There are lot, depending on your requirements. Anyway you can try: u kZK*Y9P
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1, Refractive: an achromatic as positive + plano-convex as negative; WjK[% ;Z!
OR bHx@
2, Reflective: two parabolic mirrors; |39,n~"o&
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Both of them affer nice spherical aberration performance. `[#id@Z1
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Reminder: compute the laser beam density, ensure you have enough strong optical coating. "+7E9m6I
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Good luck.