The Newtonian telescope has flat secondary mirror and a paraboloid or spherical primary mirror Gallery Tutorial TRAIL: Webel SysML Parsing Analysis example: Optical telescopes from Wikipedia: Structure and port-based light flow model Section Slide kind SysML Block Definition Diagram (BDD)
The Gregorian telescope employs a concave secondary mirror that reflects the image back through a hole in the primary mirror. Gallery Tutorial TRAIL: Webel SysML Parsing Analysis example: Optical telescopes from Wikipedia: Structure and port-based light flow model Section Slide kind SysML Block Definition Diagram (BDD)
The Newtonian telescope ... usually has a paraboloid primary mirror but at focal ratios of f/8 or longer a spherical primary mirror can be sufficient for high visual resolution. Source Wikipedia
A principal focus or focal point is a special focus: For a lens, or a spherical or parabolic mirror, it is a point onto which collimated light parallel to the axis is focused. Source Wikipedia
