Focal Ratio vs. Aperture: What Makes an Object Look Brighter Through a Telescope?

How can you make a star or deep-sky object, such as galaxies or nebulae, look brighter?

The total brightness of a star or other object is determined by the telescope’s aperture, also called light grasp. The focal ratio (also called f/number, that is, the focal length divided by the aperture) has nothing to do with it.

The f/number does affect a telescope’s magnification (with a given eyepiece), and the magnification determines the surface brightness of the view: the amount of light per square arcminute as presented to your eye. But that’s different. In fact, no telescope can ever increase surface brightness beyond your naked-eye view.

Confused? Here’s how it works. A telescope can’t funnel more light into your eye unless it also magnifies the view — that’s the way optics function. You collect more light and you see the object bigger. But with the light spread out in a magnified view, the surface brightness is diluted back down to what it was before magnification, or even less. If surface brightness was all you cared about, you’d skip the telescope!

If two telescopes have the same aperture and magnification but different f/numbers, their views will be essentially identical.

— Alan MacRobert