What is the primary visual limitation that prevents deep-sky objects like nebulae from appearing in vibrant color through an eyepiece, even with large apertures?

Answer

The human eye is poor at detecting color in dim conditions

The human eye excels in low light for motion detection but struggles significantly to perceive color when light levels are very low, meaning faint nebulae often appear gray or washed out visually, unlike photographic results.

What is the primary visual limitation that prevents deep-sky objects like nebulae from appearing in vibrant color through an eyepiece, even with large apertures?

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