The Power of a Telescope
A telescope's magnification is determined by a simple ratio: the focal length of the objective lens or mirror divided by the focal length of the eyepiece. A 1000 mm objective with a 20 mm eyepiece yields 50× magnification. But magnification alone is misleading — without sufficient aperture, you're just enlarging blur. The real power of a telescope lies in its ability to gather light and resolve fine detail.
Resolution and the Rayleigh Criterion
The angular resolution of a telescope is governed by diffraction. The Rayleigh criterion states that two point sources are just resolvable when separated by an angle θ = 1.22λ/D, where λ is the wavelength and D is the aperture diameter. A 150 mm telescope resolves details down to about 0.9 arcseconds at 550 nm — enough to split most visual double stars.
Light-Gathering Power
The light-gathering ability of a telescope scales with the square of its aperture diameter. A 150 mm scope collects about 460 times more light than the 7 mm dark-adapted human pupil. This is why aperture is the single most important specification for deep-sky observation — faint galaxies and nebulae demand photons, not magnification.
Exit Pupil: The Comfort Factor
The exit pupil — the diameter of the light cone exiting the eyepiece — equals the aperture divided by magnification. For comfortable extended viewing, an exit pupil between 2–5 mm is ideal. Above 7 mm, your eye's pupil can't capture all the light; below 1 mm, the image grows uncomfortably dim and floaters in your eye become visible. Matching exit pupil to your observing conditions is key to enjoyable astronomy.