Q:  Why do astronomers prefer to use gratings instead of prisms?


         Light must pass THROUGH the glass of a prism.  As it moves
         through the glass, light may be scattered or absorbed by
         the glass itself, or by bubbles or impurities within the
         prism.  Since astronomers often attempt to measure very
         faint sources, losing even a small fraction of the precious
         light can doom a project.

         
         But light which REFLECTS from the surface of a grating
         interacts only with a very thin layer of atoms at the surface.
         Gratings therefore lose a smaller fraction of the incident
         light than prisms, allowing astronomers to study fainter
         objects.