You have already discovered a few Constellations with the Looking Glass. This works, but there is an easier method to discovering the rest. Craft a Telescope and place it down in a high place (away from buildings and trees). This will enable you to look at the stars and find other Constellations. The telescope will let you draw out these Constellation without needing to sneak and will let you scroll left and right without having to look around.

If you are able to discover more Constellations then please do. Looking these Constellations up in your Tome will give you a bit more information on them.