What optical aberration did Isaac Newton's reflecting telescope avoid?

Isaac Newton’s reflecting telescope avoided chromatic aberration. This color-fringing problem occurs in refracting telescopes because glass bends different wavelengths of light by different amounts, causing colors to focus at different positions.

Newton reached this insight through his prism experiments. He found that a lens could not bring all the colors of white light to the same focus, so bright objects could appear surrounded by colored halos and fine detail could be blurred. His solution was to use a concave mirror as the objective instead of a large glass lens. Reflection does not disperse visible colors in the same way, so the reflector removed the main source of color distortion.

The design used a spherical primary mirror and a small flat diagonal mirror that redirected light to an eyepiece on the side of the tube. Newton believed chromatic aberration was the dominant problem in refracting telescopes, although his early reflector was not free from every optical imperfection. Spherical aberration, coma, and astigmatism are distinct issues and can still affect reflecting systems.

Later refracting telescopes used combinations of different glasses to reduce chromatic aberration, producing achromatic and apochromatic objectives. Newton’s reflector remains important because it showed that mirrors offered a practical route around the problem.