Patent Number: 
Section: claims

1. A projection optical unit configured to image an object field into an image field, the projection optical unit comprising:a plurality of mirrors configured to guide imaging light from the object field to the image field along a beam path,wherein:for at least two of the mirrors, there is no mirror between the at least two mirrors along the beam path;for each of the at least two mirrors, an angle of incidence of the imaging light with the mirror is greater than 60°; andthe projection optical unit is an obscured unit. 2. The projection optical unit of claim 1, wherein the at least two mirrors comprise exactly two mirrors. 3. The projection optical unit of claim 1, wherein the projection optical unit has an object plane in which the object field is arranged, an image plane in which the image field is arranged, and the object plane has an angle different from 0° with the image plane. 4. The projection optical unit of claim 1, wherein the at least two mirrors comprise exactly four mirrors for grazing incidence. 5. The projection optical unit of claim 4, wherein:the four mirrors arranged in a pairwise manner: andfor each pair of mirrors, there is no mirror between the mirrors in the pair of mirrors along the beam path the beam path. 6. The projection optical unit of claim 1, wherein the projection optical unit comprises at least two mirrors configured to have an angle of incidence of the imaging light is less than 45°. 7. The projection optical unit of claim 1, wherein the projection optical unit comprises at least four mirrors configured to have an angle of incidence of the imaging light is less than 45°. 8. The projection optical unit of claim 1, wherein:an overall reflectivity of the projection optical unit is a product of the reflectivity of each of the plurality of mirrors; andthe overall reflectivity of the projection optical unit is great than 9%. 9. The projection optical unit of claim 1, wherein:the projection optical unit has an image-side numerical aperture of at least 0.4;the projection optical unit has an object-side chief ray angle for the field center point of less than 7°; andthe image field has an extent of more than 13 mm along a field dimension. 10. The projection optical unit of claim 9, wherein the projection optical unit has a numerical aperture of at least 0.5. 11. The projection optical unit of claim 9, wherein the projection optical unit has an extent of the image field of more than 20 mm in one field dimension. 12. The projection optical unit of claim 1, wherein at least one of the mirrors has a reflection surface in the form of a free-form surface. 13. An optical system, comprising:a projection optical unit according to claim 1; andan illumination optical unit configured to illuminate the object field with illumination and imaging light. 14. The optical system of claim 13, further comprising an EUV light source. 15. The optical system of claim 13, wherein the optical system is a projection exposure apparatus for projection lithography. 16. The optical system of claim 15, further comprising a reticle holder configured to hold a reticle, wherein the reticle holder is configured to move the reticle in a scanning direction, and an imaging scale of the projection optical unit in the scanning direction is greater than in a direction perpendicular thereto. 17. The projection optical unit of claim 1, wherein: An EUVthe projection optical unit has an image-side numerical aperture of at least 0.4;an overall reflectivity of the EUV projection optical unit is a product of the reflectivity of each of the plurality of mirrors; andthe overall reflectivity of the EUV projection optical unit is great than 7%. 18. A projection optical unit configured to image an object field in an image field, the projection optical unit comprising:a plurality of mirrors configured to guide imaging light from the object field to the image field,wherein:for at least two of the mirrors, there is no mirror between the at least two mirrors along the beam path;for each of the at least two mirrors, an angle of incidence of the imaging light with the mirror is greater than 60°;the projection optical unit has a first imaging scale in a first direction of the image field;the projection optical unit has a second imaging scale in a second direction of the image field;the first direction is different from the second direction; andthe first scale is different from the second scale. 19. The projection optical unit of claim 18, wherein the object field has an aspect ratio of greater than in the first and second directions. 20. The projection optical unit of claim 19, wherein:the object field dimension is longer in the first direction than in the second direction; anda reducing imaging scale of the projection optical unit in the first direction is less than a reducing imaging scale in the second direction. 21. The projection optical unit of claim 19, further comprising a stop with a stop edge, an extent of the stop edge along a shorter object field dimension is smaller than along a longer object field dimension. 22. The projection optical unit of claim 18, wherein the first imaging scale is less than the second imaging scale, and the first imaging scale is less than 6. 23. The projection optical unit of claim 18, wherein the first imaging scale is greater than the second imaging scale, and the first imaging scale is greater than 6. 24. The projection optical unit of claim 18, wherein the projection optical unit has a direction-dependent object-side numerical aperture.