Let's consider the main types of spectral instruments used in astronomy. For the first time spectra of stars and planets began to be observed in the last century by Italian astronomer Secchi. After his work, many astronomers started spectral analysis. At first, a visual spectroscope was used, then the spectra were photographed, and now the photovoltaic recording of the spectrum is also used. Spectral instruments with photographic recording of the spectrum are usually called spectrographs, and with photovoltaic - spectrometers.
Currently, along with prism spectrographs and spectrometers, diffraction spectrometers are widely used. In these devices, instead of a prism, a diffraction lattice is a dispersing (i.e., decomposing into a spectrum) element. Reflective grids are the most commonly used.
The reflective grille is an aluminum mirror with parallel strokes. The distance between the strokes and their depth is comparable to a long wave. For example, diffraction gratings operating in the visible spectrum are often made with a stroke spacing of 1.66 microns (600 strokes per 1 mm). The strokes must be straight and parallel to each other over the entire surface of the grating, and the distance between them must remain constant with very high accuracy. The production of diffraction gratings is, therefore, the most difficult of the optical production.
When receiving the spectrum with the help of a prism, we take advantage of the phenomenon of light refraction at the border of two environments. The effects of the diffraction grating are based on another type of phenomenon - diffraction and light interference. Note that it gives, unlike the prism, not one, but several spectra. This results in certain losses of light compared to the prism. As a result of the use of diffraction gratings in astronomy, it has long been limited to solar research. This shortcoming was corrected by the American optician Wood. He suggested that the lattice bars should be given a certain profile, such that most of the energy is concentrated in one spectrum, while the rest are severely weakened. These grids are called directional or echellette grids.
Features of optical circuitry and design
The peculiarities of the optical scheme and design of astronomical spectral devices strongly depend on the specific nature of the tasks for which they are designed. Spectrographs built to produce stellar spectra (stellar spectrographs) differ significantly from the nebular spectra with which nebulae spectra are studied. Solar spectrographs also have their own peculiarities. The real resolution power of astronomical instruments depends on the properties of the object. If an object is weak, i.e. too little light comes from it, then its spectrum cannot be studied in detail, because with the increase in the resolution force, the amount of energy coming to each resolved element of the spectrum decreases. Therefore, solar spectral instruments naturally have the highest resolution power. In the case of large solar spectrographs, it reaches 10 6. the linear dispersion of these devices reaches 10 mm/Å (0.1 Å/mm).
A rough idea of the spectral composition of the radiation can be obtained with the help of light filters. In photographic and visual spectral regions of the spectrum, it is often used the filters made of painted glass. The figure shows the curves showing the dependence of transmission on the wavelength for some filters, combining which with this or that receiver, it is possible to identify areas not already several hundred angstrom. In stained glass filters, the dependence of light absorption (absorption) on the wavelength is used. Light filters of this type are called absorption filters.
Special Avenues
For solar research, instruments have been developed to provide monochromatic images at any wavelength. It is a spectroheliograph and spectrohelioscope. Spectrogeliograph is a monochromator with a photographic cassette behind its exit slit. The cassette moves at a constant speed in the direction perpendicular to the exit slit, and the image of the Sun moves at the same speed in the plane of the exit slit. It is easy to understand that in this case on the photographic plate there will be an image of the Sun in a given wavelength, called a spectrogram. In the spectrohelioscope, before the exit slit and after the exit slit, rotating prisms with a square section are installed. As a result of the rotation of the first prism, some part of the solar image periodically moves in the plane of the entrance slit. The rotation of both prisms is coordinated, and if it occurs quickly enough, observing the second slot in the telescope, we see a monochromatic image of the Sun.