I was just reading about measuring double stars in PRACTICAL ASTRONOMY by STORM DUNLOP. It mentions a few devices and processes that are used such as filar micrometer, binocular micrometer, diffraction micrometer, and it describes a special mask that can be used with reflecting telescopes to modify the diffraction pattern to help reveal close companions, and also using spectra. I wanted to learn more and found this video on YouTube. I would like to learn more about the mask that the book mentions and describes. From the description it is not a Bahtinov Focus Mask. The book says it typically has 6 or 8 straight sides.