How does it work?
After slewing the telescope to a bright star near the target to observe, I put the mask on the top of the telescope closed to the control plate. The brighter the star the shorter exposure time needed for the focusing operation. At that time the screen of the CCD control software showed 4 disks representing the diffraction image of the start. This picture demonstrated that the telescope was out of focus. The goal was to merge the 4 points into only one point. In general less than 5 minutes are enough to achieve the focus versus around 20 minutes in the past. To make final focusing it was necessary to remove the Hartmann mask. Once the start is too bright, it saturates the CCD and it becomes difficult to see whether the telescope is on focus or not.

The reduction of the exposure time can resolve this problem and enables to achieve the focus until a single star appears on the screen.
However, the theory is different from the reality.
Due to the air flow in the atmosphere a telescope with a diameter of 355mm never shows a stable image. Therefore, sometimes the computer screen shows 4 closepoints, sometimes 1 point. Should the points be very close it's possible to conclude that the telescope is very close to the focus as well.