The main limitation in research of weak spectral signals is a level of stray light. Stray light is mostly caused by quality of optical elements: mirrors and gratings. Besides, the stray light appears in the result of reflection of a spectrum from an entrance slit, a detector and from other structural units. The stray light can be minimized by using light absorbing partitions, by inclined installation of detectors, and by using holographic gratings.
Long-focus devices are characterized by lower level of stray light. Monochromators have lower stray light in comparison with spectrographs because the last named are used without any exit slit. Double monochromator is the best choice when the value of stray light is very important for research purposes. With a double monochromator – maximum suppression of stray light can be achieved.
The MZDD350i instrument is a double monochromator with zero dispersion consisting of two imaging monochromators MS3504i configured in one construction and the exit slit of the first monochromator is the entrance slit of the second monochromator. The device is fully automated. A set of four gratings and smoothly adjustable slits allow to operate in a wide spectral range with a required bandpass.
Zero dispersion is reached by exact correspondence of beam pass through the first monochromator to a beam return through the second monochromator. Optical parameters of the double monochromator with zero dispersion are determined by the size of the entrance slit and intermediate (middle) slit and by aberrations of the first monochromator. The size of the exit slit should be selected to avoid vignetting of the entrance slit image. The exit slit would be equal to the entrance slit, if no aberration.
Table 1 shows the design parameters of exit slit depending on size of entrance and intermediate slits for MZDD350i (diffraction grating 1200 G/mm, wavelength 546 nm).
Light at output in a single monochromator is a part of spectrum. The light leaving the double monochromator with zero dispersion is spectrally uniform at the output. This main feature, alongside with capability of maximum suppression of stray light, determines the fields of application of MZDD350i.