There are lot, depending on your requirements. Anyway you can try:
=?Md&%j R,+(JgJ 1, Refractive: an achromatic as positive + plano-convex as negative;
%ys}Q!gR OR
pDq_nx9 2, Reflective: two parabolic mirrors;
y+afUJT gK\7^95 Both of them affer nice spherical aberration performance.
~'.yhPog ucP