There are lot, depending on your requirements. Anyway you can try: %@BQv4oJ
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1, Refractive: an achromatic as positive + plano-convex as negative; ueWG/`ig
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2, Reflective: two parabolic mirrors; !P:hf/l[B
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Both of them affer nice spherical aberration performance. XhIgzaGVu
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Reminder: compute the laser beam density, ensure you have enough strong optical coating. q2hZ1o
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Good luck.