Herschelian telescope

A Herschelian telescope is a reflecting telescope, of a design conceived by William Herschel and used in his giant 48-inch instrument, in which the primary mirror is tilted so that light is focused near one side of the open end of the tube. The eyepiece then picks up this light directly, avoiding light loss from reflection by a secondary mirror. The drawback is astigmatism, unless the focal ratio is large.