An optical telescope is a telescope that keeps and focuses light, mainly from the visible part of the electromagnetic spectrum, to create a fantastic image for direct view, or to make a photo, or to collect data through electronicimage sensors.

No telescope can form a perfect image. Even if a reflecting telescope could have a perfect mirror, or a refracting telescope could have a perfect lens. In reality, perfect mirrors and perfect lenses do not exist. Image aberrations can be broken down into two main classes, monochromatic, and polycromatic.