Focusing is one of the main worries of all astro-imagers.

For my LX200 I have made this Hartmann Mask

I used a square piece of plywoord, 35x35 centimetres.
I cut 4 triangles of approximately 5x5x5 centimetres and attached 2 plastic nuts at the top.
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.

Focusing with a Hartmann Mask is quite easy, as you can see what you are doing.
For this collage I used unprocessed unstacked but cropped single frames.