I'm still trying to understand the need for a specific back focus distance and I have been Googling around to try to find a ray-diagram that I can look at.
Not specific to the EdgeHD 8", but I will explain why the back focus is critical and it helps to look at the ray trace on this link.
Note that off axis the correction is not perfect. Just about no telescope made will give a perfect off axis performance.
Also, not that Celestron does not say the field is flat. They say it is "flatter" than the standard SCT.
Now for visual use, none of this would be important, but we now have CCD cameras that can resolve the smallest deviation from a perfectly focused star. When Celestron gives back focus, the figure they give you is going to produce the smallest average abberated blur diameter across the field of the telescope.
What this means when field curvature is present is that you pick a point somewhere between the furthest focus in the field and the closest focus in the field as your best focus point. If you picked a point all the way to the outside of the field, stars on the inside of the field would be out of focus. If you picked a point a the center of the field, stars at the outside of the field would be out of focus and the more out of focus, the more likely the camera will see any aberration present. The spacing also provides the smallest astigmatic blur.
The goal is to get the smallest average aberrated blur diameter for all of the stars in the field and the back focus Celestron provides is intended reduce the size of any aberration to its lowest level and to place the curve of the field at the ideal point to keep the aberrated blur diameter anywhere in the field too small to resolve.
Edited by Eddgie, 12 July 2019 - 04:23 PM.