What phenomenon causes a distracting purple or blue fringe around bright objects in a refracting telescope?

Chromatic aberration

Chromatic aberration is a specific flaw inherent to refracting telescopes that utilize lenses to focus light. This distortion arises because lenses bend different wavelengths, or colors, of light by slightly varying degrees as they pass through the glass. Since blue light bends differently than red light, for example, the different colors focus at slightly different points along the optical axis. This differential focusing results in a visible artifact, often perceived as a false, distracting purple or blue outline or fringe surrounding very bright celestial objects like Jupiter or the Moon. While specialized lens designs exist to minimize this effect, it remains a defining characteristic of basic refracting systems.