What is the size of the exit pupil for a 5x45 telescope?

• A. 9 mm
• B. 9 cm
• C. 5 mm
• D. 45 mm

Answer: (A)

Exit pupil is the diameter of the light beam that exits the eyepiece and enters the eye. For a telescope, its size is the objective diameter divided by the magnification. With a 45 mm objective and 5× magnification, you get 45 ÷ 5 = 9 mm. So the exit pupil is 9 mm. The other options don’t fit because they would require a different ratio: 9 cm would be 90 mm (much larger than this setup), 5 mm would come from a different division, and 45 mm would be the objective size itself, not the exit pupil.