Bobby worked on the figure captions for his galaxy poster today. We also talked about the construction of reflecting telescopes. The general setup is the same for all reflectors, regardless of mirror size. A primary mirror (a paraboloid) collects the light from the emitting object, and, since these objects are at large distances, the rays can be considered to be parallel. When they hit the primary mirror, the rays are focused toward a secondary mirror. The main purpose of the primary mirror is to collect light, and the secondary mirror then focuses the rays into an eyepiece or detectors. The diameters of portable amateur telescopes range from a few inches to around 20 inches - larger than that makes them difficult to transport, while the diameters of the largest ground-based optical telescopes are currently of the order 24 - 30 feet or so.