Reflecting telescopes with spherical mirrors often have a defect called "spherical aberration." Light rays from the perimeter of the spherical mirror do not focus to the same point as rays from the centre, resulting in images with less sharpness and detail. This problem is not critical for smaller Newtonians, and other reflecting telescopes with focal ratios of f/9 or higher, where the aberration is minimal. However, it becomes more noticeable in larger Newtonians and other reflectors having ratios of f/8 or lower.

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.