A telescope with a camera behind it shows every small error in the focus. With this Bahtinov mask you can focus precisely: the mask is placed in front of the aperture; you focus on a bright star and you see three light spikes.
Turn the focus ring until the middle spike is exactly in the center between the other two. The mask fits a tube or fog cap with an outer diameter of 216 to 292 mm. Therefore, measure the outside of the front, not the opening.
Check your focus again occasionally during a long photo session, because if the temperature drops, the focus point may shift. electronic focuser This makes adjustments easier.
If you are just at the border, then compare with The mask for 139 to 216 mm or the mask for 292 to 368 mm.
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