In a 5 x 45 telescope, what does the 5 represent?

In a 5 x 45 telescope, what does the 5 represent?

• A. 5x magnification
• B. 5 mm exit pupil
• C. 5 cm focal length
• D. 5 cm object distance

Answer: (A)

Five times magnification is what the first number tells you: the view through the instrument appears five times larger in angular size than what you see with the naked eye. That means objects look bigger, not closer. The second number, forty-five, indicates the objective diameter in millimeters, which affects how much light the instrument can gather and thus brightness. From these numbers you can also infer the exit pupil is about 45 mm divided by 5, which is 9 mm—not 5 mm. The magnification isn’t a measure of a focal length in centimeters or a fixed object distance; it’s a ratio set by the optical design (focal lengths of the objective and eyepiece).

Five times magnification is what the first number tells you: the view through the instrument appears five times larger in angular size than what you see with the naked eye. That means objects look bigger, not closer. The second number, forty-five, indicates the objective diameter in millimeters, which affects how much light the instrument can gather and thus brightness.

From these numbers you can also infer the exit pupil is about 45 mm divided by 5, which is 9 mm—not 5 mm. The magnification isn’t a measure of a focal length in centimeters or a fixed object distance; it’s a ratio set by the optical design (focal lengths of the objective and eyepiece).