Had a customer the other day that balked when I suggested an 8x42 as a dual purpose, birding and
astronomy binocular. Like many people, she believed that an
astronomy binocular was a specialized binocular, designed expressly for astronomy. I explained that some
binoculars are, of course, designed with astronomy in mind, but the distinction between a conventional binocular and an astronomy binocular is largely arbitrary. In other words, there is no one feature that makes an astronomy binocular an astronomy binocular. In general we want something with good image brightness and most astronomers therefore opt for a binocular with a 4mm exit pupil (second binocular number divided by first binocular number) or larger. Most astronomers also want at least 7x magnification. Beyond that, though, any binocular has the potential to be used for astronomy, especially for a beginner, and that includes the 8x42 I suggested earlier.