Spectrographs and Monochromators
Did You Know?
The world's first spectroscope was invented by Joseph von Fraunhofer in 1814. For his discovery in the area of spectroscopy he was crowned as Knight in 1824. Spectrograph’s main function is to disperse light into its different wavelength components allowing the investigation of a specific wavelength region of the optical spectrum. It is this property that makes these instruments a vital tool for astronomic studies, from understanding the composition of Sun to analysing rocks on Mars with today’s state of the art Exploration Rovers.
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Frequently Asked Questions
In simple terms, a spectrograph is a device that separates a beam of light into its component wavelengths to be easily analyzed. Light first travels through the spectrograph to a collimating mirror which lines up the rays with a diffraction grating. Once the light reflects off this grating, the wavelengths will separate by changing their speed and direction based on their designated color. The remaining components of a spectrograph device are listed in the question below.
The standard components of a spectrograph are the entrance slit, the collimator (mirrors used to narrow particles and waves), a grating or prism, and a detector.
A monochromator is a device that produces a light beam of a selected single color with a very narrow bandwidth. It separates polychromatic light into various separate wavelengths, allowing for easier selection of specifically desired wavelengths.
Spectrographs are often used in astronomy, the collected data informs astronomers how much light of each wavelength is being emitted by their observation. This reveals important data on the atmospheres of distant planets and the composition of celestial objects (stars, nebulas, etc.). Monochromators are also commonly used in optical spectroscopy applications, and frequently utilized for measurement devices like spectrometers and microplate readers.
A spectrometer captures the entirety of the UV and visible spectrum, providing values at every wavelength. A monochromator captures only one measurement in the UV and visible spectrum, providing a single value at a specific/desired wavelength or bandwidth. Both devices offer their own advantages depending on their application.
The best choice is dependent upon what you will be using the device for; you don’t want to pay extra for features that you will not be using. Some key specs to consider when choosing your best option are the device’s monochromator type, focal length, spectral range, spectral resolution, diffraction grating, and grating blaze wavelength.
Czerny-Turner and Ebert spectrometers are both types of monochromators used for spectroscopy, but they differ in their optical designs. Czerny-Turner spectrometers use a concave mirror to focus the light onto a slit, which then passes through a diffraction grating. The diffracted light is then focused onto a detector by a second concave mirror. This design allows for good dispersion of the light and high spectral resolution, but it may suffer from off-axis aberrations that can cause distortions in the spectrum. Ebert spectrometers, on the other hand, use a curved grating as both the dispersing element and the focusing element. The incoming light is focused onto the grating, which diffracts the light and focuses it onto the exit slit. The use of a single element for both dispersion and focusing reduces off-axis aberrations and simplifies the optical design. However, Ebert spectrometers may have lower spectral resolution compared to Czerny-Turner spectrometers.