The Lens

Achromatic “Doublet” Lens. Illustration courtesy Eden Orion.

In a refracting telescope the “objective” is that lens which gathers light and brings it to a focus. The most common design of refractors is known as a “doublet” wherein two lenses are mounted surface-to-surface inside a confining ring called a cell. The lenses, made of different tuypes of glass with differing light-bending qualities, work together to focus the visible spectrum of incoming light more accurately than a single lens can. That general description fits achromatic refractor telescopes that date from the 18th century to the present. In the case of the Cooley Telescope’s objective lens, here are the basic stats: Aperture = 9 inches (229 mm), Focal Length = 131 inches (3,327 mm), F = 14.6. Lens elements: Crown glass and flint glass.

John A. Brashear, source unknown

What’s a “Brashear”?

Our telescope’s objective was manufactured by the John Brashear Company of Allegheny (Pittsburgh), Pennsylvania. John Alfred Brashear had only a public school education. From school he became an apprentice to a machinist and at the age of 20 became a master of the trade. In 1861 he went to Pittsburgh and found employment. For the next 20 years he worked as a mechanic in a steel mill. In the meantime, Brashear broadened his elementary knowledge of astronomy and made amateur telescopes in his spare time. In 1870 he built his first telescope at his South Side home and immediately opened his doors to neighbors, friends, and strangers to observe the sky. Dr. Samuel Pierpont Langley, director of the Allegheny Observatory, encouraged him to establish a workshop for astronomical instruments. The workshop became the John Brashear Company and later Contraves Corp., an internationally-known maker of superb optics. He died in 1920 leaving a legacy of craftsmanship and astronomical instruments still treasured and used today.

Lens cell with factory badge: “J.A. Brashear, Allegheny, Pa.”

Objective removed for cleaning

On August 9, 2006, the objective cell was removed from the Cooley Telescope to be taken to an optical shop for cleaning, inspecting, and testing. We believe the objective cell was removed from its telescope at least twice not long before 2006. A cloudy deposit could be seen between the elements in the bronze cell; only an expert should disassemble and clean a telescope’s objective. Out lens was hand-delivered to the skilled technicians at Brashear Division L-3 Communications in Pittsburgh, Penn. Although the company had changed ownership and organization several times over the previous century, it can trace its roots back more than a century to John Brashear Co., the maker of this lens. The Cooley Telescope’s objective cell bears the badge of its maker: “J.A. Brashear, Allegheny, Pa.” The engraved badge has worn down from contact with the telescope’s lens cover over the decades, but was still readable and photographed in 2006.

Mark Yingling inspecting objective lens elements.

Inspecting and cleaning the glass

Mark Yingling, Precision Mechanical Assembler, inspects the 100+ year-old glass from the disassembled objective. The bronze cell is in the foreground of the accompanying image. The three tiny triangular pieces above the cell are paper shims used to hold space between the two glass elements. Yingling carefully cleaned all of the glass surfaces and later reassembled the cell with the elements in the same relative positions in which he found them. He observed, however, that the elements appeared to fit best wit the crown glass element forward — toward the sky. Yingling has also worked as a volunteer with the Wagman Observatory, Pittsburgh, which owns an 11-inch Brashear-equipped refractor.

Ron Grzybek “stoning” lens chips smooth.

Stoning some chips?

There was one nasty surprise greeting technicians when the glass elements were removed from their cell: One of the 9-inch lenses, the flint, was chipped in two places along its edge. Left alone, we were told, chips like those can lead to a spreading crack and fracture of the lens! Here Ron Grzybek, Glass Shaping Specialist, skillfully “stones,” or polishes the chipped areas smooth in an effort to prevent cracking.

Hiram’s vintage lens cell mounted on optical test table

Tested on an optical table

The cleaned, repaired, and reassembled objective was subjected to modern optical tests. The cell (foreground in this image) is viewed from the “inside” face — that side facing down the telescope’s tube assembly. Among the questions asked: Will the lens perform best assembled, as was typical of Brashear objectives, “flint forward” (flint glass facing the stars) or “crown forward”? Metrologist Eileen Geary ran the tests.

Flint forward interferometric analysis

Flint-forward interferometric analysis

The cleaned and reassembled objective was mounted on an optical table and Zygo interferometric testing performed. Flint-forward means the flint glass was placed in the cell position facing the sky; the crown glass element was inside the telescope tube facing the observer. Flint-forward is a rather unusual design but typical of many Brashear Co. objective lenses.

Crown-forward interferometric analysis

Crown-forward interferometric analysis

Because there was some doubt about whether this particular objective doublet was delivered to Brashear L-3 in its original manufactured condition, the objective was reversed in the test rig to simulate “crown-forward” orientation of the lenses. Crown-forward design would have been a bit unusual for Brashear but was not unknown. The wavefront mapping shows a regular pattern that normally indicates “better” performance. We sought historical and technical advice on whether the objective should be reassembled and installed crown-forward in the case of Hiram’s Cooley Telescope. In the mean time, bowing to common historical advice, the cell was re-installed in the telescope crown-forward.

Crown Forward Simulation

Crown-forward simulation

“I raytraced a simulated BK-7 Crown and F2 Flint lens that was optimized for 486nm=blue, 587nm=green, and 656nm=red light in the visible spectrum and indeed found that for Crown forward the blue and red have minimum OPD errors leaving the green OPD errors slightly higher. When light goes through the lens in reverse with Flint forward the attached curve flips leaving the green light minimized OPD. So the only conclusion I believe is that for yellow and green light stars the Brashear flint forward design may be more appealing to the human eye.” — Andrew Clarkson, Director of Engineering, Brashear L-3 Communications

Crown-Forward Final Result

Final form

After considering information and advice provided by antique telescope experts and the good folks at Brashear L-3, and test observations made using the cleaned lens, we traveled back to Pittsburgh and asked that the objective be reassembled with the crown element forward. Mark Yingling did the work and believed the elements correctly interfaced with each other so, as discussed above, the glass assembly was essentially flipped inside the cell and reassembled. Looking at the results of modern optical testing, Brashear staff expressed their admiration of the precision work done by their predecessors who originally created the optical elements, by hand, more than a century earlier.

While the objective was apart, metrologist Geary took precise measurements of the elements. Results may be viewed by clicking the links below. The printouts are two pages each:

Crown Element – 1 ~ Crown Element – 2
Flint Element – 1 ~ Flint Element – 2

 

Brashear L-3 Communications LogoOur thanks to the good people at Brashear L-3 Communications for their advice and support in restoration and preservation of the Cooley Telescope’s optics. Photographs and graphics on this page are courtesy Brashear L-3 except where noted.