Pruyn Equatorial Telescope Restoration

Telescope Restoration

by Ron Barnell

Pruyn Equatorial Telescope

Pruyn Equatorial Telescope

This page recounts the story of Dudley Observatory’s historic Brashear telescope, both as an important observational tool, integral to the observatory’s historical mission, and also provide an insight into the importance of the ongoing restoration work being done on the telescope. The final goal of the restoration project will be the placing of this vintage 19th century instrument back into service. With the selection of a suitable location for the telescope, one that addresses both environmental concerns and accessibility, the Pruyn Telescope can take on an updated role, not only furthering education in Astronomy and the Space Sciences for the public in upstate N.Y, but also as a time-honored portal allowing a new generation of star-gazers the opportunity to experience, first hand, the visual beauty of the wonders of the celestial firmament.

“I have tested the visual combination to some extent-enough to know that the glasses are of excellent figure and good corrections”

“The blackness of the field struck me at once, and I find that this telescope is strong on faint companions of stars. It ought to do well with the satellites of Mars”

With these words, written to Allegheny Observatory Director James Keeler, Dudley Observatory Director Lewis Boss gave the first impressions of the views through the newly completed 12” telescope. The telescope was placed within the tower dome, part of the structure of the new building that was erected in 1893 on South Lake Avenue in Albany, N.Y. This refracting–lens telescope, with its optics designed and ground by the esteemed optical firm of John A. Brashear in Pittsburgh, Pa., was placed upon an equatorial mount fabricated by the engineering firm of Warner & Swasey of Cleveland, Ohio. The mount featured a special wind-up mechanical clock-drive, which enabled the large telescope to precisely track celestial objects in their nightly motions across the sky.

The telescope, a centerpiece of the new facility, had been financed by the Pruyn family in memory of Robert H. Pruyn, a prominent lawyer & civic leader in 19th century Albany. Pruyn had also been a long-time board member of the Observatory. The telescope stood in place, doing valuable astronomic and public-relations work, for over 70 years, until the closing of the facility in 1965.

The Beginning / A New Glass For Albany Skies

With the decision to close Dudley’s first observatory in Albany, it was decided that any new facility would have one of the finest large-aperture telescopes obtainable at the time. (The previous telescope was a mediocre Fitz instrument.) The majority of college and private observatories obtained their refracting telescopes from the firm of Alvan Clark & Sons, of Cambridge, Mass. Not only were they well-regarded for the high quality of their lenses, but their location seems to have given them a virtual monopoly on the institutional telescope market throughout the eastern United States.

Optical craftsman John A. Brashear was a mechanical engineer in the steel mills of Pittsburgh, while at the same time pursuing his hobby of Astronomy and amateur telescope making. It was while fabricating one of his early telescopes that it was brought to the attention of the Director of the Allegheny Observatory, Samuel Pierpont Langley. Professor Langley, being impressed with Brashear’s efforts with this early 5-inch lens, met with him, and allowed him to look through the 13-inch telescope of the observatory. After 1876, John Brashear became increasingly associated with the Allegheny Observatory, and encouraged by Langley, began to fabricate optical instruments in his home. Overwhelmed by orders after placing an advertisement in Scientific American, he made the decision to go into the telescope making business on a full-time basis with the aid of his wife Phoebe and son-in-law, James B. McDowell. Perhaps most fortuitously, Brashear was introduced to William Thaw, the railroad magnate, in Pittsburgh, who was so impressed with the craftsmanship of his instruments, that Thaw advanced him the money that allowed Brashear to set-up an optical shop in South Pittsburgh. (Many years later, John Brashear would build a large 30-inch telescope for the Allegheny Observatory, designated the “Thaw Memorial Telescope,” which remains the third largest refracting telescope in the world.) He also received aid from Andrew Carnegie who, via his philanthropic pursuits, was contributing to the establishment of science education programs. These included grants to institutions like the Allegheny Observatory to carry out scientific research programs. The newly established Carnegie Foundation played an important role in the historical advances of both the Allegheny and Dudley observatories. The year 1893 was not only the year of the Pruyn telescope construction, but also the year that Brashear and the staff of Allegheny Observatory began planning for a new observatory. (That observatory, which eventually opened in 1912, now stands on a hilltop in Riverside Park, Pittsburgh.)

With the grants now available to him, John Brashear was able to pursue the crafting of his telescope objectives and scientific optical instruments, without the need to worry about financial constraints. Two important developments took place around the same period. On a trip to Germany, he visited the newly founded optical firm of Schott and Company in Jena. Prior to this, Brashear, like many other opticians of the time, had used glass components made in France for their telescopes. The new glass the Schotts group was producing seemed to provide better optical characteristics (i.e. freedom from bubbles, striations, higher clarity, etc.) compared to the French made product. The second development was the partnership with Dr. Charles S .Hastings of Yale University. Hastings had a good record for calculating curves for several large refracting telescopes, and Brashear now gave him the time and resources that allowed Hastings to experiment with more advanced lens curvature properties . The result was a gradual spread of Brashear’s reputation among the astronomical community regarding the care he took in the fabrication of his extremely high quality instruments. Using the Allegheny Observatory as his observational test-bed, Brashear set about crafting the finest lenses and also the highest quality accessories designed to be used with both his own, and other manufacturer’s telescopes. In particular, Brashear designed and built the highest caliber of spectroscopic instruments, including the famous Congrave diffraction gratings, with which researchers were able to produce stellar spectral imaging of the highest resolution. These ruled metal plates were produced, not only with a very highly polished surface, but also featured accuracy to within a 1/5th wave-length of light. Several of these ruled gratings were used with the Pruyn telescope to obtain the stellar spectra indexes listed in the Preliminary General Star Catalog, which was published by Dudley Observatory in 1919. The many institutions that eventually utilized Brashear spectral imaging components led them to become the de facto standard for high resolution spectroscopic studies around the turn of the 19th century. (Brashear also produced optical prisms of a very high quality, and these also found their way into service at many observatories.)

Two Vintage Observatories / A Shared Search For Stellar Secrets

It was in 1891 that James T. Keeler took over the Directorship of the Allegheny Observatory from Professor Langley. (Keeler is best known for his discovery that the rings of Saturn were comprised of many small moonlets). Among the prime questions that astronomers of the time were engaged in solving was an accurate delineation of the distance scale of the Universe. Through the study of the parallaxes of distant stars and a study of their proper motion across the sky, a picture began to emerge of our place in the Galaxy (these early studies were ultimately refined with Harlow Shapley’s globular star cluster studies at Harvard, in the early 20th century.)

James Keeler now extended the program of parallax studies at Allegheny, culminating with the publication of 10 volumes of stellar parallax calculations. A great many of the later observations were made with the 30” Thaw refracting telescope, with its Brashear lens elements . At about the same time, Lewis Boss, at Dudley, was engaged in a study of the proper motion of the stars within select open-clusters, like the Hyades found in the constellation of Taurus. Over time, an exchange of correspondence between Keeler & Boss ensued, and as plans were being drawn up for the construction of the observatory, it appears that Keeler recommended to Boss that he contact the John Brashear Company regarding the building of the large telescope to be installed in the new observatory. It follows that a great part of the recommendation was to rest on the stated requirements for Boss’ proper motion studies. The specifications for the new telescope would include the requirement that the lens elements be of the highest quality in terms of the purity of glass used, lens curvature formulation, and the polishing applied to the crown and flint lenses. With the financial aid from the Pruyn family now available, Lewis Boss went ahead and placed the order for the new equatorial telescope. As specified, it was to have an aperture of 12” in diameter and a long focal length of F/15. The telescope was to be accompanied by a guide scope and a finder telescope of 3” aperture. A new plate camera was ordered, along with a special filar micrometer, that when attached to the eyepiece would allow Boss to make extremely precise measurements of stellar separation angles (i.e., between the components of binary star systems). The ability to peruse very fine and detailed astrometric work, became a hallmark of the Pruyn telescope, and was a major factor in the comprehensive programs of stellar proper motion and parallax studies being carried out. The resulting compilation of this data was to eventually find its way into several historic Dudley Observatory star catalogs which were published at the beginning of the 20th century . Those catalogs stood as an authoritative stellar motion and positional reference up until the beginning of the Space Age. It is believed that John Brashear himself traveled to Albany on several occasions in 1892 and 1893 to visit with Lewis Boss and work on detailed planning for the construction of the new telescope. However, we are still looking for probable evidence that would indicate that he attended the dedication ceremonies, held on Nov.8, 1893, for the new observatory. (At that event, celebrated Astronomer Simon Newcomb gave the inaugural address, and it is very likely that James Keeler did attend, as his written invitation is still in existence ).

The Pruyn Telescope / A Fading Role

Hemenway and Students using the Pruyn Telescope/1957

Hemenway and Students using the Pruyn Telescope/1957

Over the next several decades and well into the first half of the 20th century, the Pruyn telescope was used at the new Dudley Observatory site by Lewis Boss, and later on, by his son Benjamin Boss, who became the new Director in 1912. (Lewis Boss had died in October of that same year.) Its use in the measurement of stellar positions and attendant proper motions became a fundamental part of several seminal catalogs published by Dudley, that were important references for the astronomical community of the time. (The bulk of this work was supported by grants from the Carnegie Foundation, which created the Dept.of Meridian Astrometry at Dudley Observatory.) The telescope was also used for observations of solar system objects, particularly comets and asteroids. Among the asteroids discovered and observed from Dudley was one given the name Chaldea. Several of the staff astronomers also made sketches of Mars at several of the red planet’s close approaches to the Earth, early in the 20th century.

It was with the realization that the next stage in the advancement of astronomical studies was going to be carried out with the new large reflecting telescopes located at the Western observatories that the Pruyn, like other large refracting telescopes of the era, began to suffer from disuse. With the exception of certain specialized applications, such as the astrometric work previously mentioned, the day of the large-lens telescope was basically over. (The largest refractor was the 40” Clark Telescope, made for the Yerkes Observatory in 1897.)

Part II/-

Will outline the efforts being done to preserve and restore the Pruyn Telescope, including a side look at several other refracting telescopes that were built within the same time frame by John Brashear. With a brief recognition of the restoration efforts that were done on those few Brashear telescopes that remain in existence, it becomes possible to relate certain of those efforts to Dudley’s ongoing Pruyn Telescope restoration project.

Ron Barnell/March 2008