David W. Steinhaus

An Automatic Comparator for Measurement of Spectra

A digital computer has been programmed to find and measure spectral lines using a list of transmission values at equal intervals along a spectrum plate. The apparatus for making the list and the general computing scheme are described. The computer output consists of the position, wavenumber, intensity, and shape for each line found. Over 200 lines/min are processed by the computer. Readings are made as well as can be done by an operator using a photoelectric comparator.

PRECISION COMPUTER MEASUREMENT OF SPECTRA.

A digital computer has been programmed to do the complete job of finding and measuring each spectral line in an absorption or emission spectrum. The outputs of position, wavenumber or wavelength, log intensity, and shape of each line are similar to those obtained by a skilled operator using a photoelectric comparator, but are more consistent and are obtained much more rapidly. Many data points on each line are used, and precision measurements can be made on lines covering an intensity range of over 103. Many spectra have been processed since the first successful operation in 1962. Continuing improvements to both the apparatus and the computer program are being made. Future plans include a more direct connection to the large digital computer, programmed computer control of the apparatus, computer enhancement of the spectral resolution, and the application to spectrochemical analysis. This system is not limited to optical spectra. Any set of data with peaks or depressions can be processed so as to measure t...

Photoelectric Comparator for Wavelength and Intensity Measurements of Spectra

A photoelectric comparator is described that can be used for making wavelength measurements, intensity measurements, and observations of the shapes of spectral lines. The instrument is similar to one described by Tomkins and Fred with improvements in the optics.

Rapid Precision Wave Number Measurements from Fabry-Perot Interferograms*

In order to obtain the more accurate wave numbers needed for studies of the very rich heavy element spectra, a new measuring and calculating procedure has been developed. A modern sharp line source, such as a hollow cathode discharge or an electrodeless metal-halide lamp, is used to illuminate a vacuum Fabry-Perot interferometer (5, 10, or 20 mm spacer). The interferometer is crossed with a spectrograph resolving the free spectral range of the interferometer. The resulting interferogram is measured with a two-coordinate photoelectric comparator. The measurements are punched on IBM cards, and vacuum wave numbers are directly calculated with a high-speed digital computer. Only one standard line is needed and the index of refraction of air correction is used only to obtain air wavelengths. The phase change correction is obtained from measurements with two different spacers or from measurements on several standard lines. Only a few minutes reading time are needed for each line. This procedure is being used for a further study of the uranium spectrum with sources containing separated uranium isotopes. Over 8000 lines near 4100 A have been measured with a precision better than 0.005 cm−1.

Measurement and Removal of Ghosts in Spectra Recorded by Modern Gratings

The Lyman ghosts produced by modern plane gratings ruled under interferometric control are shown to have intensities as high as 1% of the main line. These ghosts may be removed from the spectrum by the use of a relatively large cross dispersion before the spectrograph entrance slit.

Calculation of the fractional order number at the center of a Fabry–Perot interferometer-fringe system*

With the traditional least-squares-calculation procedure, some measurements have great influence on the calculated fractional order number, whereas others contribute nothing to the final value. This is very unfortunate for the experimentalist, who would like to make full use of each measured value. To obtain the desired equal influence, the calculation procedure must be modified to use a fixed value of Δ(R2). This fixed value may come from the average of many adjacent spectral lines. Use of the modified procedure results in much greater precision without increasing the measurement effort.

Optical Design And Performance Of A Dual-Grating, Direct-Reading Spectrograph For Spectrochemical Analyses

An all-mirror optical system is used to direct the light from a variety of spectroscopic sources to two 2-m spectrographs that are placed on either side of a sturdy vertical mounting plate. The gratings were chosen so that the first spectrograph covers the ultraviolet spectral region, and the second spectrograph covers the ultraviolet, visible, and near-infrared regions. With the over 2.5 m of focal curves, each ultraviolet line is available at more than one place. Thus, problems with close lines can be overcome. The signals from a possible maximum of 256 photoelectric detectors go to a small computer for reading and calculation of the element abundances. To our knowledge, no other direct-reading spectrograph has more than about 100 fixed detectors. With an inductively-coupled-plasma source, our calibration curves, and detection limits, are similar to those of other workers using a direct-reading spectrograph.

A Simple Method for Reducing Astigmatism from Off-Axis Concave Spherical Mirrors

The advantages of mirror optics for spectrographic systems are well known, namely, that all wave lengths focus at the same place and that the reflection losses can be made very small. Most modern spectrographs use mirror optics. Some are able to use spherical mirrors while others must use off-axis paraboloidal mirrors to achieve the desired resolution. Inexpensive concave spherical mirrors are ideal for imaging the source on a spectrograph slit, except for the problem of astigmatism. Expensive off-axis corrected mirrors can be obtained, but they must then be used only at the designed off-axis angle. We have found a simple adjustable way to reduce the astigmatism from inexpensive spherical mirrors used at moderate off-axis angles.