Category: Observatories

Image credit: Gemini

The latest image taken by the Gemini telescope in Mauna Kea Hawaii demonstrates how powerful its new adaptive optics technology can be. The telescope captured an image of the globular cluster M-13, first with its normal resolution and then using the Altair adaptive optics system; the second image is crystal clear, and contains many more stars which are finely focused. The adaptive optics compensate up to 1000 times a second for distortions caused by the Earth’s atmosphere, so the light appears as if the telescope was in space. This technology is expected to revolutionize ground-based astronomy.

A razor-sharp image was released today revealing new details at the heart of a famous star cluster. The thousands of swarming stars at the cluster’s core were made visible by an innovative adaptive optics system called Altair (after the star Altair) that is currently being commissioned on the Frederick C. Gillett Gemini Telescope on Mauna Kea, Hawai`i.

Among several of the first images from Altair (Altitude Conjugate Adaptive Optics for Infrared), the high-resolution data reveal multitudes of stars with stunning clarity. The dense star cluster known to generations of skywatchers as the Great Hercules Cluster or M-13 is home to hundreds of thousands of stars that, in the center, are often blurred by our atmosphere into a great glowing mass. “The resolution obtained in these images is approximately equivalent to seeing the separation between an automobile’s headlights on the Golden Gate Bridge in San Francisco while standing 3,850 kilometers away in Hawai`i,” said Observatory Adaptive Optics Scientist Dr. Francois Rigaut.

The close-up images of M-13, with and without Altair, as well as a spectacular reference image of the entire cluster, provided by the Canada-France-Hawaii Telescope, can be viewed and downloaded at: http://www.gemini.edu/media/images_2003-2.html.

The remarkable detail in the Gemini images was made possible by Altair’s unique ability to correct starlight that has been blurred by atmospheric turbulence using adaptive optics with altitude conjugation.

Most adaptive optics systems that are currently in use correct for distortions to starlight by assuming that all of the distortions occur where starlight is collected – near the surface of the telescope’s primary mirror. In an altitude-conjugated system like Gemini’s, the distortions are assumed to be at the dominant turbulence layer of the atmosphere. By conjugating or tuning the system for a specific layer above the telescope, Altair can generate a more accurate model of the starlight’s path through our atmosphere.

“Adaptive optics with altitude conjugation is a pioneering new technique that is a powerful way to measure and fix distortions to starlight, which traveled undisturbed for vast distances through space until hitting pockets of warm and cold air in earth’s atmosphere,” said Glen Herriot, the systems engineer who managed the building of Altair in Victoria, BC at the laboratories of the National Research Council of Canada. Altair is able to precisely correct the distorted starlight up to 1,000 times per second using a sophisticated, deformable mirror about the size of the palm of your hand. “The end result is,” says Herriot, “images that rival or even exceed the sharpness of pictures taken from space.”

Working with Gemini Observatory personnel, the Canadian team headed by Project Manager Herriot and Project Scientist Dr. Jean-Pierre V?ran, have been commissioning Altair on Gemini North from late 2002 through early 2003. The instrument team, comprised of 25 scientists and engineers, guided the Gemini adaptive optics system from design to commissioning over the past six years. “Commissioning a precision instrument on a 7-story, 350-ton, sophisticated telescope is especially challenging because of the extremely intricate coordination required to make all the systems work together seamlessly,” said Herriot. Altair’s commissioning on Gemini is expected to be complete before the end of 2003.

A key feature of Altair’s sophistication is the ability to automatically monitor, adjust and optimize multiple parameters during image exposures. The idea is to make adaptive optics user-friendly for our community. When atmospheric conditions allow, simply point and click and near diffraction-limited images are delivered to a camera or spectrograph. Altair continually measures and reports on the images’ level of detail making it one of the most efficient adaptive optics systems in the world. “By routinely delivering infrared images much sharper than is currently possible even from space, Altair gives observers a tremendous advantage in probing deeper in the universe and making more accurate measurements of astronomical objects,” Dr. V?ran says.

“Altair enormously enhances the quality and power of our imaging and spectroscopy,” says Dr. Matt Mountain, Gemini’s Director. “Gemini will soon deliver diffraction-limited images in the near-infrared.” Gemini’s theoretical diffraction limit (maximum resolution) is about 40 milli-arcseconds in the near-infrared H-band (1.6 micrometers wavelength). At this point in commissioning, Altair can deliver 60-milli-arcsecond resolution in the H-band (60 milli-arcseconds is comparable to viewing one grain of sand from about 1.6 kilometers or 1 mile away).

Dr. Mountain pointed out that Altair’s commissioning means that one of the most sophisticated adaptive optics system in the world is now built-in to Gemini North as a facility instrument, and will soon be routinely available to all scientists throughout the Gemini partnership.

“This is a major achievement towards our Gemini goal of delivering space-quality images from an 8-meter, ground-based telescope,” said Dr. Mountain.

Gemini’s Associate Director Dr. Jean-Ren? Roy explains that Altair is a major step forward in Gemini’s aggressive plans to maximize the potential of adaptive optics on ground-based astronomical imaging. Dr. Roy elaborates, “Altair, representing the foundation of tomorrow’s adaptive optics technology, is important for the success of the next generation of 30- to 100-meter, diffraction-limited, infrared, ground-based telescopes now on the drawing boards.”

Future generations of adaptive optics technologies like these will undoubtedly revolutionize ground-based astronomy. For now, Altair is state of the art and provides a powerful new eye on the universe.

Original Source: Gemini News Release

Image credit: ESO

A team of engineers from the European Southern Observatory recently tested out a new adaptive optics facility on the Very Large Telescope (VLT) at the Paranal Observatory in Chile. This technology adapts images taken by the telescope to remove the distortion caused by the Earth?s atmosphere ? as if they were seen from space. The next step will be to connect similar systems to all the telescopes at the facility and then hook them up in a large array. This should allow the observatory to resolve objects 100 times fainter than today.

On April 18, 2003, a team of engineers from ESO celebrated the successful accomplishment of “First Light” for the MACAO-VLTI Adaptive Optics facility on the Very Large Telescope (VLT) at the Paranal Observatory (Chile). This is the second Adaptive Optics (AO) system put into operation at this observatory, following the NACO facility (ESO PR 25/01).

The achievable image sharpness of a ground-based telescope is normally limited by the effect of atmospheric turbulence. However, with Adaptive Optics (AO) techniques, this major drawback can be overcome so that the telescope produces images that are as sharp as theoretically possible, i.e., as if they were taken from space.

The acronym “MACAO” stands for “Multi Application Curvature Adaptive Optics” which refers to the particular way optical corrections are made which “eliminate” the blurring effect of atmospheric turbulence.

The MACAO-VLTI facility was developed at ESO. It is a highly complex system of which four, one for each 8.2-m VLT Unit Telescope, will be installed below the telescopes (in the Coud? rooms). These systems correct the distortions of the light beams from the large telescopes (induced by the atmospheric turbulence) before they are directed towards the common focus at the VLT Interferometer (VLTI).

The installation of the four MACAO-VLTI units of which the first one is now in place, will amount to nothing less than a revolution in VLT interferometry. An enormous gain in efficiency will result, because of the associated 100-fold gain in sensitivity of the VLTI.

Put in simple words, with MACAO-VLTI it will become possible to observe celestial objects 100 times fainter than now. Soon the astronomers will be thus able to obtain interference fringes with the VLTI (ESO PR 23/01) of a large number of objects hitherto out of reach with this powerful observing technique, e.g. external galaxies. The ensuing high-resolution images and spectra will open entirely new perspectives in extragalactic research and also in the studies of many faint objects in our own galaxy, the Milky Way.

During the present period, the first of the four MACAO-VLTI facilties was installed, integrated and tested by means of a series of observations. For these tests, an infrared camera was specially developed which allowed a detailed evaluation of the performance. It also provided some first, spectacular views of various celestial objects, some of which are shown here.

MACAO – the Multi Application Curvature Adaptive Optics facility
Adaptive Optics (AO) systems work by means of a computer-controlled deformable mirror (DM) that counteracts the image distortion induced by atmospheric turbulence. It is based on real-time optical corrections computed from image data obtained by a “wavefront sensor” (a special camera) at very high speed, many hundreds of times each second.

The ESO Multi Application Curvature Adaptive Optics (MACAO) system uses a 60-element bimorph deformable mirror (DM) and a 60-element curvature wavefront sensor, with a “heartbeat” of 350 Hz (times per second). With this high spatial and temporal correcting power, MACAO is able to nearly restore the theoretically possible (“diffraction-limited”) image quality of an 8.2-m VLT Unit Telescope in the near-infrared region of the spectrum, at a wavelength of about 2 ?m. The resulting image resolution (sharpness) of the order of 60 milli-arcsec is an improvement by more than a factor of 10 as compared to standard seeing-limited observations. Without the benefit of the AO technique, such image sharpness could only be obtained if the telescope were placed above the Earth’s atmosphere.

The technical development of MACAO-VLTI in its present form was begun in 1999 and with project reviews at 6 months’ intervals, the project quickly reached cruising speed. The effective design is the result of a very fruitful collaboration between the AO department at ESO and European industry which contributed with the diligent fabrication of numerous high-tech components, including the bimorph DM with 60 actuators, a fast-reaction tip-tilt mount and many others. The assembly, tests and performance-tuning of this complex real-time system was assumed by ESO-Garching staff.

Installation at Paranal
The first crates of the 60+ cubic-meter shipment with MACAO components arrived at the Paranal Observatory on March 12, 2003. Shortly thereafter, ESO engineers and technicians began the painstaking assembly of this complex instrument, below the VLT 8.2-m KUEYEN telescope (formerly UT2).

They followed a carefully planned scheme, involving installation of the electronics, water cooling systems, mechanical and optical components. At the end, they performed the demanding optical alignment, delivering a fully assembled instrument one week before the planned first test observations. This extra week provided a very welcome and useful opportunity to perform a multitude of tests and calibrations in preparation of the actual observations.
AO to the service of Interferometry

The VLT Interferometer (VLTI) combines starlight captured by two or more 8.2- VLT Unit Telescopes (later also from four moveable1.8-m Auxiliary Telescopes) and allows to vastly increase the image resolution. The light beams from the telescopes are brought together “in phase” (coherently). Starting out at the primary mirrors, they undergo numerous reflections along their different paths over total distances of several hundred meters before they reach the interferometric Laboratory where they are combined to within a fraction of a wavelength, i.e., within nanometers!

The gain by the interferometric technique is enormous – combining the light beams from two telescopes separated by 100 metres allows observation of details which could otherwise only be resolved by a single telescope with a diameter of 100 metres. Sophisticated data reduction is necessary to interpret interferometric measurements and to deduce important physical parameters of the observed objects like the diameters of stars, etc., cf. ESO PR 22/02.

The VLTI measures the degree of coherence of the combined beams as expressed by the contrast of the observed interferometric fringe pattern. The higher the degree of coherence between the individual beams, the stronger is the measured signal. By removing wavefront aberrations introduced by atmospheric turbulence, the MACAO-VLTI systems enormously increase the efficiency of combining the individual telescope beams.

In the interferometric measurement process, the starlight must be injected into optical fibers which are extremely small in order to accomplish their function; only 6 ?m (0.006 mm) in diameter. Without the “refocussing” action of MACAO, only a tiny fraction of the starlight captured by the telescopes can be injected into the fibers and the VLTI would not be working at the peak of efficiency for which it has been designed.

MACAO-VLTI will now allow a gain of a factor 100 in the injected light flux – this will be tested in detail when two VLT Unit Telescopes, both equipped with MACAO-VLTI’s, work together. However, the very good performance actually achieved with the first system makes the engineers very confident that a gain of this order will indeed be reached. This ultimate test will be performed as soon as the second MACAO-VLTI system has been installed later this year.
MACAO-VLTI First Light

After one month of installation work and following tests by means of an artificial light source installed in the Nasmyth focus of KUEYEN, MACAO-VLTI had “First Light” on April 18 when it received “real” light from several astronomical obejcts.

During the preceding performance tests to measure the image improvement (sharpness, light energy concentration) in near-infrared spectral bands at 1.2, 1.6 and 2.2 ?m, MACAO-VLTI was checked by means of a custom-made Infrared Test Camera developed for this purpose by ESO. This intermediate test was required to ensure the proper functioning of MACAO before it is used to feed a corrected beam of light into the VLTI.

After only a few nights of testing and optimizing of the various functions and operational parameters, MACAO-VLTI was ready to be used for astronomical observations. The images below were taken under average seeing conditions and illustrate the improvement of the image quality when using MACAO-VLTI.

MACAO-VLTI – First Images
Here are some of the first images obtained with the test camera at the first MACAO-VLTI system, now installed at the 8.2-m VLT KUEYEN telescope.

PR Photos 12b-c/03 show the first image in the infrared K-band (wavelength 2.2 ?m) of a star (visual magnitude 10) obtained without and with image corrections by means of adaptive optics.

PR Photo 12d/03 displays one of the best images obtained with MACAO-VLTI during the early tests. It shows a Strehl ratio (measure of light concentration) that fulfills the specifications according to which MACAO-VLTI was built. This enormous improvement when using AO techniques is clearly demonstrated in PR Photo 12e/03, with the uncorrected image profile (left) hardly visible when compared to the corrected profile (right).

PR Photo 11f/03 demonstrates the correction capabilities of MACAO-VLTI when using a faint guide star. Tests using different spectral types showed that the limiting visual magnitude varies between 16 for early-type B-stars and about 18 for late-type M-stars.
Astronomical Objects seen at the Diffraction Limit

The following examples of MACAO-VLTI observations of two well-known astronomical objects were obtained in order to provisionally evaluate the research opportunities now opening with MACAO-VLTI. They may well be compared with space-based images.

The Galactic Center
The center of our own galaxy is located in the Sagittarius constellation at a distance of approximately 30,000 light-years. PR Photo 12h/03 shows a short-exposure infrared view of this region, obtained by MACAO-VLTI during the early test phase.

Recent AO observations using the NACO facility at the VLT provide compelling evidence that a supermassive black hole with 2.6 million solar masses is located at the very center, cf. ESO PR 17/02. This result, based on astrometric observations of a star orbiting the black hole and approaching it to within a distance of only 17 light-hours, would not have been possible without images of diffraction limited resolution.

Eta Carinae
Eta Carinae is one of the heaviest stars known, with a mass that probably exceeds 100 solar masses. It is about 4 million times brighter than the Sun, making it one of the most luminous stars known.

Such a massive star has a comparatively short lifetime of about 1 million years only and – measured in the cosmic timescale- Eta Carinae must have formed quite recently. This star is highly unstable and prone to violent outbursts. They are caused by the very high radiation pressure at the star’s upper layers, which blows significant portions of the matter at the “surface” into space during violent eruptions that may last several years. The last of these outbursts occurred between 1835 and 1855 and peaked in 1843. Despite its comparaticely large distance – some 7,500 to 10,000 light-years – Eta Carinae briefly became the second brightest star in the sky at that time (with an apparent magnitude -1), only surpassed by Sirius.

Frosty Leo
Frosty Leo is a magnitude 11 (post-AGB) star surrounded by an envelope of gas, dust, and large amounts of ice (hence the name). The associated nebula is of “butterfly” shape (bipolar morphology) and it is one of the best known examples of the brief transitional phase between two late evolutionary stages, asymptotic giant branch (AGB) and the subsequent planetary nebulae (PNe).

For a three-solar-mass object like this one, this phase is believed to last only a few thousand years, the wink of an eye in the life of the star. Hence, objects like this one are very rare and Frosty Leo is one of the nearest and brightest among them.

Original Source: ESO News Release

Image credit: USNO

Astronomers from several US observatories announced that they have successfully merged the light from six independent telescopes to form a single, high-resolution image of a distant multi-star system. To create an image with this level of detail, a single telescope would need to be 50-metres across – bigger than anything that currently exists. This technique, called interferometry, has been done with pairs of telescopes before, but never with as many as six.

Astronomers from the U.S. Naval Observatory (USNO), the Naval Research Laboratory (NRL), and Lowell Observatory announced today that they have successfully combined the light from six independent telescopes to form a single, high-resolution image of a distant multiple-star system. This is the first time that this has ever been accomplished in the optical region of the electromagnetic spectrum. The Navy Prototype Optical Interferometer (NPOI) at Lowell Observatory’s Anderson Mesa site near Flagstaff, Arizona observed the triple star system Eta Virginis, located about 130 light-years away from Earth.

“This development makes it possible to ‘synthesize’ telescopes with apertures in excess of hundreds of meters,” says Dr. Kenneth Johnston, Scientific Director of the Naval Observatory. “It will lead to the direct imaging of the surfaces of stars and of star spots, analogous to the sunspots on the Sun. This technology can also be applied to space systems for remote sensing of the Earth and other objects in the solar system, as well as stars and galaxies.”

Optical interferometers combine the light from several independent telescopes to form a “synthetic” telescope whose ability to make a high-resolution image is proportional to the maximum separation of the telescopes. They are the answer to the prohibitive costs and immense technical difficulties of building extremely large, monolithic single-mirror telescopes. Since the rate at which a giant telescope aperture is synthesized with an interferometer array is equal to the number of combinations between any two telescopes of the array, the combination of the six NPOI telescopes has more than quadrupled NPOI’s capability to collect data over its competitors.

USNO and NRL, in collaboration with Lowell Observatory and with funding from the Office of Naval Research and the Oceanographer of the Navy, joined forces in 1991 to build the instrument. Stellar observations have been conducted with a three-station array since its “first light” in 1996.

However, due to the technical difficulty associated with linking even a small number of separate telescopes, the high-resolution capabilities of optical interferometers have only been used to date on relatively simple stellar sources. Basic questions, such as a star’s apparent diameter or the existence and motions of nearby stellar companions, are easily answered for such sources. However, to increase the spatial resolution and sensitivity to stellar structure, interferometers must link more telescopes together to provide an even sampling of the synthesized aperture. Three combined telescopes provide three mea-surements in the synthesized aperture, but six telescopes provide 15 combinations.

To merge the six beams, the NPOI team has designed a new type of hybrid beam combiner. In addition, new hardware and control systems have been developed to uniquely encode every possible telescope combination in the recorded data so that the information necessary for the alignment and superposition of the starlight wave-fronts and the image reconstruction may be properly decoded.

The field of interferometry is a rapidly developing one, with giants like the twin Keck 10-meter telescopes having achieved “first fringes” last year, and the European Southern Observatory’s VLTI planning to combine the light from four 8-meter telescopes. More modest but versatile imaging interferometers like CHARA, COAST, and IOTA have also been operating for a few years, but NPOI is the first to combine light from a full array of six telescopes.

In the near future, NPOI will be commissioning all of the remaining stations onto which any of the six telescopes can be mounted for a maximum array size of 430 meters, the largest baseline of all current imaging interferometer projects.

Stellar astrophysics will be revolutionized by the capability to directly image stars other than the Sun. Ultimately, when employed in space with the experience collected from ground-based experiments, optical interferometry may develop the capability to image Jupiter-sized planets orbiting distant stars.

“Remember the early days of radio interferometry and look at the world- wide arrays we routinely use today,” says Dr. Johnston. “We’ve gone from simple two-element arrays to continent-sized ones with 10 or more antennas that produce extremely fine-scale images of distant quasars. We are standing on the brink of achieving similar results for visible-light sources.”

Original Source: US Naval Observatory News Release