An illumination system comprising an array of controllable mirrors configured to direct radiation towards a pupil plane and an array of lenses configured to direct radiation sub-beams towards the array of controllable mirrors, wherein a first lens of the array of lenses and a controllable mirror of the array of controllable mirrors forms a first optical channel having a first optical power and a second lens of the array of lenses and a controllable mirror of the array of controllable mirrors forms a second optical channel having a second optical power, such that a radiation sub-beam formed by the first optical channel has a first cross-sectional area and shape at the pupil plane and a radiation sub-beam formed by the second optical channel has a second different cross-sectional area and/or shape at the pupil plane.