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Terminology
Absorption lines dark lines from a stellar or planetary atmosphere superimposed upon the spectrum background [or continuum].
Angstrom [A] a unit of wavelength measurement in a spectrum where 1A = 0.0001micron = 0.00001mm - see namometer [nm]
Atomic table list of elements from the lightest H [hydrogen] to the heavier elements like Ur [uranium] etc - excellent website + colour spectra
Autocollimator a lens that performs the dual function of both collimator and camera lens [Littrow arrangement] - the camera forming the spectrum.
Balmer series
sequence of hydrogen lines in the visible region beginning at Ha
@ 656.3nm and prominent in A-type stars like Vega or Sirius
-Lyman
sequence of hydrogen lines in the UV region beginning at Lyman a
@ 121.6nm. Often red-shifted in quasars into the visible region.
-Perchen
sequence of hydrogen lines in the IR region
Be stars stars of B-type spectra with an emission lines [b Lyrae] usually denoting exchange of material to an encircling shell of gas or companion.
Camera a lens or mirror system within the spectrograph that focuses the spectrum onto the CCD or photofilm detector.
CCD [charged-coupled-devise] a modern highly efficient electronic detector for recording direct and spectroscopic images.
Collimator a lens or concave mirror for projecting a parallel beam of light onto the dispersing medium of prism or grating.
Continuum the background spectrum [with no lines] produced by a luminous body like the surface [photosphere] of a star or a tungsten lamp.
Cross disperser a low dispersion prism or grating separating the various orders of spectra typically in an echelle spectrograph.
Dekker a shutter of variable width that limits the length of the slit.
Dispersion
the scale of a spectrum measured in A [or nm] per millimetre [10A/mm =
1nm/mm]
or A [or nm] per pixel [10A/pixel = 1nm/pixel]
-non-linear
usually via a prism which expands the blue end of the spectrum more than
the red end.
-linear
usually via a grating which gives a [near] constant scale along the length
of the spectrum making line measurement easier.
-medium
devise to form a spectrum like a prism or grating.
Doppler-shift
displacement of spectral lines due to the relative motion of an object
[and observer] used to measure line-of-sight velocities.
- broadening
blurring of spectral lines due to an object's opposing motion like rapid
rotation [eg Altair] or an explosion [novae/supernovae]
- red shift
object moving away from the observer as noted by the shift of spectral
lines towards the red end of the spectrum.
- z [redshift]
typically applied to the [cosmological] expansion of the universe since
the Big Bang. z=1 ie lines moved to double their rest wavelength.
- blue shift
object moving towards the observer as noted by the shift of spectral lines
towards the blue end of the spectrum.
Emission lines bright lines typically seen in planetary nebulae or stars like nova, supernovae, Wolf-Rayet and Be stars.
Fibre-optic feed an arrangement where the spectrograph is separated from the telescope focal plane by a fexible optical cable eg Sivo spectrometer.
Flux calibration correction of spectrum to typically remove the effects of varying detector sensitivity and earth's atmospheric absorption etc.
Fraunhofer lines, major lines in the sun's spectrum listed by letter from red [A line] to violet [K line] and lesser lines a to h by Fraunhofer in ~1815.
Grating
a series of fine parallel lines or grooves on a flat glass substate for
diffracting light into a spectrum
-transmission
a transparent grating that transmits light to form the spectrum - like
the Rainbow Optics grating
-reflection
a grating that reflects light to form the spectrum.
-holographic
a grating formed by holography onto a film-type base.
-blaze
the pitch of the grooves within the grating angled to direct light into
one efficient spectrum.
-replica
grating formed in plastic material by impression from a master grating
and mounted on glass - most economic gratings formed this way.
-ruling
another name for the parallel lines or grooves in a grating.
More lines give a higher resolution.
-ruling machine
a machine of the highest precision using a diamond tool to cut groves in
an aluminium coated glass plate to form master gratings.
-echelle
grating giving the highest dispersion and resolution usually with a cross
disperser to separate the overlapping orders of spectra.
Grism a grating + prism combo with zero deviation of the mid spectral region [staight-through view] or as a cross disperser separating orders of spectra.
Harvard spectral sequence basic classification of stars according to their spectra and temperature.
HR Diagram [Hertzstrung-Russell] demographic plot of stars against temperature [or spectral class] and luminosity [or mass].
Ionisation stripping of electrons from atoms within a stellar atmosphere revealed as changed line positions within the spectrum for a given element.
Main sequence central position on the HR Diagram that stars occupy for most of their evolution.
Nanometer [nm] typically a unit of wavelength measurement in a spectrum where 1nm = 10A = 0.001micron = 0.0001mm. See Angstrom.
Objective prism a
large dispersion prism before the telescope used to record spectra of all
the stars in the field. No slit is used.
Order [of spectra]
the multiple spectra produced by a grating. A blazed grating
will direct most of the light into a single bright spectrum.
- zero order
the real [undispersed] stellar image uneffected by the grating ie light
passes straight through or is reflected according to the grating type.
Prism wedge of transparent material like glass or quartz for refracting light into a spectrum
Radial velocity the motion of a body in the line of sight either towards or away from the observer [Doppler Shift] revealed by the spectroscope.
Resolution [R] the actual useful data that can be extracted from a spectrum. Depends of grating size, number of lines, camera lens, pixel size etc.
Rocker plate a motor driven plane glass plate moving the star image up and down the slit during exposure to widen the spectrum [on photo plates].
Slit a devise that feeds a narrow sample of light into the spectroscope which expands it sideways into a spectrum.
Spectral Class ordering of star types according to their surface temperature from 'hot' to 'cool' ie [W]OBAFGKM[RNS] with subdivisions 0 -9.
Spectro-scope
instrument for viewing a spectrum
-meter
instrument for directly measuring a spectrum usually via a micrometer screw
-gram
a record or picture of a spectrum in electronic or paper form usually shown
as a horizontal rectangle crossed with vertical lines. -graph
instrument for recording a spectrum
-Ebert a spectrograph using a single concave [usually
spherical] mirror as collimator and camera eg SBIG spectrometer.
-Littrow a spectrograph using a single lens as both collimator
and camera eg WPO spectrograph.
Spectroscopic Binary
an optically unresolved double star revealed by the cyclic red and
blue shift of the spectral lines over the obital period.
Spectrum
the separation of light into a rainbow of colours through increasing
wavelength ie UV, violet, blue, green, yellow, orange, red, IR etc.
-profile
an alternative way to show a spectrum as a graph of intensity against wavelength.
Often easier to interpret than a spectrogram.
-comparision
emission-line spectrum, typical hydrogen [H], mercury [Hg], etc introduced
at the telescope to a stellar spectrum for wavelength calibration.
-slitless
a spectrum obtained without a slit eg OG prism or Rainbow grating at focal
plane. Relies on point-like stellar images substituting the slit.
Telluric [or atmospheric] lines are superimposed onto a spectrum by oxygen and water-vapour in the earth's atmosphere like A, a, B lines in red.
Wolf-Rayet [W] stars of high mass and temperature topping the Spectral Class classification with subdivisions nitrogen [WN] and carbon [WC].
Zeeman Effect spitting
of lines in a spectrum due to strong magnetic fields.