[Wavytone]

Hi Peter,

It would help if you said exactly which model of refractor including its focal length or focal ratio.

Basically yes the simpler doublet and triplet types do have field curvature which means you'll need a flattener that is a good match for the specific scope in question.

If you want a focal reducer these are more costly.

The size depends on the type and size of your focusser - some flatteners/reducers are designed to slide into a 2" barrel much like an eyepiece while others have a screwthread intended to screw into the end of the focusser - if it has a thread - and there are several sizes in use.