Illumination optical unit for projection lithography, and method for monitoring such an illumination optical unit

An illumination optical unit, comprising:

• a transfer facet mirror comprising a plurality of transfer facets configured to guide illumination light from an illumination light source along an illumination light beam path via illumination channels specified by the transfer facets;
• an illumination specification facet mirror comprising a plurality of actuator-tiltable illumination specification facets configured to superimposedly guide the illumination light to an object field via the illumination channels, each illumination channel having one of the transfer facets and an assigned illumination specification facet, the transfer facets being imageable at least into portions of the object field;
• and
• a monitoring device, comprising:
• at least one spatially resolving monitoring sensor;
• and
• at least one satellite facet of the transfer facet mirror,
• wherein for each satellite facet:
• the satellite facet is assigned to exactly one of the transfer facets so that the transfer facet constitutes a monitoring transfer facet;
• and
• the satellite facet is configured so that monitoring light from a monitoring light source is guided via the satellite facet and one of the illumination specification facets, assigned to the monitoring transfer facet via the illumination channel, along a monitoring light channel to the monitoring sensor to monitor the illumination specification facet.