As a short refractor owner (AT60ED f/6, Evoguide 50ED) and also a C11 owner, I've been trying to learn more about field flatteners/correctors. I haven't yet found basic info on how field flatteners are designed so I thought I'd see if knowledgeable people here on CN could provide some basics. (I will note there is some info in the book Telescope Optics, but I need more help.)
For example, I have an AstroTech AT2FF field flattener that seems to work well on an AT72ED (now sold), but works significantly less well on an AT60ED (love the wider field). I can't find any (increased) spacing info various (shorter) focal length scopes and the AT2FF doesn't even seem to be spec'd for the precise focal length (or field curvature) it is designed for. I've done some experimentation with increased spacing to my M43-size sensor, but I haven't found the sweet spot yet.
There are some anecdotal reports that a Celestron 6.3 reducer/corrector works great on an AT60ED. I haven't tried that yet, but I'm wondering if that would indicate the AT60ED suffers more from coma than field curvature.
There are adjustable flattener designs out there, notably from WO and Borg. Do all those work by changing the back focus (only), or do some adjust the inner spacing of the elements themselves?
As a home machinist, I'm wondering if there is a simple way to construct your own field flattener? I've wondered about using the individual elements from an air-spaced (short focal length) achromat or even the rear group of an SLR lens.
I'd appreciate any comments or tips.