Minimum brightness is definitely the problem. For now, we're using an old incadescent lamp with 40 watt bulbs. It's way, way too bright. Maybe a good answer is to get red bulbs (like this one or that one), which seem to be favored by astronomers because they don't mess with your eyes' dark adaptation. Given that the particular lamp we're using has blue frosted glass, that would only further attenuate the light...