From the basic formulas governing echelle performance, simple expressions for echelle and groove dimensions are derived for producing a two-dimensional echelle spectrogram having any desired characteristics. Several possible mountings of Bausch & Lomb echelles having 200 grooves per inch and resolving powers in the range 200,000 to 500,000 have been tested with the objective of covering as much as possible of the spectral range 2000 to 7000A in a single exposure with high photographic speed. This type of spectrograph is of increasing importance for the analysis of complex spectra of materials available in only minute samples. The mounting thus far found most satisfactory involves making the light from a horizontal slit parallel with an 8-inch concave mirror of 10.5-ft focus, placing the echelle with its grooves horizontal in the resulting collimated beam nearly over the slit, and illuminating a 21-ft concave grating with grooves vertical with the slightly diverging parallel beams from the echelle. The vertical plate factor thus produced on 30 inches of plate set in the focal plane at the grating normal varies from 0.47 A/mm at 7000A to 0.14A/mm at 2000A, and the spectral region from 7000 to 2000A can be covered in a single exposure at such dispersion. The reduction of optical parts to mirror, echelle, and grating gives high speed, so that exposure times of from 20 to 60 sec suffice for most arc spectra. To combine the echelle and grating characteristics effectively, the spectrum below 3600A is made to overlap in the second order of the grating that from 7200 to 3500A in the first. Measured resolving powers vary from 220,000 at 7000A to 450,000 or more at 2537A. Even in complex Zeeman spectra of the rare earths, the statistical distribution of lines is found to be such that little interference results from the partial overlapping of two grating orders. The spectral images are found to be stigmatic and sharp out to 15 inches on either side of the normal to the grating.
This echelle spectrograph is found to have more than twice the resolving power of our best 35-ft concave grating mount, and from five to ten times its speed. Even with dispersion greater than that given by the grating instrument the plate length required is only 30 in. instead of 750 in. The spectrograph, which has the added advantage of being stigmatic, occupies 35 square feet instead of the 700 sq ft required by the grating mount. This echelle instrument has been used to photograph at high field intensities the Zeeman effects of erbium, holmium, terbium, gadolinium, neodymium, and praseodymium.
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