It's heavily dependent on the climate where you observe. From the humid Midwest U.S., I generally stayed at least 15 degrees above the horizon. The exceptions were objects that never rose more than 10-15 degrees. Going lower just meant getting stuck in the muck. From the high desert sites I've used in Arizona, the sky near the horizon generally isn't an issue. Dry air and excellent transparency allow you to get very nice views of objects at just 5 degrees elevation.

I decide how low to go based on the appearance of the sky near the horizon. If the sky looks milky and hazy down low, I'll stay at least one fist's width (~10 degrees) above the muck. If the sky remains uniform in appearance as it drops low and there is a sharp cutoff where the sky meets a black horizon, then I don't worry about going too low.

Bill in Flag