[h0ughy (David)]

Thanks for the reply guys. getting my head around this wasn't easy

i took the adapter off the front to look at it and it looks flush to the screw ring which roughly measures 7.5mm from the lenses

14 using the adapter and the 11mm zwo adapter
6.5 back focal to ccd
20.5 sub total
28.73 desired total
8.23bit left to find - maybe a 3mm and a 5mm or a custom 8.23mm