Helping the eye

References are made in this book to the use of binoculars or a small telescope to improve the view of the night sky. Such instruments do several things.

• They collect more light than does the naked eye (they have a greater light grasp) and so enable the user to see fainter and more distant objects.

• They magnify objects, making them seem larger.

• They can see finer detail (that is, they have greater resolution).

An ordinary pair of binoculars would be rated 10 by 50, which means magnification 10 and with lenses 50 mm in diameter. Such binoculars will magnify objects 10 times, which is about the highest useful magnification. Higher magnifications amplify the inevitable shaking of handheld binoculars to an unacceptable degree.

More importantly, the 50 mm lenses will collect 50 times as much light as the unaided eye, making visible stars down to magnitude 10 or 11. This was about the light grasp of Galileo's first telescope, though it magnified 30 times. With it, Galileo saw the mountains of the Moon, broke the Milky Way up into stars, confirmed the phases of Venus and discovered the moons of Jupiter. The brighter of the nebulae, both light and dark, will show up well with binoculars and you can separate many double stars.

As for resolution, binoculars will discern details two or three arc seconds across, provided seeing conditions are reasonably steady. This compares with the 15 or 20 arc seconds achievable with the eye alone.