| 摘要 |
A projection system provides a primary mirror with a thin nearly concentric shell which functions as a first mirror at shorter wavelengths for exposure purposes and functions as a second mirror surface at longer wavelengths for alignment purposes. The coating on the front surface of the shell is reflective to the exposing wavelengths and transmissive to the alignment illumination, and the coating on the back of the shell is reflective to the alignment wavelengths. This provides good focus and alignment of the images over two spectral regions.
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