You are presented with 2 lenses and a mounting tube. Your task is to assemble a

Question

You are presented with 2 lenses and a mounting tube. Your task is to assemble a refracting telescope. One lens has a diameter of 5 inches and a focal length of 1000 mm. The second lens has a diameter of 3/4" and a focal length of 75 mm. a. How/Where should you place the lenses in the tube to maximize the magnification power of the telescope? b. Where will an image of a star/nebula/galaxy be focused with respect to the smaller lens? c. M42, the Orion Nebula, is 60 arc-minutes in diameter at its smallest. What size will it appear to be in the telescope you have created?

Explanation / Answer

Q]

a] For a telescope, the light coming from a distant object needs to fill the lens aperture as much as possible and so a larger diameter lens is needed at the end of the telescope which faces the object.

Also, the magnification for such a telescope needs to be large and magnification m of the telescope is the ratio of focal length of objective lens (lens which faces the object) and focal length of the eyepiece (lens which faces the eye).

m = 1000/75 = 13.33

therefore, the lens of focal length 1000 mm will be facing the object and the lens of focal length 75 mm will face the eye for maximizing power.

b] The parallel rays coming from a distance object will get focused at a point which will be 1000 mm from the objective. This focused image will be at a distance of 75 mm from the second lens (eyepiece).

c]

Angle subtended by the telescope = Angular magnification x angle subtended using the eye

so, size of the image using the telescope = 13.33 x 60 arc minutes = 800 arc minutes.

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