seanh behind the OAG, I have a filter drawer that I think is 17mm. then a small spacer and then an ASI 294mc.
To allow the guide camera to focus at the same time your main camera is focused, I think the metal back of your OAG wants to be perhaps around 39 mm to 40 mm away from the main sensor (will depend on the thickness of the filter glass too).
If you are using a filter drawer from ZWO, it is 21mm thick (a 3 mm thick glass filter will add 1 mm to the optical path).
If you include the black ring of the ASI294, the flange to sensor distance is 17.5 mm. So, don't use any spacer that is longer than 1 or 2 mm between a ZWO drawer and the camera flange. If you are using some other filter drawer, just make the corresponding adjustment.
If there is a long spacer in between the drawer and the camera, you need to pull the guide camera out by the same distance as the spacer.
Chen
