Orion field flatner for small aperture F6 telescopes?

[5ash]

Has anyone any experience with these using a short focal length refractor? It looks similar to a 0.8x Orion focal reducer that i use with my ED120 and it works perfectly , without coma. Would the field flatner be useful with my SW ed65 which has a focal length of 400mm
Regards philip?

[blink138]

my brother uses the orion flattener on his f6 80cf and it seems to work very well, but i think it was made for that scope! / he also has the tv 0.8 and that works beautifully on the same scope, it worked terribly on my apm 107mm though so i do think they are focal length dependant
pat