In my first appointment after my PhD, I worked at Radcliffe Observatory in Pretoria, South Africa. That telescope also had a 1.8 meter mirror, the same light collection as the VATT. That telescope was, when built, the third-largest telescope in the world. But it needed a 30-foot long tube, and had a massive counterweight, on which I banged my head once in the dark, and was very careful ever after.
The telescope was enclosed in an enormous concrete turret about 80 feet in diameter. The VATT has the same light collecting power as the Radcliffe telescope and it produces sharper images. It is in a site with less extinction by the atmosphere, and which permits observations at infrared wavelengths. The telescope is small and unassuming. It is the Steph Curry of telescopes.
The innovations incorporated in the VATT primary mirror; internal ventilation, spin casting, and stressed lap polishing, have since been copied in the mirrors of larger telescopes, three with 3.5 meter mirrors, three with 6.5 meter mirrors, (on the MMT and the two Magellan Telescopes in Chile), three 8.3 meter mirrors, (two in the Giant Binocular Telescope and one in the Vera Rubin Observatory in Chile) More are in process of being incorporated in an additional 6.5 meter mirror of a Japanese telescope. Eight 8.3 meter mirrors are being built into the Giant Magellan Telescope.
The story is also very much the story of the Roger Angel team and the beginning of the Mirror Lab with key helpers John Hill and Buddy Martin. They let me watch their work, and sometimes I could make helpful suggestions.
Internal Ventilation is a way of holding the mirror temperature close to the temperature of ambient air and so producing sharper images. The mirror has interconnected cavities inside. And air is blown through to keep the mirror within a fraction of a degree of the air temperature outside it.
Spin-casting had never been used before the making of the VATT’s primary mirror. It is needed when the mirror focus is close to the glass, when the mirror shape is steeply curved. That shape allows the telescope and its housing to be compact. The smaller dome building for the telescope keeps its cost down. Before this, telescope housing was becoming more expensive than the telescope itself.
When the molten glass mirror is spun, the glass rides up at the edges and is depressed in the middle, approximating the final shape needed for the mirror. In this way less glass is used, and the annealing time of the glass is made much shorter.
The VATT mirror was cast before the Mirror Lab was built. There was a deconsecrated Synagogue on the University of Arizona grounds, and this was provided to allow a test of the process. In this first trial of a rotating furnace, the furnace rotated on an old gun turret. And the man who monitored the process went round and round with the glass. It all worked the first time. And so after the successful casting of the VATT mirror, the work moved to the then new Mirror Lab where larger mirrors could be cast.
While work at the Mirror Lab was focused on building the giant furnace on which all the mirrors above were to be cast, a small part of the building was set aside to learn how to polish the extremely steep optical surface of the mirror. The problem is that the center of the mirror is shaped like a sphere and the outer parts are more like a cylinder. The polishing process requires that the polishing lap surface matches the shape for the glass, and so Roger invented stressed lap polishing, where the lap edges are pulled on to change its shape as it moves over the surface.
As a result all the steepest surface mirrors have been spun cast and stressed lap polished. Even today, the VATT mirror is the steepest curve mirror in use at F1.0. Photographers will understand how extreme this is. Next is the Giant Binocular Telescope at F1.14, and the MMT and Magellan mirrors at F1.25. Eventually the Giant Magellan Mirror will achieve an overall surface of about F0.7, however using seven pieces of glass. The VATT’s mirror will remain the single steepest large piece of class, though its own small secondary mirror is even steeper.
After this wonderful first mirror was polished and tested, it was set aside to make large mirrors. But that seemed a terrible waste, so I told Fr. George Coyne about the mirror, and suggested that The Vatican Observatory could use it to have its own special telescope. The rest of the history you know. After 30 years it still stands as both a working instrument, and as a statement of where telescope technology changed.
I don’t only design and help make telescopes, but I use them too. One of the scientific problems we had was that in searching for Earth-like planets around other stars, we needed to know what the Earth looked like from the outside our atmosphere, especially we wanted to know how the gases of its atmosphere appeared in the spectrum. I used earthshine on the Moon to study this, and the first observations were made from Kitt Peak, but that observatory was too deep in the Earth’s atmosphere to show enough. So we made more observations with the VATT, and showed how water vapor, oxygen, carbon dioxide and methane all showed in the near infrared spectrum of the Earth.
It is not only the optics of the VATT which are special, but its site too.
About the Author:
Neville J. “Nick” Woolf
Professor Nick Woolf, received his PhD in Astrophysics in 1959 from Manchester University. After graduation, he moved to the U.S., first to the Lick Observatory, and then to the Princeton University Observatory. He became a National Research Council Postdoctoral Fellow at the NASA Goddard Institute for Space Studies in 1965, and the next year, he was awarded the Alfred P. Sloan Research Fellowship at the National Academy of Sciences. He became a professor at the University of Arizona in 1974, where he helped create the Mount Graham International Observatory.
Dear Brother Guy:
I wanted to be sure that at the VATT 30th birthday celebration, the story is known of the technological innovations incorporated in VATT. – Nick Woolf, August 2023
Professor Woolf is one of the four astronomers from the documentary “Star Men.”
Image from visiontv.ca
