Usually I check every hour the focus. The best situation is, when a bright star is in the field of view around the object. Then I simply refocus with that star by attaching the Bahtinov mask on the lens. After refocusing I remove the mask very very carefully.
If there is no bright star, I use 2 different methods:
1. manually refocus without the Bahtinov mask on a visible star in LiveView (10x magification). But this method is not the best… sometimes I don’t hit the focus very well
2. I use the Bahtinov mask with a bright star, which I find on the RA or DEC axis. But this method is very time consuming because I have to move one or both axes to navigate to the bright star… after refocusing I go back to the object. I need a few test shots to center the object again after refocusing…
I dont use an app. My MGEN controls the DSLR shutter. And I use LiveView on the DSLR.