 12″ telescope, which uses a large mirror to collect starlight, has a focal length of about 60″.
So if a telescope with a lens is aimed at a distant tree, for example, or the Moon, an image of the tree or Moon would be visible on a screen placed at the focal plane of the lens. At the focal plane of a lens or mirror, you can actually see a real image of a distant object. The length over which this happens is called the focal length of the objective. Once light falls onto a mirror or through a lens, it’s directed by the curvature of the optic to come to a focus at a plane some distance away. So even a modest telescope with a 100 mm aperture (about 4 inches) has (100/7) 2 = 204 times the light-collecting ability of the eye. The light-collecting ability of an objective lens or mirror is related to the square of the aperture.įor reference, the aperture of a healthy and dark-adapted human eye is 7 mm. The aperture of a lens or mirror is the diameter of its light collecting region. That’s the main tradeoff, and it’s one of the reasons not everyone has a 25″ Dobsonian reflector sitting in their garage. The cost and weight of a lens or mirror also go up proportionately, sometimes faster than the square of the aperture. So a telescope with an objective mirror of 200 mm aperture collects four times as much light as a scope with a 100 mm mirror. The light collecting ability of a telescope is directly proportional to area of the lens or mirror, which is in turn related to the square of the aperture.
#COST OF A GOOD TELESCOPE PROFESSIONAL#
Some big billion-dollar professional telescopes have mirrors with an aperture of 10 meters (400 inches), about the size of a small trout pond. A good backyard telescope for us amateur stargazers has an aperture of 80 mm to 300 mm (3.15” to 12”) or more. Aperture also influences most of the other key specifications of a telescope, including practical (but non-optical) specs like cost and weight.
More aperture makes for a brighter image. Aperture – Buckets of LightĪs mentioned in a previous article, the most important specification of any telescope is the aperture, the diameter of the main lens or mirror of the telescope.
#COST OF A GOOD TELESCOPE HOW TO#
Once you understand these 5 numbers, you will understand the similarities and differences between telescopes, and you will know how to choose the best scope for your own interests and budget. A 14″ Schmidt-Cassegrain telescope (credit: Celestron).īefore we launch into the pros and cons of the types of telescopes available to stargazers today, let’s have a quick look at 5 key numbers that describe the operation and performance of every telescope, from the junk scopes in a department store to the venerable Hubble Space Telescope.
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