I have read a lot of articles on the use of parabolic mirrors to focus light to a central tower, such as the article in 11 Dec 2010 on Gemasolar; but how can these mirrors be parabolic?  A parabolic mirror will focus towards the center of the dish, which is why solar ovens put the food chamber in the center at the focal distance and towards the sun and  TV dishes have the collecting devices in front of them towards the incoming signal.  These dishes are all around the central tower, so some will have their front to the tower, some to the side, and some to the back (so to speak) of the incoming sun.  Even shifting the angles of the mirrors would not allow the focus to be towards the central tower, but towards the location of the sun.  Since the tower can not be in many places at once, how do all of these dishes focus to the central tower?