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SEM Results: Dominion Observatory Gratings

Figure 17 - Click to enlarge
The Dominion Observatory’s prism spectrograph, ca 1930
Among the surviving optical components from Ottawa’s Dominion Observatory, now preserved in the Canada Science and Technology Museum’s collection are three spectroscopic gratings. Opened in 1905, the Observatory’s astronomers provided Canada’s primary time service and undertook astrophysical research for many decades. The astronomers used spectrographs to obtain spectrograms of the Sun and atmospheres of bright stars and nebulae.

Mount Wilson Grating (1924)

This grating was made for Dr. Ralph DeLury, for use with the 52-centimetre (20.5-inch) Brashear coelostat for a study of solar rotation. This Anderson, or Jacomini, grating (1996.0245) is clearly inferior, despite being made on their “A” engine. The lines are poorly defined, and the tops of the grooves appear rounded. The average line spacing is accurate to only ≈5%, although there are local variations of up to 7 or 8%. The blotchy patterns and cracks towards the top and left (1) are inclusions in the fine structure of the speculum metal substrate.

Figure 18 - Click to enlargeFigure 18 - Click to enlarge
This grating (CSTM 1996.0245), with electron micrograph on left, was specifically made for the Dominion Observatory at the Mount Wilson Observatory.

Tiny interline grooves (2) run parallel to the main grooves. These were the result of flaws in polishing the point of the ruling diamond, and would have caused some degrading in the quality of the spectra, as well as decreases in contrast. There was a light surface scratch (3) in the speculum, which crossed several of the grooves before ruling.

1930s Grating 

The next grating (CSTM 1996.0246) was also acquired for use by DeLury. It is blazed: it has grooves ruled with asymetric sides, as can be seen by the grooves’ sharp edges. It may have been ruled by Jacomini, who had further perfected blazed gratings. In ruling this grating, the diamond seems to have “forced” its way through the glass: the brittle nature of the glass is obvious from the cracks along the edges of the grooves. One sees distinct zones, suggesting that the ruling diamond shifted during the ruling process. “Scraping” by the diamond is indicated by the sub-micron or sub-µm (one millionth of a meter) surface structure in homogeneities of the glass itself. The inter-groove spacing is regular with errors of about 2.5%, reflecting a well-designed and -operated ruling engine.

Figure 19 - Click to enlarge Figure 19 - Click to enlarge
Ralph DeLury’s grating (CSTM 1996.0246) for solar observations and electron micrograph (left) of its surface.

1950 Babcock Grating 

The third Dominion Observatory grating (1996.0247) was apparently made by the mid-twentieth century master, Harold Babcock, during his early research. The line spacing is very regular (approx. 1% variation). One fine inter-groove line betrays a small flaw in the cutting edge of the diamond. The top of the blazed groove was left uncut. The regularity of its width demonstrates that there was good control of the ruling carriage’s path and depth during the ruling process. The groove itself is sawtooth-shaped, as one would expect in a modern blazed grating. It is surprising to see inclusions (dark blotches), which are apparently in the glass structure. At first, it seems that these could be due to flaws in the very thin aluminum coating which has been vacuum-deposited onto the surface after ruling; however, the intrusion of several blotches in the physical structure of the peaks suggests µm and sub-µm-sized flaws. This indicates that a coating of aluminum had been deposited on the glass, and that the ruling was made in that coating, rather than in the glass itself. This technique was developed by John Strong at Mount Wilson in the early 1930s. The irregular white patches are tiny specks of dust.

Figure 20 - Click to enlarge Figure 20 - Click to enlarge
Dominion Observatory grating (CSTM 1996.0247, right) and electron micrograph, apparently made by the twentieth-century master Harold Babcock