George R. Harrison

The Production of Diffraction Gratings: II. The Design of Echelle Gratings and Spectrographs1

In seeking to circumvent the difficulties that have limited production of ruled gratings having resolving powers in excess of 400,000, it is observed that resolving power at a given wave-length depends only on the ruled width of the grating and the angles of illumination and observation, and not specifically on the number of ruled grooves. The reflection echelon is a grating of high resolving power in which groove separation has been increased to a degree involving extreme sacrifice of free spectral range. It is suggested that much could be gained by producing a grating with only as many reflecting grooves per inch as are needed to give sufficient free spectral range to avoid difficulties arising from overlapping of orders. For the study of Zeeman effects, hyperfine structure, and complex atomic and molecular spectra, 20 wave numbers of free spectrum should suffice, which would require use of about 100 grooves per inch. Such gratings, having a simple step groove form that reflects light of all orders in a narrow bundle, and designed to be used at an angle of incidence greater than 45° and normal to the narrow side of the “step,” may be designated “echelles,” as intermediate in character between the echellette and the echelon. Echelles should lend themselves to production of very powerful compact spectrographs giving single-exposure coverage of broad spectral ranges. A 10-inch echelle having 1000 grooves of width 0.25 mm, each with one flat side 0.05 mm deep, gives at 5000A a theoretical resolving power of one million, a free spectral range of 5A and a plate factor of 0.2 A/mm, when used with a lens or mirror of only 250 cm focal length. It should thus be as powerful as a 10-inch concave grating of 42 ft. radius having 30,000 grooves per inch when used in the third order, and would be much faster even though used with a lens of less than 75 mm aperture. When crossed with a suitable prism or grating of low dispersion this echelle should, without overlapping of orders, produce spectrograms having a range of 10,000 wave numbers or more on a single plate, instead of the 5 to 25 plates required by the equivalent orthodox grating in the Paschen-Runge mount. The echelle spectrograph would also offer great gains in stability and portability. Problems and possibilities of the production of original and replica echelles are discussed, and it is shown that the reduction of ruling time from weeks to hours makes interferometric control of groove spacing in gratings appear feasible for the first time.

A Hollow Cathode Source Applicable to Spectrographic Analysis for the Halogens and Gases

Investigations aimed at developing methods of spectrographic quantitative analysis of the halogens and non-metals have been carried out, with emphasis on analysis for fluorine. The relative sensitivities of fluorine spectrum lines in the vacuum ultraviolet region and in the visible region were studied, and it was found that certain visible lines are, at present, more sensitive than the theoretical raies ultimes of fluorine. The greatest absolute sensitivity obtained, using a specially designed hollow cathode source, involved detection of about 0.01 microgram of fluorine, while the greatest concentrational sensitivity was slightly less than one part per million. Chlorine and sulphur were readily detected in amounts as small as 0.20 microgram and 1.0 microgram, respectively, in samples weighing 20 milligrams. Sample handling time was reduced sufficiently to permit routine analyses for these elements.

The Production of Diffraction GratingsI. Development of the Ruling Art

In this first paper of several on the design and production of diffraction gratings of high resolving power, an outline is given of the development and present status of mechanically controlled ruling engines. Gratings having notable performance are described, and problems involved in producing on a reflecting surface large numbers of parallel equidistant grooves of constant or controlled form, with position tolerances of one micro-inch or better, are analyzed. Distinctive features of the engines of Rowland, Michelson, Gale, Anderson, Babcock, Siegbahn, and others are discussed. It is pointed out that the low productivity of many ruling engines in the past should not discourage present-day experimenters, as improved surfaces for ruling, superior ruling tools, alloys less susceptible to wear, warpage, creep, and fatigue, better methods of lubrication, and servo-mechanisms capable or making a ruling engine self-correcting, are all now available. Much can be learned by studying the strong points and weaknesses of previous engine designs. A current trend in the use of gratings appears to be away from the concave and toward the plane form. Succeeding papers will deal with a suggested method for obtaining improved gratings having high resolving power, and with the modification and testing of a 14-inch ruling engine, originally designed and partially built at the University of Chicago by Gale and O’Donnell, and recently transferred to M.I.T.

Interferometric Control of Grating Ruling with Continuous Carriage Advance

A 14-inch ruling engine, whose operation with interferometer control of grating blank position between ruling strokes has been described previously, is now being operated with continuous blank advance and control. Displacement of the carriage holding the grating blank is measured in terms of the phase of a fringe system of constant inclination passing across a photoelectric pickup, which produces low-frequency ac whose phase is compared with that from a generator measuring the phase of motion of the ruling diamond. Synchronism between diamond and blank is maintained through corrections fed into a differential on the engine screw-worm by a balancing motor operated by phase differences. Grooves straight to one-tenth fringe, up to 9 in. long, can be produced by means of a cam-and-lever system which rectifies the otherwise simple harmonic motion of the diamond carriage. Continuous servo control results in improved elimination of screw errors and engine vibrations, and in simplified circuitry. The needed electronic and interferometric control systems have been found to function reliably over the long periods needed to rule large gratings.A change-gear system permits passage of any desired fractional number of fringes per diamond stroke needed to produce from about 50 000 to 2000 grooves per inch. The signal-to-noise ratio obtained when green light from an Hg-198 tube is used to illuminate the carriage-translation interferometer is found to permit control over 10 in. of carriage motion, and stepping methods are being studied to permit multiplication of this distance. Plane gratings up to 8 in. in width of ruling and 5 in. in groove length have been produced, which show acceptably low ghost intensities despite original screw errors of more than 50 times the tolerance limit. Error-of-run and fanning appear to be under good control over distances of 8 in. Rapidly occurring random errors previously found have been traced to lateral motion of the diamond carriage and eliminated. Slow irregularities in groove position are ascribed to temperature variations, to which the engine has been found to be 10 times as sensitive as necessary; these are now being removed. Continuous oscilloscope records taken during ruling show that blank positioning is being controlled by servo interferometry to within 1/40 fringe or better, and indicate sources of disturbances whose removal should result in further improvement.

A Fixed-Focus Broad-Range Echelle Spectrograph of High Speed and Resolving Power*

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.

Ruling of Large Diffraction Gratings with Interferometric Control

The 14-inch MIT ruling engine, operated under interferometric servo control as previously described, now rules excellent diffraction gratings up to 8 inches in width. The more recent of the 70 test gratings produced approach in quality the best yet ruled on any engine, showing resolving powers of about 600 000 in the green, and giving crisp spectral lines with low local background even at high angles. Rowland ghost intensities appear lower than any yet reported, ranging from 1/700 to 1/800 at 74°, corresponding to 1/25 000 to 1/29 000 in the first order of a 15 000 groove/in. grating. The balls on which the blank carriage rolls were found to introduce irregularities which were finally eliminated by a rotation-control servo mechanism which permits a carriage to be moved on curved or irregular ways over distances of 10 inches or more without rotation greater than 0.01 sec of arc. Servo controls are described which reduce both periodic and cumulative errors to new low levels, and also eliminate “fanning” of grooves.A device for compensating the variations in the control fringe field which are produced by barometric pressure changes has now been made completely automatic. No failure of electronic components has yet occurred in the thousands of hours during which the engine has been operated. The continuous control of engine motion with interferometers makes feasible a new method for ruling wider gratings.

Automatic Measurement, Reduction and Recording of Wavelengths from Spectrograms

A device is described which can be attached to any standard moving-plate comparator which automatically records the passage of the density maximum of a spectrum line, computes the wavelength corresponding to this maximum and records this value to eight figures photographically at high speed. The automatic recording, computing and measuring features can be used independently if desired. The comparator screw can be driven by hand, or by electric motor at controllable speed, through a shaft which is coupled to a wavelength shaft by means of variable ratio gears and a differential so that the speed ratio of the shafts can be rapidly adjusted to any desired instantaneous value. When measuring a plate the operator sees on a screen before him a magnified image of a portion of the spectrum; the light in this image actuates an amplifier system through three photo-cells to give records of plate density and rate of change of density with distance. When the density slope becomes zero while the density is greater than some predetermined background value a mercury arc flashes to record the wavelength dial readings. Correct wavelength readings for any desired number of standard lines can be set into the machine, or an empirical dispersion formula can be introduced. Errors due to backlash, oil film variation and temperature variation of the screw are practically eliminated, and automatic setting on most lines is found to be more accurate than hand setting. A microphotometer trace is superposed by the machine on the wavelength list so that the intensities and physical characteristics of lines can be determined directly. A ten to 200-fold gain in speed of measurement of complex spectrograms results from use of the machine. When used as a microphotometer the instrument is faster than those in common use.

Zeeman Effect Data and Further Classification of the First Spark Spectrum of Cerium—Ce II

The light emitted by cerium atoms in magnetic fields up to 96,400 oersteds has been photographed at high spectrographic resolution over the range 2500 to 7000A. Interpretable Zeeman patterns of 427 Ce II lines have been reduced, and from them g and J values have been determined for 280 levels. These data have been used to check and extend the classification of Ce II, in which 3600 lines are now assigned to transitions between pairs of 316 levels. The energy system of Ce II consists of two groups of levels which have not yet been connected. Group I is believed to be the lower of the two by approximately 5000 wave numbers. This group consists of levels arising from the electron configurations 4f26s, 4f25d, 4f26p and 4f3. Complete assignments of quantum numbers have been given to all levels in this group on the basis of Zeeman effect studies, combinations, intervals and intensities. All terms have been assigned to electron configurations and parent terms. Group II consists of energy levels believed to originate from the electron configurations 4f5d6s, 4f5d2, 4f5d6p, 4f6s6p, and others. g values to 3 figures after the decimal are well established for many of these levels, but only a few levels are given L and S assignments, which are tentative. Interactions among these levels are strong, and assignment of individual quantum numbers will probably have little significance. Data are now available for complete tests of the g sum rule in a number of cases. It is found to be exact in cases where all perturbing terms are known. g values obtained from different lines for the same energy level are consistent to within 0.003 unit on the average.

750-mm Ruling Engine Producing Large Gratings and Echelles*

A ruling engine that has produced gratings of larger size and greater power than hitherto available is described, as are the characteristics of gratings and echelles ruled by it. This MIT C engine, now fitted to rule blanks of sizes up to 450 × 650 × 125 mm weighing up to several hundred kg, was constructed by applying interferometric translation and mirror-parallelism control to a modified Moore No. 4 Universal Measuring Machine without Y or Z motion. To the base of this were added monorail diamond ways, two diamond carriages and lifters, special end-thrust bearings, mountings for lasers, and a diamond drive with heavy flywheels. Developed by use of the results of experience with our B and A engines, the new machine shows greater mechanical and thermal stability than either, and has produced well-blazed echelles and gratings of superior quality up to the 580-mm (23 in.) diagonal size, at spacings between 632 and 31.6 grooves/mm. The total length of groove per grating is limited at present to about 80 km by diamond wear, and the speed of ruling by internal vibrations and electronic noise. Ghost, satellite, and scatter intensities are at the low levels characteristic of B-engine gratings.

Large diffraction gratings ruled on a commercial measuring machine controlled interferometrically

The carriage and ways of a Moore No. 3 measuring machine were slightly modified, and to them were added monorail, diamond carriage, and suitable drive and gearing. When placed under interferometric control, the resulting ruling engine produces excellent gratings of previously unattained dimensions. A stabilized laser removes cumulative and periodic screw errors through blank translation control, and through yaw control eliminates fanning that otherwise would result from way curvature. Blanks up to 260×430 mm can be ruled. A number of echelles in sizes up to 210×410 mm (8×16 in.) have been ruled on this B engine at spacings of 10 fringes (316 grooves/mm) and 40 fringes, and at blaze angles from 62° to 79°. Many of these show the high resolving power and freedom from both error of run and Rowland-ghosts characteristic of echelles from the M.I.T. A engine, with satellites, ghosts of all types, and scattered light reduced further in intensity by 1 to 2 orders of magnitude. Still larger gratings are being ruled on the M.I.T. C engine, of capacity 450×635 mm, based on a Moore No. 4 measuring machine, which is now being improved from grating to echelle ruling quality.