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| The Bahtinov mask is a device used to accurately focus astronomical telescopes. The Bahtinov mask consists of three separate grids, positioned in such a way that the grids produce three angled diffraction spikes at the focal plane of the instrument for each bright image element (star). Bahtinov Focus Mask, Celestron NexStar 4SE |
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| Bahtinov Focus Mask, Celestron NexStar 4SE, Canon 40D, Spica Out of focus - the central spike is noticeably displaced from the |
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| Bright star near perfect focus using Bahtinov Mask. Optimum focus is achieved when the middle spike is centered on the star and symmetrically positioned between the other two spikes. Small deviations from optimal focus are easily visible. Bahtinov mask, Celestron NexStar 4SE, Canon 40D |