Wednesday, June 30, 2010


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The next generation of adaptive optics has arrived at the Large Binocular Telescope (LBT) in Arizona, providing astronomers with a new level of image sharpness never before seen. Developed in a collaboration between Italy’s Arcetri Observatory of the Istituto Nazionale di Astrofisica (INAF) and the University of Arizona’s Steward Observatory, this technology represents a remarkable step forward for astronomy.

The LBT, with its two 8.4 metre -mirrors, is the largest single optical telescope in the world. The telescope is a collaboration between institutions from the USA, Italy and Germany. Germany’s 25% participation is represented by the Max-Planck Society, the Astrophysical Institute Potsdam and Heidelberg University. The test camera for the images shown here was developed by INAF and the Max-Planck-Institute for Astronomy (MPIA) in Heidelberg.

Until relatively recently, ground-based telescopes had to live with wavefront distortion caused by the Earth’s atmosphere which significantly blurred images of distant objects (this is why stars appear to twinkle to the human eye). While there have been advancements in adaptive optics technology to correct atmospheric blurring, the LBT’s innovative system truly takes this concept to a whole new level.

In closed-dome tests beginning May 12 and sky tests every night since May 25, astronomer Simone Esposito and his INAF team tested the new device, achieving exceptional results. The LBT’s adaptive optics system, called the First Light Adaptive Optics system (FLAO), immediately outperformed all other comparable systems, delivering an image quality greater than three times sharper than the Hubble Space Telescope using just one of the LBT’s two 8.4 metre mirrors. As soon as the adaptive optics are in place for both mirrors and their light is combined appropriately, it is expected that the LBT will achieve image sharpness ten times that of the Hubble.

"This is an incredibly exciting time as this new adaptive optics system allows us to achieve our potential as the world’s most powerful optical telescope," said Richard Green, director of the LBT. "The successful results show that the next generation of astronomy has arrived, while providing a glimpse of the awesome potential the LBT will be capable of for years to come."

The unit of measure for perfection of image quality is known as the Strehl ratio, with a ratio of 100 % equivalent to an absolutely perfect image. Without adaptive optics, the ratio for ground-based telescopes is less than 1 percent. The adaptive optics systems on other major telescopes today improve image quality up to about 30 percent to 50 percent in the near-infrared wavelengths where the testing was conducted.

In the initial testing phase, the LBT’s adaptive optics system has been able to achieve unprecedented Strehl Ratios of 60 to 80 percent, a nearly two-thirds improvement in image sharpness over other existing systems. The results exceeded all expectations and were so precise that the testing team had difficulty believing their findings. However, testing has continued since the system was first put on the sky on May 25, the LBT’s adaptive optics have functioned flawlessly and have achieved peak Strehl ratios of 82 to 84 percent.

"The results on the first night were so extraordinary that we thought it might be a fluke, but every night since then the adaptive optics have continued to exceed all expectations. These results were achieved using only one of LBT’s mirrors. Imagine the potential when we have adaptive optics on both of LBT’s giant eyes." said Simone Esposito, leader of the INAF testing team.

Development of the LBT’s adaptive optics system took more than a decade through an international collaboration. INAF, in particular the Arcetri Observatory, conceived the LBT instrument design and developed the electro-mechanical system, while the University of Arizona Mirror Lab created the optical elements, and the Italian companies Microgate and ADS International engineered several components. A prototype system was previously installed on the Multiple Mirror Telescope (MMT) at Mt. Hopkins, Arizona. The MMT system uses roughly half the number of actuators as the LBT’s final version, but demonstrated the viability of the design. The LBT’s infrared test camera, which produced the accompanying images, was a joint development of INAF, Bologna and the MPIA, Heidelberg.

"This has been a tremendous success for INAF and all of the partners in the LBT," said Piero Salinari, Research Director at the Arcetri Observatory, INAF. "After more than a decade and with so much care and effort having gone into this project, it is really rewarding to see it succeed so astoundingly."

This outstanding success was achieved through the combination of several innovative technologies. The first is the secondary mirror, which was designed from the start to be a main component of the LBT rather than an additional element as on other telescopes. The concave secondary mirror is 0.91 metres in diameter (3 feet) and only 1.6 millimetres thick. The mirror is so thin and pliable that it can easily be manipulated by actuators pushing on 672 tiny magnets glued to the back of the mirror, a configuration which offers far greater flexibility and accuracy than previous systems on other telescopes. An innovative "pyramid" sensor detects atmospheric distortions and manipulates the mirror in real time to cancel out the blurring, allowing the telescope to literally see as clearly as if there were no atmosphere. Incredibly, the mirror is capable of making adjustments every one thousandth of a second, with accuracy to better than ten nanometres (a nanometre is one millionth the size of a millimetre).

(Photo: LBT Collaboration / R. Cerisola)

Max-Planck Society


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As turboprop and jet aircraft climb or descend under certain atmospheric conditions, they can inadvertently seed mid-level clouds and cause narrow bands of snow or rain to develop and fall to the ground, new research finds. Through this seeding process, they leave behind odd-shaped holes or channels in the clouds, which have long fascinated the public.

The key ingredient for developing these holes in the clouds: water droplets at subfreezing temperatures, below about 5 degrees Fahrenheit (-15 degrees Celsius). As air is cooled behind aircraft propellers or over jet wings, the water droplets freeze and drop toward Earth.

“Any time aircraft fly through these specific conditions, they are altering the clouds in a way that can result in enhanced precipitation nearby,” says Andrew Heymsfield, a scientist with the National Center for Atmospheric Research (NCAR) and lead author of a new study into the phenomenon. “Just by flying an airplane through these clouds, you could produce as much precipitation as with seeding materials along the same path in the cloud.”

Precipitation from planes may be particularly common in regions such as the Pacific Northwest and western Europe because of the frequent occurrence of cloud layers with supercooled droplets, Heymsfield says.

The study, which addresses longstanding questions about unusual cloud formations known as hole-punch or canal clouds, is being published this month in the Bulletin of the American Meteorological Society. It was funded by the National Science Foundation, NCAR’s sponsor. In addition to NCAR, the research team included scientists from Colorado State University and the University of Wyoming, as well as a retired cloud physicist.
Punching holes in clouds

Across the world, sightings of blue-sky holes piercing a cloud layer have triggered bemusement and speculation. A front-page feature on Yahoo! carried the headline “A Halo over Moscow” after photos emerged of just such a hole in October 2009.

As far back as the 1940s, scientists have wondered about the causes of these clouds with gaps seemingly made by a giant hole punch. Researchers have proposed a number of possible aviation-related causes, from acoustic shock waves produced by jets, to local warming of the air along a jet’s path, to the formation of ice along jet contrails. Indeed, the earliest observations implicated jet aircraft, but not propeller aircraft, as producing the holes.

Researchers in the 1980s observed that propeller aircraft could transform supercooled droplets into ice crystals, and experiments were launched in the 1990s to characterize the phenomenon.

But scientists had not previously observed snow as it fell to the ground as a result of aircraft until Heymsfield and his colleagues happened to fly through some falling snow west of Denver International Airport with an array of instruments. While the research team did not notice anything unusual at the time of their 2007 flight, a subsequent review of data from a ground-based radar in the area revealed an unusual echo, indicating that the band of precipitation had evolved quickly and was unusually shaped.
“It became apparent that the echo had evolved in a unique way, but I had no satisfactory explanation,” says Patrick Kennedy, a Colorado State University radar engineer who spotted the unusual readings and helped write the study.
Piecing together clues

Heymsfield and Kennedy went back through data from their aircraft’s forward- and downward-viewing camera. They noticed a hole in an otherwise solid deck of altocumulus clouds in the forward imagery, as well as a burst of snow that extended to the ground.

Since the hole was oriented in the same direction as the standard flight tracks of commercial aircraft in the region, Heymsfield surmised that a plane flying through the cloud might have somehow caused ice particles to form and “snow out” along its path, leaving a canal-shaped hole-punch cloud behind.

A subsequent review of flight track records from the Federal Aviation Administration revealed that turboprop planes operated by two airlines flew close to the hole-punch location, following a standard flight path that produced the subsequent band of snow. Snow crystals began falling about five minutes after the second aircraft flew through the cloud. The snowfall, in a band about 20 miles long and 2.5 miles wide, continued for about 45 minutes, resulting in about two inches of snow on the ground.

The researchers also examined data from onboard spectrometers that profiled the snowflakes within the band of snow beneath the hole punch. These plate-shaped crystals showed evidence of riming (accumulation of liquid water), whereas ice particles elsewhere in the cloud showed little or no riming.

“This tells us that the aircraft literally ‘seeded’ the cloud just by flying through it,” Heymsfield says.

The cloud layers outside Denver contained supercooled droplet—particles of water that remain liquid even at temperatures as low as -35 degrees Fahrenheit (about -34 degrees C). When a turboprop plane flies through such a cloud layer, the tips of its propellers can cause the air to rapidly expand. As the air expands, it cools and causes the supercooled droplets to freeze into ice particles and fall out of the clouds as snow or rain.

The research team conducted additional studies into the cooling over the wings of jet aircraft, thereby accounting for earlier observations of the impact of jets. Jet aircraft need colder temperatures (below about -4 to -13 degrees F, or -20 to -25 degrees C) to generate the seeding effect. Air forced to expand over the wings as the aircraft moves forward cools and freezes the cloud droplets.

“This apparently happens frequently, embedded in the cloud layers,” Heymsfield says. “You wouldn’t necessarily see it from satellite or from the ground. I had no idea this was happening. I was sitting in back of the plane. And then this data set just fell in our laps. It was a lucky break.”

(Photo: Alan Sealls, chief meteorologist, WKRG-TV)

University Corporation for Atmospheric Research


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Researchers at Rice University, Purdue University and the Massachusetts Institute of Technology have solved a long-standing mystery about why some fluids containing polymers -- including saliva -- form beads when they are stretched and others do not.

The findings are published online in the journal Nature Physics.

Study co-author Matteo Pasquali, professor in chemical and biomolecular engineering at Rice, said the study answers fundamental scientific questions and could ultimately lead to improvements as diverse as ink-jet printing, nanomaterial fiber spinning and drug dispensers for "personalized medicine."

Co-author Osman Basaran, Purdue's Burton and Kathryn Gedge Professor of Chemical Engineering, said, "Any kindergartner is familiar with this beading phenomenon, which you can demonstrate by stretching a glob of saliva between your thumb and forefinger. The question is, 'Why does this beading take place only in some fluids containing polymers but not others?'"

Pasquali said, "In answering the question about why some fluids do this and others do not, we are addressing everyday processes that apply to fiber and droplet formation, not just in multibillion-dollar industrial plants but also in fluids produced in living cells."

Saliva and other complex "viscoelastic" fluids like shaving cream and shampoo contain long molecules called polymers. When a strand of viscoelastic fluid is stretched, these polymers can cause a line of beads to form just before the strand breaks.

Pasquali said the explanation for why some viscoelastic fluids form beads and others do not was decades in the making. The origins of the work can be traced to Pasquali's and Basaran's doctoral research adviser, L.E. "Skip" Scriven of the University of Minnesota. Pasquali said Scriven worked out the basics of the competition between capillary, inertial and viscous forces in flows during the 1970s and 1980s. In the mid-1990s, during his doctoral research at Minnesota, Pasquali expanded on Scriven's earlier work to include the effects of viscoelasticity, which originates in liquid microstructures and nanostructures. Finally, Pasquali's former doctoral student, Pradeep Bhat, the lead author of the new study, took up the mantle nine years ago as a Ph.D. student in Pasquali's lab and continued working on the problem for the past three years as a postdoctoral researcher in Basaran's lab at Purdue.

Bhat, Basaran and Pasquali found that a key factor in the beading mechanism is fluid inertia, or the tendency of a fluid to keep moving unless acted upon by an external force.

Other major elements are a fluid's viscosity; the time it takes a stretched polymer molecule to "relax," or snap back to its original shape when stretching is stopped; and the "capillary time," or how long it would take for the surface of the fluid strand to vibrate if plucked.

"It turns out that the inertia has to be large enough and the relaxation time has to be small enough to form beads," Bhat said.

The researchers discovered that bead formation depends on two ratios: the viscous force compared with inertial force and the relaxation time compared with the capillary time.

Rice University




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