A Bahtinov focusing mask is indispensable when trying to achieve razor sharp focus, especially for astophotography. This focusing aid is proudly manufactured in the U.S.A. out of single-sheet aluminum (unlike the many less durable plastic counterparts on the market).
The aluminum is powder coated to avoid abrasion. Plastic screws and adjustable rubber grommets help the mask fit snugly on the objective of your telescope with no worry of damage. It comes in several sizes to fit most telescopes. Choose based on the outside diameter of the objective of your telescope.
These photos show how it works.
When looking at a bright star, the mask pattern produces three diffraction spikes. The first two images indicate that the telescope is out of focus, since one of the diffraction spikes is off center. The third image indicates perfect focus, because all three spikes line up and cross at the same vertex.