I was blissfully unaware that I completely don't understand the light path through my setup until I was doing some debugging for a problem.
My setup is a TS PL130 -> M68 Flattener -> M69 Extension -> M69 Rotator -> M54 Filter Wheel -> asi2600MC
(Yes, this TS Flattener is a weird M68 on scope side and M69 on camera side)
Now the setup is advertized to have an image circle that fully covers the APS-C sensor of the asi2600. And it certains does -- in my images I have stars all the way to the edge. So when I saw a flat like the below
I assumed that the bright center circle is from area in the center of the lens and that the light entering at the edge of the lens is at or beyond the edge of the image.
Now I added some tape in front of one edge to verify orientation for my camera
I expected to not really see the tape yet because it only covering the very edge of the lens which should correspond to beyond the edge of the sensor. But surprising (to me) was that this is the picture I got
Huh? I clearly don't understand how the light flows through here. If the edge of the lens appears to correspond to the fully lit circle, what's the path of the light that fills in beyond the brightly illuminated circle?
Can someone who understands this please draw me a ray diagram that shows the path leading to the fully lit circle versus out to the full image circle?
Appreciate the tutorial....