Spectroscopic Instruments

Abstract

The selection of a spectrographic system including the detector is governed by several aspects: – Wavelength region of interest – Low or high-resolution studies, survey spectra, line intensities only or detailed line profiles – Weak or strong emitter, which usually is equivalent to having a plasma of low or high density – Low or high time resolution, which basically determines the detector and only to a lesser degree the throughput of the system – Stigmatic or astigmatic image of the plasma in the exit planeSpectrometers with the exception of instruments for the X-ray region typically consist of: – An entrance slit (widthw_en, areaA_E) – A dispersive element – An optical system, which forms a spectrally dispersed image of the entrance slit in the exit plane – A detector in the exit plane Figure 3.1 illustrates a schematic layout. Dispersing elements are prisms, gratings, interferometers, and crystals. The imaging system consists usually of a lens L₁ (or mirror M₁) collimating the radiation from the entrance slit, and a lens L₂ (or mirror M₂) focusing the radiation in the exit (image) plane. Mirrors have the advantage of no chromatic aberration and can also be used at shorter wavelengths where glasses, quartz, and crystals absorb the radiation. Unfortunately, their reflectivity decreases at short wavelengths; this can be remedied to some degree by reducing the number of reflecting surfaces and employing spherical or even toroidal gratings which combine focusing and dispersing properties. The optical system (L₁, L₂) or (M₁, M₂) becomes unnecessary.