There are lot, depending on your requirements. Anyway you can try: ]xJ'oBhy
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1, Refractive: an achromatic as positive + plano-convex as negative; W;OYO
OR (:&&;]sI
2, Reflective: two parabolic mirrors; %)<oX9E
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Both of them affer nice spherical aberration performance. S<y>Y
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Reminder: compute the laser beam density, ensure you have enough strong optical coating. qXP1Q3
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Good luck.