A single-axis 4-mirror system: Large spherical primary and small fields

A catoptric corrector of modest size can be used for large spherical primaries, easily integrated at the prime focus, this corrector gives back to the system, aspect and properties of 2-mirrors classical telescopes. In the last years, progress in active and adaptative optics makes possible a lot of things,progress in measuring distances, new ideas on optical coatings, new materials and so on ina near future, all that makes the instrumentalist dreamy ! It is said that nobody knows to-day if the size of 3rd millenium telescopes will be limited or not by a theoretical, physical or technical phenomenon,thus let us imagine but with thoughtfulness because our projects will be surely restricted by financial considerations !!