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Gaertner Single Screw Measuring Engine

General Information

This measuring Otto
Struve engine was built by the Gaertner Scientific Corporation and bought by the National Radio Astronomy Observatory. It belonged to Otto Struve, the founding director of NRAO, and after Struve died it was obtained and utilized by McCormick Observatory. The engine was specifically designed for measuring the spectra of objects rather than their brightnesses or positions. By measuring the spectrum of an object, one could determine the chemical composition of that object and its radial velocity.

Operation

The spectrum of an object is taken on the telescope using some kind of spectroscope, a way to spread out the light from an object into all its component wavelengths. Spectrum
of the Sun from 1815 Usually, when one looks at the spectrum of an object, they will see either dark or bright bands. The dark bands indicate that certain wavelengths of light have been absorbed by the object and the bright bands indicate that certain wavelengths of light have been emitted by the object. Since different elements emit and absorb light of specific wavelengths, by identifying the wavelengths at which bright and dark bands are seen, the astronomer can determine which elements compose the object of study. In addition, although these wavelengths are normally at specific places in the electromagnetic spectrum, the motion of an object to or away from us on Earth can move those wavelengths to lower or higher values than we expect. For instance, if we see an object which has an emission line due to the element calcium at 393.41 nanometers and yet we know that calcium in the laboratory emits at 393.37 nanometers, we can calculate (from the Doppler equation) that the object is moving away from us on Earth at a speed of about 30 kilometers per second. Obviously, the more precise that we can measure the positions of these bright and dark spectral lines, the more precisely we can know the velocities and chemical compositions of the objects of study.

Gaertner Measuring Engine Once the spectrum is taken on a photographic plate, the plate is clamped onto the horizontal carriage (1) of the measuring engine using four long clips, shaped in such a way that every plate comes into the same plate with respect to the microscope. The plate is set in such a way that the spectrum is aligned along the direction of the measuring engine screw (3). The spectrum is placed under the microscope (6) by moving the engine carriage forward and backward using the Y-Coordinate handwheel (2) on the right side of the machine, and then the spectrum can be measured using the precision measuring screw which is attached to the X-Coordinate handwheel (4), on the left side of the machine. Similar to the first measuring engine that Frank Schlesinger designed and Gaertner constructed, the measuring handwheel has been placed on the left side to allow for ease of recording data with the right hand.

The spectrum is viewed through the microscope, which contains a single vertical reticle for accurate measuring of spectral line positions. After the plate is aligned, parallax in the eyepiece can easily be eliminated and then the focus can be finely adjusted. However, changing the setting of the focus during a series of measurements will destroy the accuracy of those measurements, so care should be taken not to move the microscope during a measurement. The operator turns the handwheel until one of the spectral lines is aligned with the reticle and then reads the screw setting using both a graduated and vernier dial (7) alongside the handwheel. This combination of dials allows for the reading of 1/1000 of a revolution of the screw, which corresponds to a fraction of a micrometer (µ). The operator then proceeds to measure all of the spectral lines in this same fashion both directly and then from 180° around, and so obtains very accurate measures of the relative positions of the lines. By comparing the experimental spectrum with known spectral line positions, one can then obtain the desired information about chemical composition and radial velocity of the object. In practice, about 15 comparison lines and 10 to 15 star lines were measured per plate, which produced a very accurate measurement.

Gaertner Single Screw Measuring Engine

General Information

Operation

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