145mm OD Bahtinov Mask for 114mm Telescopes

“The Bahtinov mask is a device used to focus small astronomical telescopes accurately.”

This thing is essentially used for astrophotography to get things in focus using diffraction. The short and sweet is that you stick this thing on the end of the scope after it's pointed at your target star, zoom in on the star in question, and adjust focus in or out until you have the three lines even, like in the picture. Take the mask off the end and you're in perfect focus.

There's a pretty good .gif on the wikipedia article that shows a mask moving through focus and back out that really helps understand what you should be seeing.

Ref: https://en.wikipedia.org/wiki/Bahtinov_mask