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All refractors have field curvature apart from the FSQ series as they have a built in focal reducer field flattener. I would have to check out the 120ED but it would be fairly standard I would imagine. You really only need FF when you have a full frame camera. Sorry I am wrong to get a truely flat field a carefully placed focal reducer (high quality achromat or better) is a must.
Thats if you want close to some sort of perfection!
Bert
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