A red arrow at a finite distance from a gregorian telescope focused in the same plane (F) as if it was at infinite distance by moving the elliptical secondary mirror (M2) the distance d away from the parabolical primary mirror (M1). Green dotted lines for construction of primary image and blue dotted lines for constructing secondary image from primary image. Yellow line shows an actual light-ray through telescope. Red line shows optical axis. Red contour shows location of secondary mirror if the object was at infinite distance, so that focus of primary (F1) and close focus of secondary (F2) coincide and distant focus of secondary (also marked F2) coincide with wanted projective plane of telesocpe (F). Black zig-zag lines symbolize a lot of "cropped distance". Vertical size of secondary mirror and aperture in primary mirror exaggerated, else the lines would have been indistinguishably cluttered, and the figure is cluttered enough as it is.
Own work
Episcophagus