Differential refraction as light pass through a lens. Problem with refraction telescope: focal length of red, green and blue light differs, cannot build really large ones because the lens cannot be supported properly, all large modern telescope are reflectors o, images and detectors. Charge coupled devices (ccds) are electronic devices that store light intensity. Spatial detail is lost by the grid resolution of the ccd: larger telescopes are desirable. Better resolution the separation of distinct features. Doubling the radius of telescope gives four times the light gathering power: angular resolution limit is given by. Small angular separation requires: high resolution astronomy. Atmospheric blurring: due to movement of air. Bends light effect is worst for visible light. Solutions: telescopes on mountain tops in deserts, telescope in space, active optics- control mirrors based on temperature and orientation, adaptive optics: track atmospheric changes with laser, adjusts mirrors in real time.