There are many quantitative methods to determine the faintest “magnitude” or degree of darkness for the night sky at your location for astronomy. For the most part, this is mostly a matter of determining how much light pollution you have in your area, but it can also reveal some subtle differences in the quality of your site as to transparency and air stability, both of which are variables at even the darkest site on any given night.
Why is sky darkness so important? The darker the sky, the fainter the objects that can be seen with your
astronomy binoculars or
telescopes and this translates into more objects seen as well as more detail seen. From experience, I can tell you that even a little bit of light pollution can make many binocular or telescopic objects disappear. So, what is the cheapest and most effective way to improve the performance of your telescope? Buy new
telescope eyepieces? Nope. The quickest and most effective way to turn your telescope into a larger and better performing telescope is to pack it up and travel to a truly dark sky.
How do you know when you’ve found a dark sky without resorting to methods that require you to count stars in a given section of sky? Simple. Let the Milky Way be your guide. You have an excellent sky when you can clearly and easily see the band of light that is the Milky Way stretch across the sky. This requires a location that is basically free of light pollution. If you can only faintly see the Milky Way, you still have a site worthy of
setting up the telescope.
Do your telescope or astronomy binoculars a favor and see what they can do under a dark sky.