The accompanying picture shows a refractor telescope, that is mounted on an equatorial mount. The celestial equivalent of longitude on Earth, is Right Ascension. It is abbreviated to RA, and is measured in hours, minutes and seconds instead of degrees. One minute later starts the cycle over at 0 hours. Schmidt-Cassegrain A type of reflecting telescope that also uses a lens, called a corrector plate.
Light enters the telescope through the corrector plate.
It then travels to the rear of the telescope where it is reflected and focused by the primary mirror onto the secondary mirror. The secondary mirror, mounted in the center of the corrector plate, reflects the light back to the rear of the telescope. The light travels back through a hole in the center of the primary mirror, out of the rear of the telescope, where it then passes through the eyepiece . Most of the time a 90 degree diagonal is attached to the rear of this kind of telescope before the eyepiece. This diagonal allows for more convenient viewing.