Bahtinov mask for 4 inch William Optics refractors (102 and 103 mm) | William Optics

Focusing on a star seems easy, until you find that your photos don’t turn out quite right afterwards. A Bahtinov mask makes small differences visible that you miss on a screen. This mask is made for William Optics refractors with an aperture of 102 or 103 mm.

You place it in front of the telescope opening. Three diffraction spikes form on a bright star, and the middle one is exactly in the center when the image is sharp. Check beforehand whether it fits your model, for example by measuring the outer diameter of the dew cap. If you have a different telescope, then there is a Bahtinov mask tailored to your tube.

Focus on a bright star, lock the focus if possible, and remove the mask before you start to take photos. For more information on how to start photographing, read our astrophotography course.

If you want to autofocus later, check the Electronic focusers for astrophotography.