Hi HT,
If you have an old manual Nikon lens most likely it's a Nikon F mount lens which has a 46.5mm flange focal distance. (if it's a cine or enlarger lens then the FFD is not 46.5mm)
Taking it as an F mount lens (the most probable) and given you have a 1600mm (backfocus = 6.5mm) and the 11 mm ZWO supplied black spacer, then you need a further spacer with thickness equal to.....
46.5 - (6.5 +11) =
29mm
29mm is the thickness of the ZWO to Nikon F mount adapter I have - the filter
drawer type.
See...
https://www.bintel.com.au/product/zw...iABEgJpAvD_BwE
There is also a ZWO to Nikon F mount adapter - screw in filter type with the same
29mm thickness. It also has an extra 5mm spacer which you don't use with the 1600mm.See...
https://www.bintel.com.au/product/zw...SABEgIq6fD_BwE
There is also a
much thinner Nikon lens adaptor to use between a Nikon Lens and
ZWO filter wheel.
Without the filter wheel the spacing would be incorrect. Perhaps that's the type you have tried to use and couldn't get focus.
Best
JA