stronomer Scott Sheppard runs through his checklist as he settles in for a long night of skygazing at the Subaru telescope atop Mauna Kea, Hawaii. The air above the summit: clear. The telescope: working smoothly. His 3-terabyte hard drive: emptied and ready to accept a flood of fresh data in the hours to come.On a wall in the observing room, three clocks track the hours in Hawaii, Tokyo and Coordinated Universal Time. Screens display every tic of the weather above the summit: wind direction, temperature and the dreaded humidity levels that could end this November night of observing if they were to rise. But for now, conditions are nearly perfect, especially when it comes to a characteristic known as seeing -a measure of how stable the stars above look. "Seeing is point-five-five, " says David Tholen, an astronomer at the University of Hawaii in Manoa. "It doesn't get much