If you have no choice and temps are getting close to freezing, chop em down. In my experience, some strains can take a night or 2 of near freezing temps while others don't do well with temps under 40. Either way.........don't let them get frostbite. Even a light frost can do enough damage to make you cry.
Slim's right, for 10 or 12 bucks the Radio Shack hand held microscope is a good deal. Takes some getting used to but well worth the effort. I'd guess from the pics (big guess cause it's hard to say) that they could go a little longer.
Nice job on the grow, she's a beauty!