The quality of the data we gather depends on two things: the incidence angle, which is the angle between a normal to the surface and a line from the sun to the surface (angle i in this picture) and the emission angle, which is the angle between a normal to the surface and a line from the spacecraft to the surface (angle e in this picture). The sum of these two is known as the phase angle. In order for the data we get to be useful, the emission angle should be low -- preferable less than 30 degrees. We can change the emission angle by changing where the spacecraft point, but, as you can see . . .