Achromatic Lens
A compound two (or more) element lens so designed to bring two (or more) wavelengths to the same focus point (focal length). Necessary to combat chromatic aberration.

Active Optics
Mechanical system of thrusters to compensate mirror stresses caused by temperature changes and mirror orientation.

Adaptive Optics
Electrooptical feedback system via active optics that compensates for atmospheric "seeing"

Angular Resolution
Roughly the angular separation of two equally bright stars that can just be seen as two distinct objects. Approximately wavelength/aperture (radian=206265"). Relates to the finest detail that can be seen in a telescope, the smaller the better.

Cassegrain Focus
Reflecting telescope optical arrangement wherein light from the concave primary mirror is directed back through a hole in the center of the mirror by a convex secondary mirror. Very common.

Charge-Coupled Device (CCD)
Solid state array of photon detector elements (image detector). Each element acquires a charge (or discharg) proportional to the photon flux (integration phase) then the charge on rows of detectors are transferred "bucket brigade style" to the last column (readout phase) and the charge on each element stored or displayed.

Chromatic Aberration
Due to the dispersive nature n=n(wavelength) of all refracting materials the focal length of a lens depends on wavelength. Usually blue light is focussed more strongly than red. The bane of refracting optics. See dispersion curve.

Coudé Focus
Reflecting telescope optical arrangement wherein light from the concave primary mirror is directed back through the polar axis through a complicated optical path (Cassegrain secondary-newtonian flat out the dec axis-flat down the dec support arm-flat to polar axis-flat down polar axis) whew! The big advantage is the focal point is fixed in space. Useful for bulky detectors such as large spectrographs.

Electromagnetic Spectrum
The gamut of electromagnetic radiation from very low frequency through radio, microwave, infrared, visible, ultraviolet, X, gamma rays.

Exit Pupil
Real image of objective. All light from objective passes through the exit pupil. The exit pupil must not exceed the size of the eye pupil.

Eyepiece Lens
Lens used to examine the real image formed in the focal plane by the objective lens.

Focal Length
The distance from a lens that an image of a distant object is formed. The focal length of the objective is related to the size of the telescope.

Focal Plane
The locus of points that form the image of an object (focal length).

Focal Point (Focus)
Any point in the focal plane.

Gamma Ray
High energy electromagnetic radiation, E>100keV.

Infrared Radiation
"Heat". Elecctromagnetic radiation less energetic than visible light from micron to hundreds of microns wavelength.

Interferometry
The art of using the phase information contained in electromagnetic waves to reconstruct an image from scant information such as an array of radiotelescopes or two or more optical telescopes separated by a baseline many times the objective diameters. A way of getting high resolution from several collectors with wide separation. VLBI.

Light Gathering Power
A measure of the collecting area of the objective. Goes as aperture squared.

Magnification
The number of times larger an object appears when seen through a telescope. Angular magnification. Goes as objective focal length divided by eyepiece focal length. Goes as aperture divided by exit pupil.

Newtonian Reflector
A rflector design wherein light from the primary is directed back along the optic axis and is intercepted by a 45 degree (Newtonian) flat mirror that directs the light out the side of the telescope.

Objective Lens
The lens closest to the object. The big Kahuna.

Photon
Quantum of light, energy equal to Planck's constant times frequency E=hf=hc/w

Pixel
Picture element. One element of an array of detectors such as a CCD.

Primary Mirror
Reflecting telescope objective. Light bucket.

Prime Focus
The direct focal plane of a reflector, on axis, and blocking the path of the incoming light. Just beyond where a Newtonian flat or Cass secondary would go.

Radio Telescope
Radio analogue of optical telescope. Usually Cassegrain design.

Radio Wave
Electromagnetic radiation having wavelengths ranging from sub millimeter to hundreds of meters in length (a few Hz to tens of GHz).

Reflecting Telescope
Telescope design having a reflecting or mirror objective.

Reflector
Reflecting telescope design having a reflecting or mirror objective.

Refractor
Telescope design having a (usually achromatic) lens objective.

Refraction
The bending of light rays toward the normal when passing from a fast (eg air) to a slow (eg glass) medium

Schmidt
Reflector having spherical primary with a (smaller) corrector plate that yields a fast (f/2 or better) coma free image over a wide field.

Secondary mirror
Mirror placed inside or outside the prime focus to direct the light out of the path of light coming from the object. Newtonian, Cassegrain, etc.

Seeing Disk
Size of star image caused by atmospheric "seeing" effects. Atmospheric turbulence causes distortion of the incoming wavefront resulting in an interference pattern or speckle in the focal plane which, averaged over time, looks like a blur. Limits resolution to around a second of arc.

Speckle
Diffraction image of a star caused by atmospheric "seeing".

Spectrum (spectra)
A graph or photograph of the intensity of the electromagnetic spectrum as a function of frequency or wavelength.

Spherical Aberration
Penalty paid for using spherical surfaces. Rays above the optic axis come to a focus closer to the lens than do axial rays.

Twinkling
Flickering of starlight due to "seeing" effects, interfrence of starlight reaching eye along paths of slightly differing optical path length.

Ultraviolet UV
Electromagnetic radiation that lies between visible(400-770nm) and X (100nm) radiation.

Very Long Baseline Interferometry VLBI
Radioastronomy technique wherein radio signals from distant radiotelescopes is combined (with phase information) to form a very high resolution image of a radio source.

Wavelength
The distance between two succesive peaks in any wave.

X ray
Electromagnetic radiation that lies between "hard" UV and "soft" gamma radiation.