You need to go down as far as the lower keys, such as Bahia Honda state park or around Big Pine Key. The annual Winter Star Party is hosted on Scout Key, right beside Bahia Honda, every year in February, and it is one of the major amatuer star parties in the US. It's a wonderful drive anyhow, and I would do it in a heartbeat myself if I could.
Here is one I took myself at the WSP this past February. 18mm Zeiss, D800, tracked exposure (about 3 minutes if memory serves me correctly) on a Byers-Cam track. All one shot, I did not cut and paste tow images together.
One other thing, the whole of the Keys, at least I find, is a wonderful place to go for photography. Bahia Honda is incredible, and for what it is worth, an extra hour's drive down to Key West and the old architecture there is great. Just sayin.