Glossary
225
Coma A problem with a reflector whereby light incident to the edge of the mirror
is brought to a slightly different focus from light at the centre, resulting in the
image becoming smeared out.
Coordinated Universal Time A standard time based on GMT. The different
between GMT and UT is currently minor.
Cosmic Ray A high-energy charged particle. The source of cosmic rays is not yet
fully understood, although high-energy events such as supernovae are thought to
be likely candidates.
Count A dimensionless instrument-specific unit, indirectly related to flux.
Dark Adaptation A combination of physiological responses that increase the
ability to see in low light levels. Dark adaptation takes about 20 min to reach
its optimal level. It is lost almost instantly when the eye is exposed to bright light.
Dark Current The noise caused by thermal electrons.
Dark Frame An image of the same exposure time as the light frame but with the
shutter closed. Used to remove the dark current.
Diagonal An optical device that changes the pathway of light as it leaves the
telescope.
Diffraction Halo A halo that appears around bright objects due to diffraction.
Diffraction Spike A crosslike spike appears around bright stars in images. Often
added by amateurs in Photoshop.
Digitisation noise Noise caused by an ADU that is a digital device.
Dithering Moving the frame a few pixels between exposures so that some sources
are not exposed to the same pixels.
Dome Flats Flat frames that use the inside of the dome as the illumination source.
Ecliptic The path along which the Sun appears to travel.
Elliptic Coordinates A coordinate system using the ecliptic as the fundamental
plane.
Emission Lines Bright lines in a spectrum caused by the presence of atoms and
molecules in the emitting source.
Emission Nebula Any nebula that emits light, normally due to embedded stars.
Equation of Time The translation between mean solar time and apparent solar
time.
Equator The great circle on a sphere perpendicular to the meridian and equidistant
from the poles.
Exoplanets Planets orbiting stars other than the Sun.
Extended Objects A resolved object, for example a planet or a galaxy.
Extinction Coefficient The level of extinction.
Eye Relief A measure of the comfort of an eyepiece.
Field Curvature A problem associated with the projection of a curved surface
(the sky) onto a flat one (the CCD).
Field of View The amount of sky visible through the instrument, measured in
degrees or arc minutes.
Field Stop The aperture of an eyepiece.
Filters A device designed to allow only a range of specific wavelengths to pass
through to the detector.
225
Coma A problem with a reflector whereby light incident to the edge of the mirror
is brought to a slightly different focus from light at the centre, resulting in the
image becoming smeared out.
Coordinated Universal Time A standard time based on GMT. The different
between GMT and UT is currently minor.
Cosmic Ray A high-energy charged particle. The source of cosmic rays is not yet
fully understood, although high-energy events such as supernovae are thought to
be likely candidates.
Count A dimensionless instrument-specific unit, indirectly related to flux.
Dark Adaptation A combination of physiological responses that increase the
ability to see in low light levels. Dark adaptation takes about 20 min to reach
its optimal level. It is lost almost instantly when the eye is exposed to bright light.
Dark Current The noise caused by thermal electrons.
Dark Frame An image of the same exposure time as the light frame but with the
shutter closed. Used to remove the dark current.
Diagonal An optical device that changes the pathway of light as it leaves the
telescope.
Diffraction Halo A halo that appears around bright objects due to diffraction.
Diffraction Spike A crosslike spike appears around bright stars in images. Often
added by amateurs in Photoshop.
Digitisation noise Noise caused by an ADU that is a digital device.
Dithering Moving the frame a few pixels between exposures so that some sources
are not exposed to the same pixels.
Dome Flats Flat frames that use the inside of the dome as the illumination source.
Ecliptic The path along which the Sun appears to travel.
Elliptic Coordinates A coordinate system using the ecliptic as the fundamental
plane.
Emission Lines Bright lines in a spectrum caused by the presence of atoms and
molecules in the emitting source.
Emission Nebula Any nebula that emits light, normally due to embedded stars.
Equation of Time The translation between mean solar time and apparent solar
time.
Equator The great circle on a sphere perpendicular to the meridian and equidistant
from the poles.
Exoplanets Planets orbiting stars other than the Sun.
Extended Objects A resolved object, for example a planet or a galaxy.
Extinction Coefficient The level of extinction.
Eye Relief A measure of the comfort of an eyepiece.
Field Curvature A problem associated with the projection of a curved surface
(the sky) onto a flat one (the CCD).
Field of View The amount of sky visible through the instrument, measured in
degrees or arc minutes.
Field Stop The aperture of an eyepiece.
Filters A device designed to allow only a range of specific wavelengths to pass
through to the detector.