Comment by doetoe

The most important is that they capture enough light, for which the lenses must have a large diameter. 50mm is typical. Magnification around 10x is good. This is referred to as 10x50. I have a Celestron Skymaster 15x70 myself, which is specifically for night sky observation. The 70mm is very good, but the weight and the magnification make it difficult to hold still without a tripod, though you can still use it without, e.g. lying on your back