Double stars
The other great work of the 26-inch’s illustrious career has been its observations of double stars. In fact, the first orders to Matthew Fontaine Maury, the first superintendent of the USNO, in 1846 specifically mentioned the need to observe double stars. But in the telescope’s early history, this took a back seat to solar system work.
Other than the work of Hall, the 26-inch contributed few double star observations until nearly the 1960s, when Kaj Strand came to the USNO. Strand, who became the observatory’s scientific director in 1963, had been trained by Ejnar Hertzsprung in the use of photography for measuring doubles.
Shortly thereafter, at a meeting of the double- and multiple-star division of the International Astronomical Union in Hamburg, Germany, Lick Observatory announced it wished to divest itself of the responsibility for maintaining the IAU’s double star database, the Index Catalog of Visual Double Stars (IDS). Strand volunteered the USNO for this task and in August 1964, the IDS was transferred to the USNO and renamed the Washington Double Star Catalog (WDS).
Charles Worley, a former member of Lick’s staff, was tasked with maintaining it. After the tailpiece of the 26-inch was modified to accommodate the photographic double-star camera in 1958, the telescope began a new era, with an emphasis on double stars that continues to this day.
Worley and the 26-inch continued to evolve their techniques. In 1981, the photographic program ended. But filar micrometry, in which an observer uses an eyepiece with a reticle to measure the separation of a double star, continued. Worley was particularly skilled at this art, which had been used on the 26-inch since 1873. But in the twilight of his career, he abandoned it for the superior technique of speckle interferometry, which extracts information from the small spots or “speckles” that appear around an image of a star due to interference effects caused by atmospheric turbulence. In a 1987 interview, 10 years before his death, he quipped, “I’m the last generation of visual observers, and I should be the last generation of visual observers. I think it’s time to do this kind of astronomy in different ways.”
Speckle observations of double stars continue on the 26-inch to this day. The USNO’s first speckle camera, with an intensified CCD detector, was installed in 1990; in 2019, it was retired in favor of a newer, more advanced electron-multiplying CCD.
As a result, in terms of total number of measured relative positions, the 26-inch is the most productive telescope for double-star astronomy ever. The WDS is legendary in the field and remains the gold standard for double-star catalogs: As of August 2022, it contained over 155,000 double stars.
After a century and a half, the venerable 26-inch shows no signs of giving out or letting up. As Worley said in 1988: “The truth of the matter is that this is often rather a good climate, as far as the steadiness of the atmosphere goes, so I think the telescope has the potential of continuing to produce good astronomical observations.”
Happy birthday, telescope!
The era of the great refractors
Historically, refracting telescopes were limited in size due to aberrations. The first large telescope to correct for this was the 9.6-inch Great Dorpat Refractor in Tartu, in modern-day Estonia. Constructed in 1824 by Joseph Fraunhofer, it was used by German astronomer Friedrich Georg Wilhelm von Struve. What followed was a technology contest throughout the 19th century to build the largest telescope — the era of what we now call the great refractors. Telescopes built by Alvan Clark & Sons held the top spot five different times, and the USNO’s 26-inch was the largest in the world from 1873 to 1880.
