Question on the physics of flatteners

[focaldepth]

I have been reading up on flatteners and there is something I can't quite grasp.

OK so my understanding goes:

A refractor telescope focuses it's image of distant objects on to a plane, but due to lens design, that plane is curved so when doing photography we get focus/distortion issues toward the edge of the frame. (This is true of camera lenses as well only perhaps less noticeable). I could also imagine that other aberrations each have different curved planes needing different corrections. Some detailed camera lens testers go as far as to measure these issues.

A flattener is designed to distort the curved plane into a flatter one to make more of the image less distorted and better focused.

So I understand that a corrective lens could be designed to correct a given telescope and produce a flat field. So I would expect one flattener model would work with one and only one telescope model.

What I do not understand is how flatteners can be sold to cover a range of telescopes. e.g. TS 2inch Field Flattener claims to cover F5 to F7.5 and SW FF from F5.5 to F6. There is rarely any mention of

focal length, aperture or lens configuration.

Am I missing something obvious?

[brianb]

The flattener simply alters the curvature of the focal plane ... by an amount dependent on the distance between the flattener and the focal plane. The curvature that requires to be corrected is essentially only a function of the focal length, in the case of a doublet or triplet objective with the elements touching or almost so. In other words, aperture and focal ratio are irrelevant, except in so far as they affect the focal length. Providing, of course, that the diameter of the flattener is such that, when mounted at the correct distance from the focal plane to correct the curvature, it is large enough not to vignette the image formed in the focal plane.

[focaldepth]

Thanks that helped quite a bit.

So now my simple mental model is...

A simple lens (telescope) focuses onto a spherical surface r=focal length.

A flattener bends light into an inside-out sphere (cant think what to call it).

So if we pick the correct flattener to image distance we can (hopefully) cancel out the 2 spheres.

I think I can work with that unless any one has a better mental model.