Abstract: An optical apparatus includes an interchange mechanism and an optical assembly of an illumination system or a projection objective. At least one of the plurality of optical elements of the optical assembly is selected from among a plurality of ones selectable from the interchange mechanism which facilitates exchange of one for another in the beam path. To reduce transmission of vibration from the interchange mechanism to the optical assembly, the interchange mechanism is mounted on a structure which is substantially dynamically decoupled from the housing, and a selected selectable optical element is located at an operating position at which it is separate from the interchange mechanism.