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A simple weak plano-convex doublet placed in front of the focal plane will flatten the field of a simple achromat or triplet refractor. If you nominate the focal length of your objective, the desired focal length of the flattener can be calculated, it doesn't have to be exact either, +/- 20% would be fine.
And it also acts as a modest reducer.
NB this is the reason why the simple doublet "reducers" work so well with small refractors and cassegrains.
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