In a reflecting telescope the secondary mirror prevents or obstructs a portion of the incoming light from reaching the primary mirror. The central obstruction is the proportion of obstructed light expressed as a percentage of the incoming light.
For example, an 8" f/6 Newtonian reflection has a central obstruction of 18%. A 10" Schmidt-Cassegrain (f10) scope has a much larger obstruction of 37% due to its shorter and more compact optical tube.
