I added two 1mm spacers after the QuadTCC and the curvature got down to 13% from 21%.
This is good enough for me.
The stars on the corners are not perfectly round but they are very good.
I'm thinking about replacing the copper tape shims with a CTU to remove the last remaining tilts.
I was following Roland's instruction to know if I need to add or remove spacers.
Fortunately I had to add spacers.
I needed to move the focuser inward by around 0.23mm from the perfect focus position for the central stars to get the stars on the corners focused.
I was wondering if I had to increase the spacing by 0.23mm so I posted a question about it on the AP user group.
Roland said I need to add 2mm given my scope and the sensor size and the 0.23mm inward travel.
He was correct!
When I tried 1mm or 3mm extra spacing, the results was much worse than 2mm.
What I'm surprised is how much the spacing was off even though I got a custom adapter based on the published backfocus distances information.
Edited by Denimsky, 22 July 2020 - 01:25 AM.