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NightSky Bahtinov Mask for 245mm Outside Diameter Tubes
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How to use it?
1.Place the Bahtinov Focus-Mask over the end of your telescope, then locate a bright star near your target, take a 3-5 second picture and view the diffraction spikes.
2. Change your focus, take another picture and again, view your diffraction spikes. Is the center spike moving further from center?
3. Then change the direction your focusing and try another picture. Watch the direction the spikes are moving and when the center spike passes the middle of the other two spikes, change direction and fine tune your tuning knobs until the center spike is in the middle. Your now in perfect focus.