1. A lithographic apparatus arranged to project a pattern from a patterning device onto a substrate and having an illumination system configured to condition a radiation beam and direct the radiation beam onto the patterning device, wherein the illumination system comprises: a first reflective component and a second reflective component, the first reflective component being arranged to direct radiation of the radiation beam onto the second reflective component and comprising a plurality of movable reflective elements, each movable reflective element being movable between at least a first position and a second position so as to change an illumination mode, and the second reflective component being associated with a pupil plane of the illumination system; and a control system arranged to set the plurality of movable reflective elements to respective desired positions in order to effect a desired illumination mode and further arranged, in the event that a first one of the movable reflective elements is defective and cannot be set to the respective desired position, to set a second one of the movable reflective elements to a corrective position, different than its desired position, to at least partially ameliorate a deleterious effect of the first one of the movable reflective elements, wherein the control system is arranged such that the second one of the movable reflective elements is a movable reflective element that in its desired position would direct radiation to a second point in the pupil plane that is opposite to a first point in the pupil plane to which radiation would have been directed by the first one of the movable reflective elements, and wherein the second point is opposite to the first point relative to an optical axis of the illumination system or relative to an imaginary line lying in the pupil plane and passing through the optical axis of the illumination system.
2. The lithographic apparatus according to claim 1 , wherein the control system is arranged such that, in the event that the first one of the movable reflective elements directs radiation to a first point in the pupil plane, the second one of the movable reflective elements is set to a corrective position such that it directs radiation to a second point in the pupil plane that is opposite to the first point.
3. The lithographic apparatus according to claim 1 , wherein the control system is arranged to, in the event that the first one of the movable reflective elements directs radiation so as to cause a first telecentricity error, set the second one of the movable reflective elements to a corrective position to direct radiation so as to cause a second telecentricity error that opposes the first telecentricity error.
4. The lithographic apparatus according to claim 1 , wherein the control system is arranged to, in the event that the first one of the movable reflective elements directs radiation in a direction such that it does not reach the substrate, set the second one of the movable reflective elements to a corrective position such that it directs radiation such that it does not reach the substrate.
5. The lithographic apparatus according to claim 1 , wherein the control system further comprises: a memory configured to store information identifying a defective movable reflective element and a defective position of the defective movable reflective element; and a comparator configured to compare the defective position of the defective movable reflective element to the desired position of the defective reflective element, and wherein the control system is arranged to set the second one of the movable reflective elements to a corrective position only if the defective position does not equal the desired position of the defective movable reflective element.
6. A lithographic method for manufacturing a device, the method comprising: directing a radiation beam onto a first reflective component so that it is reflected thereby and incident on a second reflective component, the radiation beam being incident thereafter on a patterning device, and the first reflective component comprising a plurality of movable reflective elements, each movable reflective element being movable between at least a first position and a second position so as to change an illumination mode of the patterning device; in the event that a first one of the movable reflective elements is defective and cannot be set to a desired position to define a desired illumination mode, setting a second one of the movable reflective elements to a corrective position, different than its desired position to define a desired illumination mode, to at least partially ameliorate a deleterious effect of the defective one of the movable reflective elements, wherein the second movable reflective element in its desired position would direct radiation to a second point on the second reflective component opposite to a first point on the second reflective component to which radiation would have been directed by the first movable reflective element, and wherein the second point is opposite to the first point relative to an optical axis of an illumination system of which the first and second reflective components form a part or relative to an imaginary line lying on the second reflective component and passing through the center thereof; patterning the radiation beam using the patterning device; and projecting the patterned radiation beam onto a substrate.
7. The lithographic method according to claim 6 , wherein when the first movable reflective element directs radiation to a first point on the second reflective component, the second movable reflective element is set to a corrective position such that it directs radiation to a second point on the second reflective component that is opposite to the first point.
8. The lithographic method according to claim 6 , wherein when the first movable reflective element directs radiation so as to cause a first telecentricity error, the second movable reflective element is set to a corrective position to direct radiation so as to cause a second telecentricity error that opposes the first telecentricity error.
9. The lithographic method according to claim 6 , wherein when the first movable reflective element directs radiation in a direction such that it does not reach the substrate, the second movable reflective element is set to a corrective position such that it directs radiation such that it does not reach the substrate.
10. The lithographic method according to claim 6 , further comprising: storing information identifying a defective movable reflective element and a defective position of the defective movable reflective element; comparing the defective position of the defective movable reflective element to the desired position of the defective reflective element; and setting the second movable reflective element to a corrective position only if the defective position does not equal the desired position of the defective movable reflective element.
