The Tubes

   


A telescope focuses the image being viewed by changing the distance between the lenses. A convenient way to do this is to have one tube sliding in or out of another tube. In this kit there are three tubes sliding in or out of each other.

Lens and wavefronts


Baffles and Stop Rings

Stops are rings which stop light coming through the edges of the objective lens from reaching the eyepiece. The edge of a lens is usually where it is bent the most and may not be bent evenly depending on the quality of the lens near the edges.

Baffles help to reduce light from sources well away from the viewed from bouncing off the insides of the tube and reaching the eyepiece. These extraneous light sources could be bright objects near the viewed object or street lights near to the observer. This unwanted light would interfere with the image of the object being viewed.

A tip: It helps to paint the inside of the tubes with a non-reflective black paint. A typical matt-black spray paint would be ideal.

Magnification

The magnification of this telescope is 10x. This means that the object being viewed appears to you to be 10 times closer than what it really is. The image appears 10 times bigger. Try to view the same object looking directly at it with one eye and simultaneously looking through the telescope with the other eye. With a bit of practice you can see the images superimposed over each other and can estimate how many times the smaller image would fit into the larger, magnified image.

Light Path

Consider a basic refracting telescope with familiar biconvex lenses. The objective lens focuses the light at its own focus length The eyepiece then looks at the objective focus point at its own (the eyepiece’s) focus length. Moving the tubes in or out of each other brings the focus points of the objective and the eyepiece lenses to the same point. The total length of the telescope when focussed is the sum of the focal lengths of the two lenses.

Light Path

Consider a basic refracting telescope with familiar biconvex lenses. The objective lens focuses the light at its own focus length The eyepiece then looks at the objective focus point at its own (the eyepiece’s) focus length. Moving the tubes in or out of each other brings the focus points of the objective and the eyepiece lenses to the same point. The total length of the telescope when focussed is the sum of the focal lengths of the two lenses.

Galilean Light Path

A few useful links :

The Galileo Project: The Telescope
Constructing a Galilean refractor
Lens (optics) - Wikipedia, the free encyclopaedia


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