[The_bluester (Paul)]

Originally Posted by AdamJL

So I tested it tonight. Bahtinov mask without a filter then mask with. There was a definite difference in focus which was interesting

Adding or removing a filter between scope and camera will move your focus point by about a third of the thickness of the filter glass. Add a filter and you need to rack the focuser out slightly, likewise, remove one and you need to rack in.