Optical Telescopes

Just some notes on optical telescopes and refracting versus reflecting

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Refracting telescopes
A refracting telescope uses two converging lenses
The objective lens converges the rays from the object to form a real image
The eye lens acts as a magnifying glass on the real image to form a magnified image
If the object is assumed to be at infinity, then the light rays from it are parallel, and the real
image is formed on the focal plane
A telescope is set so that the principal focus of the objective lens in the same position as
the principle focus of the objective lens is in the same position as the principle focus of the
eye lens, so the final magnified image appears to be at infinity
The magnification of a telescope can be calculated in terms of angles of in terms of focal
length. The angular magnification is the angle subtended by the image 1, over the angle
subtended by the object 0, at the eye
i
M= e

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Reflecting telescopes
A reflecting telescope uses a concave mirror and a converging lens
A parabolic concave mirror (the primary mirror) converges parallel rays from an object,
forming a real image and an eye lens magnifies the image as before
The focal point of the mirror (here the image is formed) is in front of the mirror, so an
arrangement needs to be used where the observer doesn't block out the light.…read more

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