It is an observation I have made before, but one that continually amazes me… Each Keck telescope consists of three hundred tons of steel and glass, with one simple purpose, to hold a few grams of aluminum in the perfect shape necessary to collect the light from distant stars and galaxies.
Looking into the optics of the Keck 2 telescopeEach segment of the primary mirror is covered with a very thin coating of pure aluminum, about 100nm thick, this is 1/10,000 of a millimeter or 0.000004 inches. Aluminum is used in the Keck telescope as it reflects over 92% of the light across a wide wavelength range extending from the UV well into the infrared.
The layer is just thick enough to reflect nearly all of the light, any thinner and too much light would penetrate the mirror, any thicker and small variations in the coating would begin to distort the shape of the mirror.
How much aluminum?
Density of Al…
2.70g/cm³
Area of a Keck Primary…
36 x 2.598 x (0.9m)² = 75.75m²
Mass of Al…
2.70g/cm³ x 75.75m² x 100nm x 1,000,000cm³/m³ = 20.45g
20.45g = 0.71oz (if you prefer imperial)
Just how much aluminum is really on each Keck primary mirror? Simple enough to calculate… just multiply the surface area of thirty six hexagons by the thickness of the aluminum layer to figure the total mass of metal used.
The figures are found in the sidebar, and the answer is surprisingly little, about 20.5g. In comparison, an empty 12oz soda can weighs about 15g, thus it take a bit more than one soda can of aluminum to cover the Keck’s 10 meter primary mirror.
There is much more to a telescope than just one simple layer of aluminum. But that one component is critical. It is the mirror that gets a great deal of the attention. The primary mirror is what gives a large observatory the ability to capture light from the earliest eras of the universe, billions of years in the past.
In the tropical climate of Hawaiʻi small creatures are commonly found among one’s belongings. Spiders, centipedes, scorpions and more are a fact of life and can be found in anything that has not been carefully stored. Telescopes are often housed in the garage, particularly the large dobsonians that are too large to easily be carried into the house.
A gecko in the center of a 24″ telescope mirrorIn the process of setting up telescopes at the Mana Kea VIS we found another critter that had sought a home. Upon unpacking the 24″ scope belonging to Cliff Livermore we were all amused to find a gecko atop the primary mirror when opening the cover of the box.
One can only imagine the poor creature’s predicament. A seemingly nice quiet, warm and dark place now subject to a great deal of motion and upheaval. Then the cover is removed to allow admission of cold mountain air. Hardly the safe refuge expected when the animal crawled into the box.
A gecko in the center of a 24″ telescope mirrorThe discovery of this poor creature evoke an immediate response, a crowd of laughing humans quickly gathered round the mirror box.
It was notable that everyone’s reaction was the same. Not the revulsion that finding a cockroach or centipede would have caused in the same situation. A gecko is almost universally liked, the results were laughter and amusement.
Instead of being hunted down and squished, this gecko was carefully caught to be given a ride back down the mountain to the warm tropical climate of Waikoloa. The next day he was released into the new rock wall I am building in the back yard for a vegetable bed. With the voids among the rock, this wall is a perfect gecko habitat.
At the Mauna Kea Visitor Information Station telescopes are available every clear night for the public to enjoy the wonders of the night sky. Every evening a set of telescopes ranging from 102mm to 16 inches is setup in the patio beside the VIS. The gear is used heavily, every night of the year, the wear on the telescopes does exact a toll. The abuse is constant, kids hanging on the eyepiece, volunteer operators who have never used a telescope, rain, fog, blowing cinder dust. Conditions that were never foreseen by the designers and far beyond what most telescopes encounter. Sometimes the condition of the equipment is embarrassing, dirty eyepieces, groaning mounts that refuse to track, much of the gear just looks worn and tired.
An 8" telescope mirror covered with a ridiculous amount of dustIt is hard for me to see this, but at least I can do something about it, I do, after all, fix telescopes for a living. It is not unusual for me to spend an evening repairing a telescope and I have made a point of getting some more extensive maintenance accomplished.
My first effort last year was to clean and repair the small dobsonians used by visitors every night. Two eight inch, a six and a 4.5″ Orion dob are put out for anyone to use, from adults to children. After years of use they were in horrible shape, bearings and focusers were coming apart, collimation gone, moisture dissolving the woodwork, a finder attached with duct tape, the mirrors so covered with dust it is surprising there was much of an image to see. One of the eight inch scopes and the 4.5 inch were in pieces in the warehouse after a fix attempt by another volunteer. It took a few days of work to put all to right. Stealing parts from an older scope, repairing what could be saved, cleaning and pounding out a couple dents. Clean, re-install and re-collimate the optics. Four dobs back in service and in better shape than they had been in quite some time.
Parts of a Losmandy G-11 telescope mount spread across a table topThe 16″ Meade LX200 should be the flagship of the equipment used at the VIS. But for all too long it refused to work properly, it would not track. A trip back to the manufacturer failed to correct the problem, despite nearly a thousand dollars in shipping fees for factory service the telescope still would not work most of the time. Most volunteers would not use it, having given up in frustration. Surprisingly the issue was obvious, just listening to the scope indicated gears not fully meshed and grinding on one another. An hour’s worth of dismounting the scope, opening the bottom panel and re-seating a motor mount had the telescope back on sky and slewing from target to target. The scope has failed since, but the problem was even simpler, a loose connection found after a half hour of poking around.
Currently, one of the three Losmandy G-11 mounts belonging to the VIS is in my garage, spread across the table in many parts. I spent a few hours yesterday dismantling the mount and cleaning the grease and cinder dust out of the bearings. It is in pretty good shape, a good cleaning, re-seat the worm gears and some new clutch pads and it will be ready for a few more years of service. I need to get some more grease before I can reassemble the mount, but otherwise everything is ready to put back together. Finish this one and there are two more like it in sore need of maintenance.
One thing at a time, of course by the time I get through it all it will be necessary to start over again…
There is an article on the TMT in yesterdays’s Honolulu Advertiser. While the article shows growing community awareness of the Thirty Meter Telescope project and puts forth some of the basics, it concentrates on some of the political issues. Those are, as typical for Hawai’i, rather fractious, showing the very polarized nature of political discourse in the state.
An overhead view of the proposed Thirty Meter Telescope, credit TMT Observatory CorporationIt is the comments that make the most interesting reading. These show the wide range of feelings in the community. There also seems to be a misunderstanding about the roles of large telescopes in general. The last two decades has seen an enormous advance in our understanding of the universe and its history. We now know the age of the universe, we have discovered planets around other stars, we have learned that ordinary matter makes up only a small fraction of the universe. These represent huge leaps in our understanding and these revelations have come, to the greater degree, from ground based astronomy. Many of these discoveries have been made, or greatly assisted, by the telescopes atop Mauna Kea.
One of the themes that popped up in the comments several times was the idea that space telescopes are where to put the money and the belief that ground telescopes were “obsolete’, thus projects like TMT are not needed. This could not be further from the truth. Yes there are advantages to space telescopes with respect to interference from the atmosphere. But space telescopes face several severe and inherent disadvantages.
