Behind Thornton, at the bottom of the image, is Endeavour's open payload bay. Above and behind her is one of the old pairs of solar arrays that were replaced. It is partly wound back into its cylindrical container but would not retract completely because of a bent part.
The HST itself is further back in this picture, mounted on the floor of the payload bay, and pointing towards the Earth. On its special mounting, the telescope could be turned on command to the side the astronaut wanted to work on was facing him or her. The tube could also be tilted downwards. An electricity cable through the mounting supplied power to roll up the old solar arrays and roll out the new ones, and for other uses. Endeavour's crew could switch as needed between this power supply and the HST's own six nickel-hydrogen batteries.
This image was taken during the daylight part of the orbit. The HST takes only about 90 minutes to circle the Earth during which time it goes through both daylight and darkness.