HIGH RESOLUTION GRATING SPECTROGRAPH

 

The optical system of the spectrograph consists of:(a) 25 μm entrance slit (2.4 mm height);(b)concave mirror (f = 500 mm); (c) 600 lines/mm diffraction grating (24 x 102 mm of working area); (d)  plane mirror with adjustable tilt;-(e)detector (photodiode array or CCD camera).

After passing the entrance slit, a light beam is reflected from a concave mirror. The parallel beam falls onto the grating at the sliding angle. After the grating, light is redirected by a plane mirror back onto the grating which reflects light under some other sliding angle to the concave mirror. A selected part of the spectrum is imaged onto the focal plane, i.e. on a detector. This part of the spectrum is determined by diffraction order and the angle between the plane mirror and the grating. Large angular dispersion under sliding incidence and the double dispersion on the grating result in high spectral resolution of such a compact device. A precise mechanism with a stepping motor controls the tilt angle of the plane mirror. Choosing a proper tilt angle and the most convenient diffraction order, one can set the spectrograph  to each wavenumber within 7087 -– 52350 cm^-1. Working at the blaze angle (54.1 degrees, when the grating reflecting efficiency is the highest) one chooses the highest difraction order for the larger selected wavenumber. Second diffraction order corresponds to the minimal wavenumber and the order of twelve corresponds to maximal one in all the measurement range. 

The change of the plane mirror tilt angle within any diffraction order influences: (i) the center wavenumber value of the spectral zone; (ii) the spectral zone width; (iii)  the effective resolution of the spectrograph.

A d value is determined by the grating resolution (the chosen diffraction order) and by the detector spatial resolution

(1 array pixel = 25 mm).

Spectrograph optical system aperture ratio: f/22.

Spectrograph dimensions: 23x 23x 46 cm.

Slit position above spectrograph base: 26cm.

 

 

 

 

 

On the right,  nitrogen laser spectra obtained by:

The ½-meter spectrograph with a photodiode array (resolution Δ λ = λ /502000) (top);

A 14-meter Czerny-Turner spectrograph (resolution Δ λ = λ /670000) (bottom).