I put this - home made from hard plastic - Hartmann Mask at the sky end of my telescope [there are two pins at the back side of the mask]. When I aim at a bright star I will see 4 triangles on my screen. When I improve the focus with the electric focuser these triangles move to each other. When they have merged completely I have reached focus.

Note: for my LX200 I have made a similar Hartmann Mask