Primary mirror:
The main mirror of the Hubble Space Telescope, measuring 2.4 meters (94.5 inches) in diameter. The primary mirror captures light from objects in space and focuses it toward the secondary mirror.

Secondary mirror:
A smaller mirror (0.3 meters, or 12.2 inches) that redirects the light coming from the primary mirror through a hole in the center of the primary mirror and onward to the science instruments.

Focal plane:
Where Hubble's images are formed. The mirrors focus the starlight onto the focal plane, which is roughly the size of a dinner plate. The light is then picked up by the science instruments.

Corrective optics:
Corrective apparatus "worn" by each of the early science instruments to compensate for the imperfect shape of the primary mirror. Later science instruments had corrective measures built-in, eventually making the Corrective Optics Space Telescope Axial Replacement (COSTAR) redunant. COSTAR was removed in 2009 during Servicing Mission 4.

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