I don't understand this based on what I have seen. Starizona even says it only delivers a 30mm image circle. I will have to try my 5DMKIV camera.
Just to be clear, I'm not using an Apex. I was just commenting on the size of the Esprit 100's image circle, and that it's greater than 40mm.
The Esprit 100 documentation does say "40 mm" image circle in one place, and it says "full frame" in another.
My experience is the Esprit fully illuminates the ASI6200 sensor (43.3mm I believe) but I haven't found a backspacing where I can get clean stars in all four corners. I'm attaching an example of what my corners generally look like. (Single typical unprocessed H-alpha sub with auto-histogram.)
There's some vignetting at the corners, but it's only about 15% light falloff vs. the center. The bigger issue is the star shapes as you approach the corners.
In any case, I'd expect the Apex-L (based on what Starizona says) to fully illuminate the ASI2600 with an Esprit 100, but the corners might not be pretty.