Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A space telescope is an instrument in outer space which is used for observation of distant planets, galaxies, and other outer space objects. Performing astronomy from the Earth's surface is limited by the filtering and distortion of electromagnetic radiation (scintillation or twinkling) due to the atmosphere. A telescope orbiting the Earth outside the atmosphere is subject neither to twinkling nor to light pollution from artificial light sources on the Earth. Space-based astronomy is even more important for frequency ranges which are outside of the optical window and the radio window, the only two wavelength ranges of the electromagnetic spectrum that are not severely attenuated by the atmosphere. For example, X-ray astronomy is nearly impossible when done from the Earth, and has reached its current importance in astronomy only due to orbiting X-ray telescopes such as the Chandra observatory. Learn more about some of the most important scientific tools in the history of space exploration, the the space telescopes, in this book.