question about field flattener

I am using a Newport PAC040, (you should be able to find data through lens catalogs), and the major aberration is Petzval. I am thinking to use a negative lens near image plane as field flattener, but do not how to design such a lens. generally, what principle should I follow?

thanks in advance!

noboday answer this?
Anybody can give a guildline on how to choose lens?
when I need pick some lens from catalogs, I only know Focus length and Diameter, but there is so many of such lens with same F and D...

well, if I design lens, I probablly can set 2 faces as variable and optimize them. however, in real world, you would not find such lens. so my question is: during the system design, how to choose lens from catalogs to minimize aberration?

what puzzles me is that there are so many different lens from different companies but all have same focus length or similar diameter. I am sure they are all different, maybe different glass V# or different curvature...how do I select a suitable lens? do I need look into all these detail spec?

Thank you very much, ukyochen!
now I can see it needs lots of work to select good lens during the system design.