Description and configuration of spectrograph NP250-2M

The NP250-2M Imaging spectrograph manufactured by SOL instruments is a manual-control device with two independent spectral channels and astigmatism compensation, designed on the modified Hill scheme.

The aperture ratio (F/number) of the device is 1/4.2. Each of channels is an imaging spectrograph with a focal length of 270 mm and F/number of 1/6.1. The used optical scheme includes parabolic mirrors, which allows a high-performance spectral line imaging. It provides both high spatial and spectral resolution.

The entrance slit may be illuminated both directly and by means of fiber optics. The maximum acceptable height of the entrance slit is 4 mm. If the light enters the slit directly, a special height-limiting diaphragm should be used to set the height limit in 4 mm. When using the optical fiber, an adapter should be used to match apertures of the optical fiber and spectrograph. There is one more additional requirement in such a case, the total fiber optical bundle height along the slit should be less than 4 mm.

The images of the entrance slit are formed in two channels simultaneously. Two height spaced spectra can be observed in the focal plane of the NP250-2M spectrograph. The distance between the centers of two spectral tracks is 4.8 mm. The height of each spectral track in its central part is equal to the entrance slit height due to the small level of aberrations in the system and its vertical magnification of 1. The maximum height of the spectral track in every spectral channel, which may be achieved with the largest slit height, is 4 mm. It requires a matrix detector with a height of 8.8 mm at least.

Two independent spectral channels in the NP250-2M, each of which is an imaging spectrograph, extend user opportunities and provide the following benefits:

1. Registration of an extended range (doubled spectral range, practically). The spectral ranges of the channels are selected so that the long-wavelength cutoff of the first channel is the short-wave boundary of the other one. In this case, diffraction gratings with the same groove densities should be used.
2. Simultaneous detection with different spectral resolution of the same spectral line. Diffraction gratings are selected with very different groove densities, usually. In one channel, a broadband spectrum (so-called panoramic spectrum) is acquired. A spectral line can be chosen in the panoramic spectrum, and its wavelength may be set to a second channel, which allows recording a narrow region of the panoramic spectrum, but with a higher resolution. Thus, it is possible to observe all interesting spectral regions of the panoramic spectrum in the high-resolution channel.
3. Simultaneous registration of two spectral regions, which are sufficiently separated in wavelength. In this case, diffraction gratings can be used not only with the same grooves density but also with different them. It depends on the desired task to be solved.

Through the use of parabolic mirrors with a vertical magnification of 1, the NP250-2M two-channel Imaging spectrograph is ideal for multi-channel spectroscopy.

The light from several sources may be delivered to the device entrance slit using a multi-channel optical fiber. The height spaced spectra (in accordance with the fiber structure) are formed in the focal plane of each channel. Spectra are acquired with a matrix detector, simultaneously.

As opposed to the conventional multi-channel spectroscopy, when all of the spectra recorded in the same spectral range, the presence of two independent channels in the NP250-2M device allows to record spectra from different sources in two spectral ranges simultaneously.