While I was out last night with the ST120, I also tested my AT72EDII with the TS Flat2. This was using the Baader 2" ClickLock Mirror Diagonal with barrel removed. The field was flat viewing with the 8 Ethos and 10 Ethos.
According to the TS specs, the AT72EDII needs 128mm spacing, since it's focal length (430mm) is less than 450mm. The spacing for my test was 94mm and 105mm. Maybe tonight I'll retest with the diagonal barrel attached, for a spacing of 120mm and 131mm, much closer to the 128mm that TS suggests.
Compared to the ST120, however, the AT72EDII has a relatively flat field even without the TS Flat2.
Edited by Sarkikos, 28 July 2019 - 12:25 PM.