What fundamental optical flaw affected early refracting telescopes, causing colors to fail convergence?

Answer

Chromatic aberration

The technological hurdles immediately facing astronomers after the initial shock of the telescope's capabilities involved purely optical issues inherent in the early refracting designs. The primary technical limitation described was chromatic aberration, a phenomenon where different colors of light entering the lens system failed to focus or converge at the same precise point. This resulted in blurred images that hindered clarity, even as astronomers attempted to gain marginal improvements by building increasingly long instruments, such as the massive 46-meter device constructed by Johannes Hevelius by 1647.

What fundamental optical flaw affected early refracting telescopes, causing colors to fail convergence?
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