For the tests I've done this mask is a marvel when it comes to focus the camera, as it nt only is more luminous than Hartmann mask and clearly shows when you reach exact focus, but also you can clearly see which side of focus you are at all times.You can make a custom template for this type of mask here .
The following video of a focusing sequence clearly shows the exact focus and also how we move from one side of the focus to the other side.
Being so luminous, this kind of mask can be used to focus my 60mm guider with 4 seconds exposures.
The following image shows the guider camera in focus, with the center line in a position symmetrical between the diagonals.
In summary, this mask is much better than a Hartmann mask but it is harder to make as it has many openings to be cut accurately.
You can use the software Bahtinov Grabber (freeware) to help excact focus detection using this mask.
