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diffraction grating
subject			fact

diffraction grating	has definition Polished metallic surface (usually a metallic mirror on a block of glass or quartz) on which has been ruled a great number (in thousands) of parallel lines, used to split light to produce a spectrum.
	has definition A system of parallel slits, where the slit width is of the same order as the wavelength of the incident radiation, which is capable of dispersing light into its spectrum.
	has definition An optical device containing thousands of very fine parallel grooves which produce interference patterns in a way which separates out all the components of the light into a spectrum.
	is a kind of spectrograph dispersing element
spectrograph dispersing element	has dispersion
	has wavefront modification wavelength
instrument	has operator
artifact	has creator
	has creation date
	has purpose
	has owner
physical object	has location or center of gravity
	has angular momentum
	has mass
	has velocity
	has momentum
	has temperature
	has volume
	has extent
	has material

Kinds of diffraction grating :

• blazed grating (2 facts) - Diffraction grating so ruled that the reflected light is concentrated into only a few orders, or even a single order, of the spectrum.
• echelle (2 facts) - A type of diffraction grating with groove angles of 90°. With the grating at an angle of 45° the grooves resemble a staircase.
• grism (2 facts) - This is a right-angled glass prism with a transmission diffraction grating deposited on the hypotenuse surface. The spectrum produced by the grating is deflected by the prism to remain on the optical axis and the apex angle of the prism is chosen to get a certain wavelength in the center of the detector. Grisms can be placed in a filter wheel.
• objective grating (2 facts) - A coarse grating placed in front of the telescope objective.

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