The Bahtinov mask is used focus small telescopes accurately. Although masks have long been used as focusing aids, the distinctive pattern was invented by Russian amateur astrophotographer Pavel Bahtinov.
Simply place the mast in front of the main optic of your telescope and take a picture. Look at the diffraction spike pattern and continue until the spikes are even. Using a Bahtinov mask is the easiest way to get into focus.
This 180mm mask is great for 150 - 165mm class telescopes
Fits dew shields with an outside diameter of 185-210mm