The best focusing device EVER invented!
Let me first say that this device was invented by a respected Russian astrophotographer Pavel Bahtinov. His web page is http://astrophoto.chat.ru/ but it’s rather outdated. I believe he didn’t update it since 2002.
Most of you have probably heard about (or used) Hartmann mask (http://cometman.com/Mask.html). It’s a whole aperture mask with 2-3 holes (triangular or round) that is supposed to facilitate focusing. Frankly, I always found these mask close to useless, as when 3 stars come close to each other it’s almost impossible to tell if you are before focal point, after or right in it.
Pavel’s invention looks like so: