Installing a new ZWO ASI 290MM Mini as a guide camera on my QSI 683 wsg-8 camera. The mini has a back focus of 8.5 mm, and I find that even with the 4 mm M28.5-CS adapter ring that comes with it, the mini is still inside focus on the OAG. I read somewhere in this forum that the back focus needed is 12.5 mm, so it looks as if I need to get another M28.5-CS adapter ring from ZWO and I'm all set.
When I set this up and tried to get focus it did seem like about 4 mm more outwards is in the ballpark.
There is more than 4 mm of travel on the big threads that the C-adapter screws onto, and a set screw for its position, but I did not try that and I am uncertain if the sensor will remain perpendicular to the light path.
So has anyone had experience with this same thing, and can confirm or correct my assumption that if I get a M28.5-CS adapter it'll be what's needed to get focus on the QSI OAG?