On an early autumn day of 1608, Hans Lipperhey, a spectacle maker from Middelburg, in the Netherlands' coastal province of Zeeland, applied before the States General of The Hague for a patent on an "instrument for seeing far". By that time, use of small rounded glass disks to aid the natural eyesight wasn't new. Those bulging out on both sides, resembling lentil - or "lens" in Latin - have been used to correct for farsightedness since the mid 13th century. The idea of a device for magnifying distant objects may have been already grasped for some time as well. But this was the beginning of something else. In the summer of 1609, Galileo, Harriot, and others, turned the new Dutch invention - the "spyglass" - toward the night sky. The telescope was born.

This tale grew old, but our fascination with telescopes has not. Following text is an attempt to give more of an insight into their inner workings. More specifically, how do they form images and what factors determine their quality.

Somewhat unusual, the main aspect is that of the optical wavefront, as opposed to the geometric (ray) "interface". That deliberate choice, while perhaps somewhat less convenient, allows for more accurate qualitative assessment, and should throw more light at the underlying physical fundamentals. Main reference sources are as follows: (1) Astronomical Optics, Daniel J. Schroeder, (2) Aberration Theory Made Simple, Virendra N. Mahajan, (3) Optical Imaging and Aberrations I and II, Virendra N. Mahajan, (4) Optics, Eugene Hecht, and (5) Telescope Optics, Harrie Rutten and Martin van Venrooij. Most of raytracing plots and routine checkups are by ATMOS, Massimo Riccardi, and OSLO, Sinclair Optics. Most of diffraction patterns were generated by Aberrator, Cor Berrevoets.

For additional reading, see extensive coverage of telescope optics and related subjects at Bruce MacEvoy's Astronomical optics (descriptive), and Solo Hermelin's Optics (mathematical). For related graphic aspects, Wyant College website.