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The incident light traveling at an angle (i) reflects off a surface at the same angle (r) between the reflected ray and the perpendicular (p).

The change in the direction of light as it passes through a medium.  Light slows as it passes through a denser medium than a vacuum.

Glass that is shaped to make distant objects closer and brighter.

Glass that is coated with silver, aluminum, or a similar highly reflective substance that reflects light waves.

Measures the spectra of an object, not recording an image of an object.  A diffraction grating is used to break up light into a spectrum.

A telescope that orbits the earth at 600km.  It can measure any wavelength of light from 115 nm to 1 micro n.

The bending of light by the earth’s atmosphere.

An observer or instrument is placed directly at the focal point, within the barrel of a telescope.  Used only in large reflecting telescopes.

Has a convenient, accessible focal point in a telescope.  A hole is drilled directly through the center of the primary mirror, and a convex secondary mirror placed in front of the original focal point reflects the light rays back through the hole.

A quantity that gauges how well fine details of objects can be seen.  It is the angle between 2 stars.  The smaller the angle, the sharper the resolution.

A system that compensates for atmospheric turbulence, so that the angular resolution can be smaller than the size of the seeing disk.  As turbulence causes a star to “dance” around, sensors detect the “dancing” and adjust the mirror accordingly.

A convex mirror that is placed in front of the original focal point in a reflecting telescope.  It reflects light rays from the primary mirror to an observer, instrument, or another mirror.

A technique designed to improve angular resolution.  The idea is to have 2 widely separated telescopes observe the same object at the same time, then use fiber optic cables to pipe the light signals to a central location, where they are combined to form a single image.

The measurement of the brightness of stars and other astronomical objects.

A reflecting telescope that has a flat mirror at a 45 degree angle in front of the focal point in the barrel of a telescope.  This secondary mirror reflects the light rays to one side of the barrel, where an eyepiece lens is placed to magnify the image.

The light-gathering power of a telescope is directly proportional to the objective lens, which is proportional the square of the lens diameter.

A concave mirror that that reflects light rays to a focal point.