Astronomy in Arizona
By Anna Edmonds
On our recent trip to Arizona we visited two important astronomy workshops, the Lowell Observatory in Flagstaff and the Steward Observatory Mirror Lab at the University of Arizona in Tucson.
For the delight that our 5-year-old granddaughter found in the displays, the Lowell Observatory ranks high on our “must experience” list. The interactive exhibit hall there kept her exploring with her mother the many programs on the computers. Then she danced up the “Pluto Walk,” stopping only to be sure we were following. We’d already shown her the rare display of five naked eye planets in the western sky. She will remember the fun, but I doubt that she will associate the night sky with the brass markers in the walk.
The Mirror Lab, located underneath the U. of Arizona football stadium, gave us a more specialized study. There we were treated to a presentation on the Lab’s projects of casting and polishing large, lightweight mirrors. We were guided in part by Bob Crawford, an undergraduate optics major who had helped with BPAA’s exchange of the large Zerodur blank for a working telescope.
Most of Arizona’s mirror blanks are honeycombed rather than solid glass giving them a number of advantages: 1) The borosilicate glass (similar to Pyrex) is affected only minimally by changes in temperature. 2) It can be molded at a relatively low temperature. 3) It is less expensive than Zerodur. 4) It is not damaged when the reflective metallic coat is cleaned off and renewed every year. That means it does not have to be repolished frequently.
The honeycombed substrate 5) makes the glass both remarkably stiff and relatively lightweight. 6) With less mass, the glass adapts more quickly to changes in temperature. Also, 7) conditioned air circulates quicker and more completely through the honeycomb than around a solid piece, making it easier to keep the temperature of the glass balanced.
The mold of the mirror blank is made in a pattern of hexagonal ceramic core boxes fixed in a cylindrical tub. Twenty and a half tons of borosilicate glass in five pound chunks are melted in the tub to a temperature of 1180° C. At the same time, the tub is spun so that the glass not only fills the area around the boxes, it also laps up evenly on the edge, forming a parabolic upper surface. When the blank has been molded (more than two years of heating and cooling), the boxes are washed out with a high pressure water spray. What is left is first, to give it the final polishing and lapping to an accuracy of more than one millionth of an inch, and then to aluminize it to give it the reflective surface.
Two additional ideas for telescopes are being developed in Arizona. The Large Binocular Telescope involves a collaboration among 13 communities, among them four in Arizona, six in Germany and one in Italy. The LBT on Mount Graham northeast of Tucson is a combination of two 8.4-m mirrors on a common mount which gives the equivalent resolution of a 22.8-m telescope.
The second is the use of liquid mercury as the reflective surface of a mirror. Although this mirror is able to point only directly overhead (the liquid cannot be tipped) the Earth’s tilt and rotation permit a reasonable fraction of the celestial sphere to be viewed. Its focal length can be changed as desired in minutes by merely changing the speed of its rotation. As with the glass mirrors that are spun, the rotation gives the mercury its paraboloidal surface.
With its high mountains and many nights of clear skies, it’s obvious why Arizona is a Mecca for astronomers.