This is a cross-section of a Cassegrain-type reflecting telescope. It consists of a parabolic mirror with focus F (blue) and a small hyperbolic mirror (red), part of a hyperbola with foci F and F' and a vertex at A. This is one way to move the focus outside the telescope.[br][br]Rays coming from the telescope's entrance E, traveling parallel to its axis, are reflected by the primary mirror towards the primary focus F. But before reaching it, they are intercepted by the secondary mirror and reflected towards the secondary focus F', where they finally converge. Rays that reach the secondary mirror from behind are not reflected and are lost.[br][br]You can move all points E, T, F, F', and A to change the telescope's configuration. The slider n allows you to change the number of rays displayed. If you stop the animation using the control in the lower left, you can manually change the value of t from -1 to 1. At t = -1, the rays enter the mirror; at t = 0, they reach the focus F'. The animation then proceeds as if the rays originated from F', although this is no longer a realistic situation. In the input bar, you can assign any value to t, for example, [t = 0] (without brackets).[br][br]This is the original graph: https://www.geogebra.org/material/show/id/zKxtg6Um