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Why theoretically the images produced by a reflector is not as sharp as that by a refractor of same diameter of aperture?

- David (24.11.2001)

Smaller the point a telescope can focus, the better its optical performance. As the secondary mirror and the spider vanes in Newtonian reflector blocks in front of the primary mirror, the diffraction pattern¡° of the star image is then enlarged. You may treat the image of a celestial body, say Jupiter, as numerous tiny points of light. These points are smeared so the contrast is degraded.

But practically, if the diameter of the secondary mirror is small enough (below 25% of the telescope diameter) and the spider vanes is thin enough (cross section area less than 0.5% the area of aperture), the decease in contrast can hardly be detected. If all other common problems of reflectors (such as thermal inequilibrium, spherical aberration and coma) are eliminated, this ideal reflector should perform just as good as an ideal refractor.¡@

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Diffraction pattern

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¡°When light passes through an obstacle (e.g. the rim of telescope tube), its traveling direction deflects slightly. This phenomenon is called diffraction. Diffraction will cause the point-like star image to become a set of concentric circle.

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LAU Kai-ip
Asst. Curator, Hong Kong Space Museum
30.09.2002