The diagram shows a Newtonian reflector telescope viewing an object at infinity.

Question

The diagram shows a Newtonian reflector telescope viewing an object at infinity. Based on the specs provided on the diagram.

a ) How far outside the tube will the rays come to a focus?

Newtonian telescopes are very long, which can make them unwieldy to move around and point at different objects in the sky. Describe how a Cassegrain telescope could be designed to decrease the total length of the telescope.

How far will the image point move if we use the telescope to observe a bird 50 m from the mirror?

Explanation / Answer

1) Object at infinity, Image at focal point, Image Distance from concave lens,v = 285cm.

A Plane mirror deflects the image. But total distance remains same.

Thus, CD = v - BC = 285cm - 225cm = 60cm

.

.

2) Now, u = -50m, f = -2.85m

1/v - 1/u = 1/f => v = fu(f+u) = -2.7 m

Image Distance = 2.7m.

Distance to plane mirror from concave lens = 2.25m

Thus, image point,D is at a distance of 2.7 - 2.25 m = 45 cm from C

Image point moves 15cm towards C

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