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H

Hydrogen-Alpha Filter - A type of narrowband filter.  There are two types of Hydrogen-Alpha (H-alpha) filters.  One is used for solar observing and allows viewing of the prominences and solar flares in the sun's chromosphere.  The other type of H-alpha filter is used for CCD imaging of nebulae.  They are very different filters and cannot be used interchangeably, but they both allow transmission of the hydrogen-alpha portion of the visible light spectrum, so they are referred to by the same name.  The H-alpha line is at a wavelength of 656.3 nanometers, in the red part of the spectrum.

Hand Control - This is the small hand-held controller used to control a computerized telescope.  Also, some non-computerized telescopes with motor drives also have hand controllers, used to make small adjustments to the telescope's position when viewing.

Hyperbolic Mirror - An aspheric mirror shape that is used to eliminate certain optical aberrations.  Hyperbolic mirrors are difficult to make and hence are seen only in specialized, expensive telescope designs, notably the Ritchey-Chrétien design.