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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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