Echelle mounting

We have observed three subgiants HD 23249 (KO), HD 198149 (KO), HD 222404 (Kl) and three dwarfs HD 10780 (KO), HD 4628 (K2), HD 201091 (K5), on 2002 November 28 and 29, with the high-resolution cross-dispersed echelle spectrograph SOFIN, mounted on the Nordic Optical Telescope (NOT). They are in the solar neighbourhood (< 15 pc), are very bright (V < 6) and have modest projected rotational velocities (v sin i < 4 km s 1) to limit blends between spectral lines. They also do not present any evidence for emission (or a moderate one, as in the case of the three dwarfs) in the core of Ca II H and K lines. 

The LPA instrument of Mount (42) uses a XeCl laser to monitor absorption near the Qi(S) line group. The laser beam, expanded in a telescope to an initial area of 150 cm2, is reflected from a retroreflector array for a total optical path of 20.6 km. The returned beam and a portion of the outgoing beam follow symmetric paths through an echelle spectrograph to a pair of photodiode array detectors, thus providing both I and Z0 spectra for the... 

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]). 

Obviously, such a high-resolution monochromator requires active wavelength stabilization in order to avoid drift problems. This has been accomplished through an internal neon lamp, mounted on an adjustable stand in front of the intermediate slit between the pre- and echelle-monochromator, so that it can be moved into the beam automatically if necessary. The neon lamp emits many relatively narrow lines in the 580-720 nm range, and, in the absence of any pre-selection, these are separated by the echelle grating into various superimposed orders. This means that without pre-dispersion at least two neon lines for every grating position surely fall on the detector, and can be used for stabilization. The precision of this stabilization is only limited by the stepper motor for grating adjustment, and is better than one-tenth of a pixel width (see Welz et al. [10]). 

Fig. 20. Echelle mounting in optical set-up for CCD-ICP-OES (Optima)(Perl

Fig. 94. Echelle spectrograph with internal order separation in a tetrahedral mounting. (1) Entrance slits, (2) spherical collimator mirror, (3) prism, (4) Echelle grating, (5) spherical camera mirror, (6) focal plane (A)...

Several systems for automation of spectrometers have been discussed. A computer-controlled Echelle monochromator allowed wavelength increments of 0.01 nm. A wavelength-scan and lamp-intensity control scheme for the popular Bausch and Lomb high-intensity monochromator has been described. The accurate synchronization of monochromator wavelength-scan and chart-recorder speed, and the possibility of rapid scanning allowing spectra to be displayed in real time on an oscilloscope, has also been discussed. Details have been provided for the modification of a commercially available mirror mount (Oriel model 1450) for use as a stepper-motor controlled grating mount. 

The spectrometers used are adapted either for sequential or simultaneous multi-element measurements. Commonly used grating spectrometers in plasma AES include (i) spectrometers with the Paschen-Runge mount, (ii) echelle spectrometers, (iii) spectrometers with Ebert and Czerny-Turner mounts, (iv) spectrometers with Seya-Namioka mounts, and (v) double monochromators. Also Fourier transform spectrometers may be used in plasma AES. 

Figure 121 illustrates a sequential echelle spectrometer. Light from the entrance slit is directed by the collimating mirror to the echelle grating, and the diffracted beam is then reflected to the prism lense (order-sorter) and focusing lense. A two-dimensional spectrum is formed in the focal plane of the system. An aperture plate, which contains preset apertures, is mounted at the focal plane. Slits and mirrors are placed at appropriate positions on a... 

Since spectra of different orders may coincide or overlap, some method of separating orders is required. Separation of orders is achieved by use of a second dispersing element, either a prism or conventional grating, mounted so its dispersion is in a direction perpendicular to that of the echelle. This combination of dispersing elements produces a two-dimensional, spectral pattern. An example of such a pattern is shown in Figure 3-18, which was... 

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