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Journal of the Optical Society of America

Journal of the Optical Society of America

  • Vol. 23, Iss. 5 — May. 1, 1933
  • pp: 178–181

Focus of a Concave Grating Spectrograph

DAVID L. MAcADAM  »View Author Affiliations

JOSA, Vol. 23, Issue 5, pp. 178-181 (1933)

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The problem of mounting a twenty-one foot concave grating and slit at oblique incidence, so that the focus will lie on a previously established track having less than one inch adjustment range, is discussed. A simple technique, employing only a good transit, is described. The development of this method depends on a precise knowledge of the nature of the general focal curve. This information is found by a brief examination of grating theory. A table of permissible departures of the slit from the Rowland circle is given, such that the error in path length to the sharpest image is less than λ/4. Application of classical grating theory to the case of oblique incidence and diffraction results in an expression for maximum useful grating width. This expression is shown to differ only by a factor of 1.06 from the expression derived in a recent article by Mack, Stehn and Edlén.

DAVID L. MAcADAM, "Focus of a Concave Grating Spectrograph," J. Opt. Soc. Am. 23, 178-181 (1933)

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  1. Mack, Stehn and Edlén, "On the Concave Grating Spectrograph," J. Opt. Soc. Am. 22, 260 (1932).
  2. A communication from Dr. Mack advises that the paragraph quoted was not intended to be a discussion of the general focal curve. According to that author, the quoted paragraph should begin somewhat as follows—"By the Upsala method of adjustment, the photographic plate and the slit are placed quite accurately on a circle of radius ρ/2, tangent to the grating circle." The remainder of the paragraph then discusses correctly the errors in phase introduced by the special misadjustment described there.
  3. This result is mentioned with others in the discussion of the Rowland mounting inKayser, Handbuch der Spectroscopie, Vol. 1, 443, p. 468. The discussion of various slit adjustments in 448, p. 476 is quite complete, and applicable to most mounts.
  4. This comparison is also embodied in reference 8 of Mack, Stehn and Edlén's paper. The limiting useful values of H (directly proportional to grating width) as calculated by Rayleigh's criterion and by the new generalization of that criterion are shown to be 1.120 and 1.177. which bear the ratio of 1: 1.053.

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