What specific optical feature made early Galilean refractors problematic for faint astronomical objects?

Answer

Chromatic aberration (color fringing)

Although the initial Galilean design, featuring a convex objective and a concave eyepiece, conveniently produced an upright image suitable for terrestrial viewing, it had a significant failing when applied to astronomy: chromatic aberration. This optical defect manifests as color fringes or halos appearing around the edges of objects because the simple lenses failed to focus all wavelengths of light (all colors) to the exact same focal point. This inherent imperfection in the glass led some contemporary astronomers, who suspected deception, to dismiss the startling visual discoveries—like the texture of the Moon—as mere optical illusions generated by the flawed lenses themselves, rather than physical realities.

What specific optical feature made early Galilean refractors problematic for faint astronomical objects?

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