November 10, 2011
By Andrew Cooper
A telescope is all about the primary mirror. Be it a lens or a curved mirror, the size of the primary matters. The larger the primary the more light that can be gathered, allowing astronomers to observe and analyze distant cosmic objects like exceedingly faint galaxies at the far reaches of time and space.
All telescopes, from the very smallest, to the very largest, are all about ?aperture,? or the size of that primary optic, whether you are using a 40 mm spotting ?scope, or the eight and 10-meter telescopes atop Mauna Kea. This is a case where size truly matters!
The largest glass lenses ever used in refractor telescopes were around 40 inches across, beyond this they simply become impractical. That?s why all large telescopes built over the last century use reflecting mirrors for the primary optical element. But mirrors also have their limitations. The largest mirrors that can realistically be made as a single piece of glass are around eight meters (26 feet) in diameter. The Gemini and Subaru telescopes atop Mauna Kea use single-piece mirrors eight meters across.
Any telescope larger that this eight-meter size must use a different approach. The two Keck telescopes each use an array of 36 hexagonal segments to assemble two giant 10-meter (33 foot) primaries.
Each Keck mirror segment is coated with a thin layer of aluminum. This aluminum allows excellent reflection of light from ultraviolet wavelengths, through the visible spectrum, well into the infrared. The layer of aluminum is quite thin, about 100 nanometers, or about one thousandth the thickness of a sheet of paper. One can say that all three hundred tons of steel and glass that make up each Keck telescopes exists to hold less than a single ounce of aluminum in just the perfect position. This is a bit more aluminum than what makes up a single empty soda can.
Telescope mirrors are coated on the front surface, not the rear surface like bathroom or automotive mirrors. Passing through the glass would distort or absorb much of the light, unacceptable for the precise optical surface used in a telescope. This also means that the glass does not protect the aluminum coating; it is exposed to the wind and dust while the telescope operates through the night.
Over time and exposure to the elements, the aluminum coating on the surface of the mirror degrades. About every two years it is necessary to strip the old aluminum coating and reapply a fresh coating. For a telescope with a mirror made from a single piece of glass this is a laborious process that consumes several weeks, during which the telescope is out of service. With a segmented telescope individual segments can be swapped out with prepared spare segments in an operation that takes less than a day so that the Keck Telescopes are prepared for astronomers each and every night of the year.
Segment exchange days are a challenge for the entire summit crew. A large team assembles to carefully remove a segment from the primary mirror and replace it with a clean one, each person doing their part in the process with precision and care. Long checklists are followed to insure each step is completed properly. When everything goes smoothly, Keck?s skilled crew will remove and replace three segments in a day. And it is a long day, with overlapping teams either arriving at the summit early or staying late into the afternoon.
Re-coating a mirror segment is a painstaking process of stripping the old coating, carefully cleaning the mirror, then placing it in a vacuum chamber to deposit a new aluminum coating onto the glass. The re-coating process takes several days per segment, with one full time technician dedicated to the task, with a little help to handle some of the more intense parts of the process.
With thirty-six segments in each Keck primary it takes twelve days of segment exchange to give the mirror a complete makeover. We schedule these days throughout the year, allowing the spare segments to be re-coated and prepared between each exchange. A constant shuffle of segments keeps the technicians responsible for re-coating and preparing the segments very busy. As of this month we have completed working through the primary of the Keck I telescope, in 2012 it will be Keck II?s turn to receive a set of fresh segments.
The proposed Thirty Meter Telescope will use a segmented design similar to Keck. In place of Keck?s 36 segments the TMT will use 492 hexagonal segments. Where Keck schedules a handful of days each year, the TMT will have to replace a few segments each week to keep up with a two-year cycle. Bigger telescopes are bigger challenges, but in return they offer bigger rewards for science.