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For larger Newtonians, such as Sky-Watcher the spherical surface of the mirror is altered during polishing to bring the light rays onto a common focal plane. By "parabolising" the mirror, the images become sharp and detailed, and free of spherical aberration effects. Sky-Watcher parabolic mirror reflectors also feature a four-arm, secondary mirror bracket with fine supports to reduce diffraction spikes.
Accurate parabolising, although more difficult and time-consuming, is essential for large Newtonians. If you have opted for a telescope of lower f/ratio or with an aperture of 150mm or more, it is important to look for "parabolic mirror" or "paraboloidal mirror" in the specifications.