Spectrograph
Encyclopedia
A spectrograph is an instrument that separates an incoming wave into a frequency spectrum
. There are several kinds of machines referred to as spectrographs, depending on the precise nature of the waves. The term was first used in 1884.Merriam-Webster Dictionary: Spectrograph
A spectrograph is closely related to a polychromator
.
as the detector. The star spectral classification
and discovery of the main sequence
, Hubble's law
and the Hubble sequence were all made with spectrographs that used photographic paper. The plant pigment phytochrome
was discovered using a spectrograph that used living plants as the detector. More recent spectrographs use electronic detectors, such as CCD
s which can be used for both visible and UV
light. The exact choice of detector depends on the wavelengths of light to be recorded.
The forthcoming James Webb Space Telescope
will contain both a near-infrared spectrograph (NIRSpec) and a mid-infrared spectrometer (MIRI).
s, rotated 90 degrees with respect to each other and placed close to one another. Therefore an entrance point and not a slit is used and a 2d CCD-chip records the spectrum. Usually one would guess to retrieve a spectrum on the diagonal, but when both gratings have a wide spacing and one is blazed
so that only the first order is visible and the other is blazed that a lot of higher orders are visible, one gets a very fine spectrum nicely folded onto a small common CCD-chip. The small chip also means that the collimating optics need not to be optimized for coma or astigmatism, but the spherical aberration
can be set to zero.
Frequency spectrum
The frequency spectrum of a time-domain signal is a representation of that signal in the frequency domain. The frequency spectrum can be generated via a Fourier transform of the signal, and the resulting values are usually presented as amplitude and phase, both plotted versus frequency.Any signal...
. There are several kinds of machines referred to as spectrographs, depending on the precise nature of the waves. The term was first used in 1884.Merriam-Webster Dictionary: Spectrograph
A spectrograph is closely related to a polychromator
Polychromator
A polychromator is an optical device that is used to disperse light into different directions to isolate parts of the spectrum of the light. A prism or diffraction grating can be used to disperse the light. Unlike a monochromator, it outputs multiple beams over a range of wavelengths simultaneously...
.
Stellar & solar spectrograph
The first spectrographs used photographic paperPhotographic paper
Photographic paper is paper coated with light-sensitive chemicals, used for making photographic prints.Photographic paper is exposed to light in a controlled manner, either by placing a negative in contact with the paper directly to produce a contact print, by using an enlarger in order to create a...
as the detector. The star spectral classification
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...
and discovery of the main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
, Hubble's law
Hubble's law
Hubble's law is the name for the astronomical observation in physical cosmology that: all objects observed in deep space are found to have a doppler shift observable relative velocity to Earth, and to each other; and that this doppler-shift-measured velocity, of various galaxies receding from...
and the Hubble sequence were all made with spectrographs that used photographic paper. The plant pigment phytochrome
Phytochrome
Phytochrome is a photoreceptor, a pigment that plants use to detect light. It is sensitive to light in the red and far-red region of the visible spectrum. Many flowering plants use it to regulate the time of flowering based on the length of day and night and to set circadian rhythms...
was discovered using a spectrograph that used living plants as the detector. More recent spectrographs use electronic detectors, such as CCD
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
s which can be used for both visible and UV
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
light. The exact choice of detector depends on the wavelengths of light to be recorded.
The forthcoming James Webb Space Telescope
James Webb Space Telescope
The James Webb Space Telescope , previously known as Next Generation Space Telescope , is a planned next-generation space telescope, optimized for observations in the infrared. The main technical features are a large and very cold 6.5 meter diameter mirror, an observing position far from Earth,...
will contain both a near-infrared spectrograph (NIRSpec) and a mid-infrared spectrometer (MIRI).
Echelle spectrograph
An echelle spectrograph uses two diffraction gratingDiffraction grating
In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as...
s, rotated 90 degrees with respect to each other and placed close to one another. Therefore an entrance point and not a slit is used and a 2d CCD-chip records the spectrum. Usually one would guess to retrieve a spectrum on the diagonal, but when both gratings have a wide spacing and one is blazed
Blazed grating
A blazed grating is a special type of diffraction grating.Blazed gratings produce maximum efficiency at a specified wavelength; that is, a diffraction grating that is "blazed at 250nm" will operate most efficiently when light with a wavelength of 250 nm passes through the grating.Like standard...
so that only the first order is visible and the other is blazed that a lot of higher orders are visible, one gets a very fine spectrum nicely folded onto a small common CCD-chip. The small chip also means that the collimating optics need not to be optimized for coma or astigmatism, but the spherical aberration
Spherical aberration
thumb|right|Spherical aberration. A perfect lens focuses all incoming rays to a point on the [[Optical axis|optic axis]]. A real lens with spherical surfaces suffers from spherical aberration: it focuses rays more tightly if they enter it far from the optic axis than if they enter closer to the...
can be set to zero.