Other Links
In addition to the Astronomy Links page, here are some additional and useful web sites that I recommend for more advanced users, especially if you are learning the mapping systems used for astronomy. There are also links to several pages on hardware for telescopes and mounts.
Mapping systems -
Astronomical coordinate systems - Used for specifying positions of celestial objects. Note there are two prominent systems for this - Horizontal & Equatorial
Horizontal coordinate system - A celestial coordinate system that uses the observer’s local horizon as the fundamental plane to define two angles of a spherical coordinate system - altitude and azimuth. Since this relies on the observers perspective using the Earth’s surface as reference, it is a more intuitive way of observing the sky.
Equatorial coordinate system - A celestial coordinate system is based on a projection of the Earth’s lines of latitude and longitude on a celestial sphere. This system essentially maps the entire heavens on to the inside of this virtual sphere. The equatorial coordinates are not dependent on time or location of the observer and are thus more widely used to specify the positions of celestial objects.
Celestial sphere - Wikipedia page with the history & concept of an abstract sphere concentric to Earth’s globe.
Celestial sphere - An advanced look, from NASA’s Basics of Space Flight learning resource
Optical Telescope Types -
Refractor - Design that uses lenses to form a focused image for the viewer
Reflector - Design that uses an arrangement of mirrors to bring light to a focus
Catodioptric - Design using mirrors and lenses combined to form a focused image
Telescope Mounts -
Altazimuth mount - Simple two-axis mount for supporting and rotating an instrument
Equatorial mount - Mount for instruments that compensates for Earth’s rotation by having one rotational axis
Originally published on Feb 21, 2009; Updated on Sep 13, 2026