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| ==== Aperture masks ==== | ==== Aperture masks ==== | ||
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| Aperture masks have proven to be very useful tools in astrophotography for focusing and for testing the optical quality of telescopes. They use the physical principle of diffraction to determine the exact focus position of a telescope. | Aperture masks have proven to be very useful tools in astrophotography for focusing and for testing the optical quality of telescopes. They use the physical principle of diffraction to determine the exact focus position of a telescope. | ||
| Masks with two apertures are usually called **Scheiner masks**, whereas masks with more than two apertures are called **Hartmann masks**. The masks are mounted in front of the telescope aperture. | Masks with two apertures are usually called **Scheiner masks**, whereas masks with more than two apertures are called **Hartmann masks**. The masks are mounted in front of the telescope aperture. | ||
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| To find the focus, the telescope is pointed at a bright light source (e.g. a bright star). Because the light passes through different apertures, several images of the source appear when the telescope is defocused. By adjusting the focus, these images gradually overlap and finally merge into a single point. | To find the focus, the telescope is pointed at a bright light source (e.g. a bright star). Because the light passes through different apertures, several images of the source appear when the telescope is defocused. By adjusting the focus, these images gradually overlap and finally merge into a single point. | ||