An update : well I received the spacers and had a bit of a testing session. In summary, there was good news and bad news.......
I estimated that I required around 30mm extra spacing between the Pflat-4 and the QHY8L. However, on both the ED80 and the ED120, I could not achieve focus. I double and triple chaecked, and basically I had to have less backfocus!! I played around with different configurations and I finally received some decent results with the ED80 and a 6mm T thread spacer between the flattener and CCD with the flattener set at "86" (I will need to post the test image....). I could achieve focus with the ED120 and the 6mm spacer and the flattener set at "66", however I was getting very egg shaped stars in the corner... far worse than I get without a flattener.
So in summary, I am getting some decent results with the ED80 however cannot achieve the same with the ED120. This is starting to hurt my head. Maybe I am doing something wrong..... I don't really want to cut the tube on the ED120 (just kidding
).
Just need some more clear nights to keep testing.......
Cheers,
Daniel