Flattener advice please

[pmrid (Peter)]

I've made a cute 80mm F5 shortie achromat out of bits and pieces I have cobbled together. It's probably going to be used for a bit of lunar on the go but even so I would also like to flatten out the field a bit so I can have the option to use NB filters (so CA is less of a problem) for a bit of deep-sky imaging as well but those streaky stars are a real PITA that I'd like to eliminate.

But I don't want to bust the bank on this either.

Does anyone have any suggestions for a cheap generic flattener? I already have a Hotech Field Flattener but I just can't get the spacing right so I'm looking for alternatives.

[Camelopardalis (Dunk)]

Bottom if a coke bottle?

[By.Jove (Jove)]

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.