(and hence far away) objects would remain invisible. What was needed was large aperture cameras possessing wide fields of good imaging properties ("definition"), and fast focal ratios to decrease exposure times. Unfortunately, the only large aperture wide-field telescopes before Schmidt were ordinary reflecting telescopes of short focal ratio (about f/3), and these presented images which while sharp at the middle of their fields of view, quickly lost their definition away from the field center. Star images became bloated and comet-shaped, with the head of the "comet" pointing to the middle of the photographic field. This bloating results mainly from the optical aberrations (i.e. errors) called "coma" and "astigmatism". Before Schmidt it was impossible to build a large, fast reflector telescope which was not plagued by these errors. Schmidt was well aware of this and had been pondering possible solutions during the late 1920s. According to Baade, he had abandoned at least one solution already, when finally he hit upon his ultimate design, which involved a novel, indeed bold departure from traditional optical designs. Schmidt realized that by employing a large spherically shaped mirror (instead of the normal paraboloidal mirror of a reflector telescope) and a smaller apertured diaphragm placed at the center of curvature of the mirror, he could at a stroke eliminate coma and astigmatism. He would be left, however, with spherical aberration which is just as damaging to image sharpness. Schmidt realized that he could eliminate the spherical aberration by placing a thin, very weakly curved aspheric lens (now called the "Schmidt corrector plate") at the same center of curvature as the apertured diaphragm. This aspheric lens has a complex curve that is convex near its middle and concave near its periphery that creates the opposite spherical aberration of the spherical mirror it is paired with, canceling out the mirror's spherical aberration. In this way, very neatly and simply he could construct a large camera of f/1.75 or even faster, that would give sharp images across a field more than 15 degrees in diameter, making it possible to image large swathes of sky with short exposures (on the order of a few minutes versus an hour or more with a conventional reflector). His first camera had an aperture of about 360 mm or 14.5" in diameter, and a focal ratio of f/1.75. It is now housed in a museum at the Hamburg Observatory. Schmidt's combining of diverse optical