My rule of thumb for scopes in low light goes like this:

You want a scope with at least a 7mm exit pupil. This is how tightly concentrated the image coming out of the scope is at the viewer's end of the scope.

You determine the exit pupil diameter by dividing the objective size (in mm) by the magnification. So, let's say you have a 42mm objective, and you have a 10X scope. Well, you have an exit pupil of 4.2mm, which means that the scope is costing you light. Your eye's pupil can dilate to about 7mm in full darkness, a bit less in evening dimness. You want an exit pupil at least as large as the opening that allows light into your eye.

Lots of scopes with higher magnification and smaller objectives will end up costing you useful light.