[JohnH]

In my (very) limited understanding of optics SA is the error that causes the focal plane to be curved, all refractors will suffer from this, so yes this is what flatteners are intended to correct. An FR is another animal though and will make curvature worse (more of the outer parts of the field where this distortion is at its worst will now fall on your ccd). Some devices do both (WO spring to mind). It seems to be a bit of a black art with no system being perfect for all uses, so you may need to experiment and/or check what other users of the scope are doing for FR/Flatteners.. I used a 127ED with a mono ccd for narrowband it worked out ok but I always found Ha sharper than Oiii and with RGB B was always softer. Only way out of that is a mirror or large $$$....(shields up!).