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Old 11-05-2021, 06:01 PM
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mental4astro (Alexander)
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mental4astro is offline
 
Join Date: Jun 2008
Location: sydney, australia
Posts: 4,959
Werner,

As Marc has said, the sharp straight corners of a square or rectangular shape in a telescope's light path will create a diffraction pattern, and genrate additional scattering of light which reduces contrast.

The other thing about telescope mirrors is they are "first surface mirrors". In other words they are reflective from the first surface the light touches. Everyday mirrors, such as your shaving mirror, are second surface mirrors as the main reflection comes from the silvered surface below the glass. These mirrors are no good for telescopes. Not only because of the glass that the light goes through, but they are not smooth/flat enough as needed for telescopes. Flat mirrors for telescopes are much more expensive than ordinary everyday mirrors because the manufacturing techniques used are much more expensive.

The ellipse shape of these mirrors is both to reduce diffraction patterns, but the shape is geometrically optimised for the optical path. When you look down into a Newt the secondary mirror looks perfectly round.

Alex.
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