Question

Suppose you were to look at a star in the sky with your naked eye. The star's light enters your eye's pupil, which has a diameter of 0.6 cm. Now suppose you look at the same star through the Carleton observatory 14-inch (35.6 cm) diameter telescope. How many times larger is the light gathering power of the telescope compared to that of your naked eye?

Answer 1

The light gathering power of a telescope is determined by the diameter of its primary or objective mirror. Telescopes are basically light buckets, the wider they are in diameter, the more light they can collect, and therefore the fainter the objects are that we'll be able to see. Our telescope is 14 inches (355 mm) in diameter. As the light leaves the telescope's eyepiece at the spot where the image comes into focus (known as the exit pupil), it is about 7 mm in diameter, so the telescope's primary mirror is about 50 times wider (350mm/7mm) than the exit pupil. That means that the mirror has 2500 times more surface area than the exit pupil (50 x 50). So, using our 14 inch telescope, we'll be able see objects that are 2500 times fainter than we would with our naked eye. This means that we should be able to use this telescope to see objects that are about 14th magnitude.

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