one optical item to check is...
Your eyes!
Surprisingly, many people only discover they have a vision problem when they are looking at the unusual lighting conditions of a star/planet in a telescope. It makes some mild conditions more evident.
One author suggests that Percival Lowell, for instance, had high blood pressure and the bulging blood vessels in the beck of his retina where faintly visible overlain on the disk of Mars - hence his canals!