The auctioneer was known for his malapropisms. He was the kind of man who might aspire to own a tantrum bicycle. Or announce that Rome wasn’t burned in a day. At the auction of the restaurant my parents were selling, the auctioneer tried to add value by including the view in the sale. He said there were magnificent sea views for ‘as far as the eye could see’. Eyes rolled, then checked the view before acknowledging he was correct. He had viewed the price and repriced the view.
As far as the eye can see isn’t quite as impressive as how far the eye can see. Or so I thought when I stumbled over some notes I made few years ago about astronomical telescopes. I was interested in array sizes at the time. The bigger the antenna, the more data could be collected. So I thought astronomical telescopes might help some budget holders see why I was requesting astronomical quantities of money for geophysics.
I was attempting to design a geophysical array of 400 square kilometres extent. Forty years earlier, 24 and 24 active sensors were the standard deployment and were capable of creating fuzzy, two-dimensional acoustic profiles of the subsurface. Each aperture was perhaps 5 kilometres long and the aperture could be rolled to create overlapping transects ranging from 10 to 100 or 1000 kilometres collected during weeks of operations. By the time I came to need 400 square kilometres of three dimensional acoustic data, 100,000 sensors could be deployed at once. But the scale of my design needed 10 million active sensors for three months of operations or 100,000 for several years. Unless we could come up with a clever alternative, we’d have to wait another decade to make it affordable. But there wasn’t a better idea and then, the business case evaporated. And while I’m here, I should clarify that my field of geophysics was reflection seismology. To catch a reflection implies the existence of a signal. This need doubles the effort. The sensors are deployed among active energy sources that radiate sound waves into the subsurface whose echoes the sensors are destined to detect. It’s like using a flash for your photos but on an enormous scale with sound not light.
My notes (and what you’ll read on Wikipedia) say that the telescope at the Great Paris Exhibition in 1900 was the largest refracting telescope ever constructed. It was 1.25 m across and pretty much useless because it was too heavy to manoeuvre to aim at anything beyond Paris.
If you’d accept that 1.25 m is your maximum arm-extended width at service in tennis, and maybe it isn’t, I’d like to use tennis courts to illustrate the scale of telescopes through the ages. Tennis and basketball analogies aren’t my idea, that’s also something I stumbled over a few years ago.
The point is that gathering low levels of signal requires huge sensors. Just like in cameras, the bigger the sensor surface area, the more light can be collected. And like a camera, the intensity of light gathered varies with surface area and time. The twin Keck telescopes in Hawaii are the largest telescopes in operation today and together, the two sensors are about the size of a tennis court. Built on Mauna Kea between 1992 and 1996, they are each nearly ten times bigger than the Paris telescope. Each ten metre Keck instrument is composed of 36 mirrored glass segments and the twins are are steerable for aim and focus under computer control.
And the twin Keck telescopes are able to see fainter and further into the cosmos than any other current research facility. They use 1990s technology designed in the 1980s.
Next comes The Thirty Metre Telescope also in Mauna Kea. TMT will be 12 times sharper than Hubble which isn’t even on the planet but in low orbit around it. TMT will be 2020s technology designed in the late 2000s.
And around the same time, the European Extremely Large Telescope will be commissioned high in the Chilean mountains. ELT will be 16 times sharper than Hubble. It will start work about 12 years after it was scoped out, so to speak. The ELT will become the largest optical/near-infrared telescope in the world in 2025. It will gather 13 times more light than the largest optical telescopes that exist today.
Based on surface area alone, ELT will be about 7 million times better at gathering light than Galileo’s 1609 telescope. But surface area is only a small part of the story. The sharpness will be relatively awesome . These modern telescopes can correct for all of the atmospheric distortions and are mounted as high as possible. Gallileo would expect no less.
Hopefully, we’ll get a better handle on dark matter, alien life, space weather, asteroid impact, black holes, the births and deaths of galaxies and our own universe. Perhaps you want to share some of my more uncomfortable thoughts.
I have to mention two other things while on the subject of how far the human eye can see. The Large Zenith Telescope was built in British Colombia in 2003 and is a 6 metre bowl of liquid mercury. You can’t tilt it so it’s unblinking eye gazes up only. If you spin it slowly, carefully, the mercury forms a paraboloid that is more perfect than glass or any polished metal. But the eye isn’t quite unblinking, the local weather was too bad and the site was decommissioned recently.
And my mind has been much focussed on such things because of my latest chapbook. I decided to make one scene the subject for April. And that took me to imaging places I’d never visited before.
Quarried, the chapbook, is with the printer.
Further excitement: The Hubble successor is the James Webb Space Telescope and it should be launched this year.
‘We are opening the infrared treasure chest, and surprises are guaranteed’ said Dr. John C. Mather, Senior Project Scientist for the Webb mission and Senior Astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
I too have been thinking about infrared but at a more personal scale.
Afterword: Despite how adaptive optics has enabled surface based telescopes to have better resolution than those in orbit, we still need space based telescopes. That’s because our atmosphere absorbs nearly all incoming ultraviolet light. The most common elements in the universe – hydrogen, helium, carbon, nitrogen, oxygen and silicon – all leave spectral signatures in the ultraviolet. Without an extraterrestrial viewing platform, we are blind to the hot cores of active galaxies, quasars, energetic stars, and the clouds of dust that surround black holes. (Summarised after a NASA fact sheet).