Abstract

The newer ruling engine of the Mount Wilson Observatory is now producing sizable diffraction gratings of high quality in all respects. Several recent gratings are 714 inches wide with grooves 534 inches long, and one is 8 by 514 inches. Curved-edge ruling diamonds, developed here, have been used in blazing these gratings to specifications for astronomical use. High luminous efficiency is combined with practically complete absence of scattered light, either general or local, in the spectrum. Resolving power of 500,000 is achieved. Rowland ghost intensity is held to about 0.00004 in the first order of 15,000 line per inch rulings. Most ruling to date has been at 300, 400, 600, or 900 grooves per millimeter, but other spacings are available. The rather considerable modifications of the Rowland-type engine are described, with particular reference to the monorail diamond carriage, the coupling of the nut to the grating carriage, the end-thrust bearing of the screw, the use of Nitralloy steel ways, and the spacing mechanism.

That the principles of this entirely mechanical, single-screw machine are thoroughly sound is attested by the quality of its products. Blazed plane gratings have almost entirely supplanted prisms in fast stellar spectrographs of both short and long focus.

Our methods of testing gratings are outlined and a formula is proposed for the evaluation of gratings.

© 1951 Optical Society of America

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References

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  1. Physical Papers of H. A. Rowland (Johns Hopkins University Press, 1902), p. 691.
  2. J. A. Anderson, Glazebrook’s Dictionary of Applied Physics 4, 30 (1923).
  3. Graphitar, a product of the U. S. Graphite Company, Saginaw, Michigan.
  4. John Strong, J. Opt. Soc. Am. 41, 3 (1951).
    [Crossref]
  5. John Strong, Phys. Rev. 43, 498 (1933); Astrophys. J. 83, 401 (1936).
    [Crossref]
  6. H. D. Babcock, J. Opt. Soc. Am. 34, 1, 1 (1944).
    [Crossref]
  7. W. S. Adams, Astrophys. J. 93, 11 (1941).
    [Crossref]
  8. Astrophys. J. 21, 197 (1905).
  9. E. Ingelstam, Arkiv för Fysik Bd.  2, Nr. 13, p. 105, (1950).
  10. C. F. Meyer, Diffraction of Light, X-Rays, and Material Particles (University of Chicago Press, 1934), p. 185.

1951 (1)

1950 (1)

E. Ingelstam, Arkiv för Fysik Bd.  2, Nr. 13, p. 105, (1950).

1944 (1)

1941 (1)

W. S. Adams, Astrophys. J. 93, 11 (1941).
[Crossref]

1933 (1)

John Strong, Phys. Rev. 43, 498 (1933); Astrophys. J. 83, 401 (1936).
[Crossref]

1923 (1)

J. A. Anderson, Glazebrook’s Dictionary of Applied Physics 4, 30 (1923).

1905 (1)

Astrophys. J. 21, 197 (1905).

Adams, W. S.

W. S. Adams, Astrophys. J. 93, 11 (1941).
[Crossref]

Anderson, J. A.

J. A. Anderson, Glazebrook’s Dictionary of Applied Physics 4, 30 (1923).

Babcock, H. D.

Ingelstam, E.

E. Ingelstam, Arkiv för Fysik Bd.  2, Nr. 13, p. 105, (1950).

Meyer, C. F.

C. F. Meyer, Diffraction of Light, X-Rays, and Material Particles (University of Chicago Press, 1934), p. 185.

Strong, John

John Strong, J. Opt. Soc. Am. 41, 3 (1951).
[Crossref]

John Strong, Phys. Rev. 43, 498 (1933); Astrophys. J. 83, 401 (1936).
[Crossref]

Arkiv för Fysik (1)

E. Ingelstam, Arkiv för Fysik Bd.  2, Nr. 13, p. 105, (1950).

Astrophys. J. (2)

W. S. Adams, Astrophys. J. 93, 11 (1941).
[Crossref]

Astrophys. J. 21, 197 (1905).

Glazebrook’s Dictionary of Applied Physics (1)

J. A. Anderson, Glazebrook’s Dictionary of Applied Physics 4, 30 (1923).

J. Opt. Soc. Am. (2)

Phys. Rev. (1)

John Strong, Phys. Rev. 43, 498 (1933); Astrophys. J. 83, 401 (1936).
[Crossref]

Other (3)

Graphitar, a product of the U. S. Graphite Company, Saginaw, Michigan.

Physical Papers of H. A. Rowland (Johns Hopkins University Press, 1902), p. 691.

C. F. Meyer, Diffraction of Light, X-Rays, and Material Particles (University of Chicago Press, 1934), p. 185.

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Figures (9)

Fig. 1
Fig. 1

General view of the “B” ruling engine. A 6 × 7 1 2 -inch rectangular grating rests on the levelling table of the grating carriage. Above this is the light-weight diamond carriage fitted with inverted V-blocks of Graphitar which slide on the cylindrical steel monorail. The screw may be seen between the ways of the grating carriage; on its right-hand end is the 900-tooth spacing gear. The crank-shaft is at the rear.

Fig. 2
Fig. 2

Lapping nuts and accessories used in finishing the screw. Note the two interferometer plates for mounting on short nuts.

Fig. 3
Fig. 3

Operating nut, before threading. The steel shell is provided with centrifugally-cast inserts of hard nickel babbitt. At left is one of the extensions attached to the ends of the screw during lapping; this permitted considerable overtravel of the lapping nuts.

Fig. 4
Fig. 4

Spacing mechanism and period compensator. Intermittent rotation of the cylindrical cam, at right, is transmitted by the spur gears to the worm, producing incremental rotation of the large gear and the screw. Any residual periodic error in the spacing, arising from the end-thrust bearing of the screw or from eccentricity of the spacing gear, is compensated by a small longitudinal oscillation of the spacing worm; this motion is generated through a lever by the large disk attached obliquely to the hub of the gear.

Fig. 5
Fig. 5

Diamond support. The rocking parts are carried by two thin, flat, steel springs at each end of the axis. At the right is a clamp holding a ruling diamond in its shank of rivet steel.

Fig. 6
Fig. 6

(a) Direct-intensity tracing of the Hg spectrum formed by a 10,000-groove-per-inch grating in a low-resolution scanning spectrometer. Note the very low intensity in all spectral orders except the second and third. The line of greatest intensity in each order is λ4358. The envelope of the blaze is indicated by the dotted line; it is, ideally, the diffraction pattern of a single groove of the grating. The step-tracing was made by opening the slit in equal increments; it proves that the deflections are linear. (b) Tracing made with a filter transmitting only the line λ4358. This line is strong in both second and third orders, as the blaze is at λ4000 in the third. At the far left is the deflection due to the total incident light of this wavelength, when the receiver is aimed directly into the collimator. A comparison of the deflections shows that the luminous efficiency of the grating, I/I0, is 67 percent.

Fig. 7
Fig. 7

Two examples of cross-rulings. The width of each section corresponds to about three turns of the screw, the grooves running vertically. The marks at the bottom are one turn ( 1 1 2 mm) apart. The detail in the cross-bands, which repeats at marked intervals, is enhanced by making the second ruling shallower than the first. At the left, the amplitude of the periodic error is about 1/40 of a grating space and would result in Rowland ghosts having an intensity 6×10−3 as great as that of the parent line in the first order spectrum. The sharpness of the fine cross-lines testifies to the practically complete absence of accidental errors and random errors in the ruling. In the test at the right the periodic error is negligible; Rowland ghost intensity is less than 4×10−5 in the first order.

Fig. 8
Fig. 8

Profiles of (a) the green Hg line, λ5461, and (b) the blue Hg line, λ4358, showing hyperfine structure and isotope effect. These profiles were obtained in the fifth and sixth orders, respectively, of grating 46B by photoelectric scanning; deflections are proportional to intensity. Below are plotted to scale the components as measured with the interferometer (Burger and van Cittert, Physica 5, 177, 1938).

Fig. 9
Fig. 9

The green (a) and the blue (b) Hg lines. These patterns were made by cutting the profiles of Fig. 8 from white paper and copying them photographically on a moving plate. Direct photography of the patterns in the spectrograph was avoided because the apparatus, while well adapted for visual tests, is not sufficiently stable for long exposures.

Equations (3)

Equations on this page are rendered with MathJax. Learn more.

M = W 2 L ( s / 6000 ) 1 2 R r p R l R l s R g R d .
a = a + n α .
α = 1 / R ψ ,