WO61 and Hotech SCA flattener woes

[symmetal]

I thought I'd try a Hotech SCA flattener on my WO ZS61 and ASI1600 in place of the WO flattener as I obtained a second one at a good price.  I already have one on my FLT98 which works very well. This is the result at the recommended spacing of 55mm + 0.7mm for the filters. It seems to exhibit a 'coma' like distortion towards the edges. I tried it at 52mm and 58mm and there was little change, if any in the 'coma'. The scope is f5.9 and the Hotech is intended for scopes from f5 to f8.

I tried the new one on my FLT98 in place of the existing one and it was fine. Any tilt or spacing errors just gave the usual elongated stars at the edges.

Can anyone explain why it might show this 'coma' like effect on the ZS61? I put the dedicated WO flattener back on with a tilt adjuster attached as the WO flattener's  varying length depending where you measured it caused tilt. It's still much better than the Hotech. ?

Alan