What optical defect, particularly associated with faster telescope designs, causes stars near the edge of the viewing field to look stretched or like tiny comets?

Answer

Coma

Coma is a technical disadvantage where stars positioned away from the optical center of the image appear distorted, resembling small comets pointing outward from the image center.

What optical defect, particularly associated with faster telescope designs, causes stars near the edge of the viewing field to look stretched or like tiny comets?

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