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Glossary
Array Size The size of a CCD or CMOS in pixels.
Astigmatism A problem caused by the incorrect alignment of optical components.
AU The mean distance between the Sun and Earth, 149,597,870,700 m. Used
mainly for working with objects within the solar system.
Background In photometry, the unwanted signal arising not from the source, but
from the sky, internal sources of noise, etc.
Bahtinov Mask A mask that causes a distinctive pattern when the telescope is in
focus.
Balmer Series A series of hydrogen lines caused by the transition of an electron
down to the n = 2 energy level.
Band A range of wavelengths. For example, the R-band is the wavelength over
which a Johnson R-band filter is transparent.
Barlow A device to increase the effective focal length of a telescope.
Bias The signal within a CCD frame that comes from the read process and other
such sources.
Bias Frame A zero-second exposure image used to remove the bias.
Binning The joining of pixels to form one large pixel.
Black Body A perfect emitter and absorber of radiation.
Blooming The spike effect caused by reading a saturated pixel.
Bolometric Overall wavelengths. For example, the bolometric magnitude is the
magnitude integrated over the entire spectrum.
Broadband Filter A filter that covers multiple spectral lines.
Cassegrain A telescope using a hyperbolic secondary mirror placed parallel to
the parabolic primary, reflecting the light down the tube and through a hole in the
centre of the primary with the focal plane lying just outside the tube at its rear.
Catadioptric A telescope design that uses a correcting lens (known as the correcting plate) in front of the primary mirror.
CCD Charge-coupled device. The light-sensitive part of a CCD camera.
Celestial Equator The Earth’s equator projected onto the sky.
Charge Diffusion The loss of electrons during the CCD transfer process.
Chromatic Aberration The appearance of coloured rings in refractors, due to
different wavelengths being brought to different focal points.
Circularly Polarised Polarised to the rotational orientation of the photon’s electric field rather than its linear orientation.
Circumpolar An object that is either visible to an observer all year round or never
visible.
Cold Pixel A pixel reporting low at all times.
Collimation The alignment between the optical components in a reflecting telescope. A telescope out of collimation has elongated point sources.
Colour Index The ratio of two fluxes in different bands. Colour is a distanceindependent value.
Colour–Magnitude Diagram (CMD) A plot of magnitude versus colour (for
example, B versus B-R).
Glossary
Array Size The size of a CCD or CMOS in pixels.
Astigmatism A problem caused by the incorrect alignment of optical components.
AU The mean distance between the Sun and Earth, 149,597,870,700 m. Used
mainly for working with objects within the solar system.
Background In photometry, the unwanted signal arising not from the source, but
from the sky, internal sources of noise, etc.
Bahtinov Mask A mask that causes a distinctive pattern when the telescope is in
focus.
Balmer Series A series of hydrogen lines caused by the transition of an electron
down to the n = 2 energy level.
Band A range of wavelengths. For example, the R-band is the wavelength over
which a Johnson R-band filter is transparent.
Barlow A device to increase the effective focal length of a telescope.
Bias The signal within a CCD frame that comes from the read process and other
such sources.
Bias Frame A zero-second exposure image used to remove the bias.
Binning The joining of pixels to form one large pixel.
Black Body A perfect emitter and absorber of radiation.
Blooming The spike effect caused by reading a saturated pixel.
Bolometric Overall wavelengths. For example, the bolometric magnitude is the
magnitude integrated over the entire spectrum.
Broadband Filter A filter that covers multiple spectral lines.
Cassegrain A telescope using a hyperbolic secondary mirror placed parallel to
the parabolic primary, reflecting the light down the tube and through a hole in the
centre of the primary with the focal plane lying just outside the tube at its rear.
Catadioptric A telescope design that uses a correcting lens (known as the correcting plate) in front of the primary mirror.
CCD Charge-coupled device. The light-sensitive part of a CCD camera.
Celestial Equator The Earth’s equator projected onto the sky.
Charge Diffusion The loss of electrons during the CCD transfer process.
Chromatic Aberration The appearance of coloured rings in refractors, due to
different wavelengths being brought to different focal points.
Circularly Polarised Polarised to the rotational orientation of the photon’s electric field rather than its linear orientation.
Circumpolar An object that is either visible to an observer all year round or never
visible.
Cold Pixel A pixel reporting low at all times.
Collimation The alignment between the optical components in a reflecting telescope. A telescope out of collimation has elongated point sources.
Colour Index The ratio of two fluxes in different bands. Colour is a distanceindependent value.
Colour–Magnitude Diagram (CMD) A plot of magnitude versus colour (for
example, B versus B-R).