Some of us start the night shift early. The dome is opened well before sunset to help equalise temperatures inside and outside the dome. Sometimes the IRTF, pictured above, is opened early as it's observing a planet that's only visible during the day.
Other observatories tend to wait until after the sun has set, the Kecks being an example. Although I'm not familiar with the Keck's policies, one thing you must avoid doing if you open early is having direct sunlight on any of the telescope structure. Even at sunset or sunrise when the sun is low in the sky, the light from the sun will warm the structure it strikes and some expansion will take place. It'll be tiny, a few microns perhaps, but enough to effect image quality.
Shortly after I took the top picture of the IRTF, I saw them rotate the dome so the aperture was pointed directly at the sun. There may have been a good reason for this, but it's certainly not something I'd feel comfortable doing. Then again, the IRTF was designed to observe planets in the infrared and sometimes that means pointing close to the sun.