Refracting Telescope

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A refracting telescope can be a simple telescope produced by using two lenses.
The lenses are a large long focal length convex lens and a small short focal length convex (magnifying or one where the centre bulges out)lens.
For a class practical the lengths need to be reasonably small. 400mm (+2.5D)and 50mm (+20D) respectively. Other values may be suitable but as will be shown the total of the focal lengths must be less than 1m for convenience.
The class practical does not require good quality lenses but they should be reasonably sound. Various methods are used to mount the lenses and some comercial kits are popular. Convex or plano-convex (one flat side)lenses are equally useful
As the lenses are used to look at distant objects the light may be thought of as coming from infinity. The front lens or "Objective" lens will focus the light at the near (eye side) focus. The focus is then looked at with the second "ocular" or eyepiece lens. For the light entering eye to be parralel the focus of this lens has to correspond to the focus of the Objective lens.
The simplified diagram shows how this works. This is not the normal ray diagram for an Astronomical telescope as it does not show how a magnified image is produced. It does, however, show the inverted image and the location of the focii. It can be seen that having too long focal lengths can result in a large, unweildy telescope.
An astronomical telescope can be made into a "terrestrial telescope" with the addition of a third lens. Terrestrial telescopes create images that are not inverted. The "extra" lens (another convex) is insrted between the objective and ocular. For a corrected image to be formed the lens should be place at two of its focal lengths from the focus of the objective and two from the focus of the ocular. (image to follow)

For an alternative try making a refecting telescope.

External links telescope practical
(categories and external links to be added)
--D.B.Ferguson 20:43, 1 February 2008 (GMT) * Back to: Physics Experiments
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