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

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

• A. 45 mm objective lens diameter (entrance pupil diameter)
• B. 45x magnification
• C. 45 cm focal length
• D. 45 mm exit pupil

Answer: (A)

The number after the “x” in a small telescope spec is the objective diameter in millimeters, which is the size of the light-gathering front lens and roughly the entrance pupil. In a 5 x 45, the 5x tells you the magnification, and the 45 means the objective is 45 mm across. This larger front lens collects more light and helps with brightness and resolution. Why the other ideas don’t fit: the magnification isn’t 45x—it's 5x. The second number isn’t a focal length, so it isn’t 45 cm (450 mm). The exit pupil would be the entrance pupil divided by the magnification, so here it’s about 45 mm / 5 = 9 mm, not 45 mm.

The number after the “x” in a small telescope spec is the objective diameter in millimeters, which is the size of the light-gathering front lens and roughly the entrance pupil. In a 5 x 45, the 5x tells you the magnification, and the 45 means the objective is 45 mm across. This larger front lens collects more light and helps with brightness and resolution.

Why the other ideas don’t fit: the magnification isn’t 45x—it's 5x. The second number isn’t a focal length, so it isn’t 45 cm (450 mm). The exit pupil would be the entrance pupil divided by the magnification, so here it’s about 45 mm / 5 = 9 mm, not 45 mm.