What key hardware installed during SM1 acted as corrective eyeglasses for the telescope?

Answer

COSTAR (Corrective Optics Space Telescope Axial Telephoto)

COSTAR was the essential piece of hardware installed during the first servicing mission; it contained five small, precisely shaped mirrors designed to intercept the flawed light and redirect it, compensating for the spherical aberration.

What key hardware installed during SM1 acted as corrective eyeglasses for the telescope?
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