Littrow

The Littrow prism is also widely used. It is a 30° prism with a mirror back face the light passes through the prism to the mirror face and is reflected back through the prism, the total path being equal to a 60° prism. The prism mounting results in a fairly compact instrument (Section 17.7). 

Liquid junction potential 63, 549 Literature of analytical chemistry 6, 122, 156, 251, 253, 498, 499, 640, 641, 813, 815 Lithium, D. of as aluminate, (g) 459 Litmus 265 Litre xxix, 78 Littrow mounting 661 Logarithms four figure, 843 Lovibond comparator 655 Low voltage d.c. arc 763, 771 Lubricants for glass stopcocks 85 Lyophilic colloids 419 Lyophobic colloids 419 stability of, 419... 

Cary 83 2.0 — Littrow double gratings 0.4 1200 lines/mm, blazed at... 

Apart from using the grating in zero order so that it works like a mirror, maintaining a (compact) round beam at the camera (A = 1) requires that the spectrograph adopt the Littrow configuration for which Tf = 0 and the incident and diffracted rays are parallel. From equations 6 and 9, the resolving power at blaze in the Littrow configuration is... 

Absolute elliciency of fitted CMOS gratings (Littrow, unpolarised)... 

Figure 4. The efficiency of gratings used in the Gemini Multiple Object Spectrograph (CMOS) from measurements by the vendor in Littrow configuration.

Figure 11. Left Littrow configuration for an echelle grating. Right example of the layout of orders (labelled by m) on the detector showing the wavelength ranges covered.

Figure 24.6, shows the schematic diagram of a Littrow Type spectrograph which essentially has the following components, namely ... 

It is interesting to observe that a typical large Littrow Spectrograph having a single Quartz prism covers a wavelength range from 2000 to 80000 A. 

Littrow mounting spect The arrangement of the grating and other components of a Littrow grating spectrograph, which is analogous to that of a Littrow quartz spectrograph. li tro, maunt-ir ... 

Littrow quartz spectrograph spect A spectrograph in which dispersion is accomplished by a Littrow quartz prism with a rear reflecting surface that reverses the light a lens in front of the prism acts as both collimator and focusing lens. li,tro Ikworts spek-tr9,graf ... 

For clarity and precision in the following discussion, the 5-m Littrow spectrometer at the University of Tennessee at Knoxville will serve as the prototype (Jennings, 1974). The optical diagram is presented in Fig. 1. The system is used as an absorption spectrometer most of the discussion also applies to acquisition of emission spectra. 

As the beam leaves the prism predisperser, it is focused on the entrance slit of the grating monochromator. The slit is curved, has variable width, and opens symmetrically about the chief ray (optical center line of system). The monochromator itself is of the off-axis Littrow variety (James and Sternberg, 1969 Stewart, 1970 Jennings, 1974) and uses a double-pass system described by McCubbin (1961). The double-pass aspect of the system doubles the optical retardation of the incident wave front and theoretically doubles the resolution of the instrument. The principal collimating mirror is a 5-m-focal-length, 102-cm-diam parabola. 

The grating equation for both single- and double-pass Littrow monochromators is given by... 

The following definitions are required 6g is the angle between a ray parallel to the collimator axis and the grating normal, the angle of incidence 8 = 0g + Si9 and the diffraction angle 9d = 9g — 5d. In the symmetrical Littrow case 5 = — <5d, and Eq. (3) follows from the general grating equation... 

The data illustrated in Fig. 4(a) are methane absorption lines (0.02 cm-1 wide) observed with a four-pass Littrow-type diffraction grating spectrometer. For these data also, 256 points were taken. The data were obtained at low pressure, so that Doppler broadening is the major contributor to the true width of the lines. The straightforward inverse-filtered estimate with 15 (complex) coefficients retained is shown in Fig. 4(b). Figure 4(c) shows the restored function. The positions and intensities of the restored absorption... 

Figure 14.12 —Schematic of an instrument showing deuterium lamp background correction. Perkin Elmer, model 3300 with a Littrow-type monochromator. This double beam assembly includes a deuterium lamp whose continuum spectrum is superimposed, with the aid of semitransparent mirrors, on the lines emitted by the hollow cathode lamp. One beam path goes through the flame while the other is a reference path. The instrument measures the ratio of transmitted intensities from both beams. The correction domain is limited to the spectral range of the deuterium lamp, which is from 200-350 nm. (Reproduced by permission of Perkin Elmer.)...

Littrow prism A prism with a reflecting back surface. 

Most available infrared instruments use the Littrow mount for the prism, the beam being reflected from a plane mirror behind the pnsm and thus returning it through the prism a second time. This doubles the dispersion produced. Actually, a double-pass system is also used so that the beam goes through the pnsm four times. Other design modifications include those with single beam and double monochromator, double beam and double monochromator, and related combinations, See also Infrared Radiation. 

Figure 10.6. Remote-sensed spectra of representative areas on the Moon s surface (from Gaddis et al., 1985). Left telescopic spectral reflectance scaled to unity at 1.02 i.m and offset relative to adjacent spectra right residual absorption features for the same measurements after a straight line continuum extending from 0.73 pm to 1.6 pm has been removed, (a) Highland soil sampled at the Apollo 16 landing site (b) high-Ti mare basalt at the Apollo 17 landing site (c) low-Ti mare basalt at Mare Serenitatis and (d) pyroclastic deposits at Taurus-Littrow.

Heitmann et al. [11] designed a very compact double monochromator, consisting of a 300 mm prism pre-monochromator and a 400 mm echelle grating monochromator, both in Littrow mounting, which is shown schematically in Figure 4.3. The prism monochromator selects the part of the spectrum that is of interest, and the echelle monochromator provides the high dispersion of the selected spectral interval, which is better than 2 pm per pixel at 200 nm (see Welz et al. [10]). 

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