AO (Adaptive Optics)

From the outset, adaptive optics (AO) was incorporated into the design of the LBT. Adaptive optics correct for the distortion in the light waves caused by the turbulence in our atmosphere. Similar to air rising from the ground on a hot day causing a heat mirage, hot and cooler air mixing above a telescope blurs the resulting images. Adaptive optics (AO) systems can compensate for much of the degrading effects of the atmosphere. The AO system on the LBT is a function of the two adaptive secondary mirrors (ASM), each 91 cm (36 inches) in diameter and 1.6mm (0.063 inches) thick, with 672 computer-controlled actuators, changing the shape of the mirror over 1000 times each second to overcome the atmospheric turbulence that would otherwise blur the image.

A picture of the movable secondary mirror during its installation in the Arcetri lab. The image shows the 672 tiny magnets spread over the back of the mirror. The reflecting face of the mirror is face down. The upper portion contains the electro-mechanical devices that control the magnets. (Photo by R. Cerisola)

www.lbto.org/AO/AOpressrelease.htm

 

Click here to read: Calibration of Force Actuators on an Adaptive Secondary Prototype by D. Ricci, A. Riccardi and D. Zanotti

 


Side view of the Adaptive Optics system during lab testing. The deformable reflecting surface of the 91-cm concave mirror is visible, along with some of the 672 circuit cards that control the magnetic forces shaping the surface

 

The first Adaptive Secondary Mirror saw first light in May, 2010 using natural guide stars, and provided astronomers with a new level of image sharpness never before seen. Until relatively recently, ground-based telescopes had to live with wavefront distortion caused by the Earth's atmosphere that significantly blurred the 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.

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 meter mirrors. When 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. 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 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. These results were achieved using only one of LBT’s mirrors and it is expected that the images will have even greater clarity when both mirrors are used with adaptive optics. When 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.

Watch an Adaptive Optics Video from August 2010

Watch some more Adaptive Optics Clips:

www.lbto.org/AO/RealTime.mpg

www.lbto.org/AO/Transition.mpg

 

Development of the LBT’s adaptive optics system took more than a decade through an international collaboration. A prototype system was previously installed on the Multiple Mirror Telescope (MMT) at Mt. Hopkins, Ariz. 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 produced the accompanying images. 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 meters in diameter (3 feet) and only 1.6 millimeters (0.063 inches) 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 nanometers (a nanometer is one millionth the size of a millimeter).



Strehl ratios of 85% with magnitude 8 stars (0.8 arcsecond seeing) were demonstrated using a pattern of Airy diffraction rings with a logarithmic stretch (the best performance of other systems is 50-60%). Strehl ratios measure the degree to which an optical system approaches “diffraction-limited” perfection, or the theoretical performance limit, of the telescope. It is the simplest meaningful way of expressing the effect of atmospheric distortions on image quality.  Adaptive Optics Secondary Mirror #2 is currently undergoing lab commissioning. Adaptive Optics science is scheduled to begin in late 2011, with the commissioning of the LUCI 2 instrument.

Adaptive Optics at the World's Biggest Optical Telescope by M. Hart, S. Esposito, S. Rabien (Article Not Available)

 

A double star as observed with the LBT in standard mode (left), and with the adaptive correction activated (right). Because of atmospheric blurring, the fainter companion of the star cannot be identified in the images taken in standard mode, while it is easily visible when the adaptive module is activated. A third faint star also becomes visible in the upper right part of the frame, thanks to the increased sensitivity of the telescope in adaptive mode.



Left Image: Imaging Cluster NGC 6341 (M92) Hubble Space Telescpe H Band 20 minute exposure
Right Image: M92 at 1.6μm as observed with the Hubble Space Telescope (left) and the LBT in adaptive mode (right). It is immediately clear that the resolution and depth acheived with LBT surpasses even those of the that particular Hubble image.

 

However, using a natural (real) star for adaptive optics correction of atmospheric distortion of the telescope's images has it's limits as such guide stars must be very bright and very near the telescope's target, and there are few such stars available. Also, this system limits the best optical correction to a single point on the sky and correction quality degrades away from that location. Half to two-thirds of the total atmospheric turbulence is found within the first kilometer above a telescope, which is common to every point on the sky. Correcting for only this ground-layer turbulence will therefore lead to a dramatic improvement in imaging over the full field of view facilitated by the guide sources. This can be done by using multiple stellar or laser sources that are always pointed in the same direction as the telescope, thus enabling many types of wide-field AO correction.

Thus, following the pioneering work done at the 6.5 m Multiple Mirror Telescope on Mt Hopkins, the LBT is currently developing adaptive optics based on using powerful laser light to stimulate a layer of sodium atoms which are about 45 miles above the earth. This system is called ground-layer adaptive optics (GLAO)

Click here to read: Adaptive Optics Using Multiple Laser Guide Stars by C. Baranec

The multiple laser-guide-star ground-layer adaptive-optics (GLAO) system in operation at the 6.5m MMT (formerly the Multiple-Mirror Telescope) in southern Arizona. (Images courtesy Thomas Stalcup.)

 

The light reflected back to the telescope from these sodium atoms will be distorted by our atmosphere's turbulence, but the telescope's adaptive optics will be able to correct for this turbulence because it knows how the undistorted sodium atoms' light should appear.

Comparison of stellar images in the near-IR without (left) and with (right) ground-layer AO (GLAO) correction. With GLAO, the image width is reduced from 0.70 to 0.33 arcseconds and the peak intensity is increased by a factor of 2.3. Each grid point represents 0.107 arcseconds on the sky. From: Adaptive optics using multiple laser guide stars Christoph Baranec 2 April 2009, SPIE Newsroom. DOI: 10.1117/2.1200903.1570


Another more advanced adaptive optics system also being developed currently is called ARGOS (Advanced Rayleigh Ground-layer Adaptive Optics System), an innovative system whose primary goal is to correct the atmospheric turbulence induced distortions and thus enhancing the imaging and spectroscopic capabilities for LUCI 1/LUCI 2 over a wide field of view.

The parts of ARGOS are colored

http://www.mpe.mpg.de/ir/projects/LBTLaser/index.php

 

Click here to read the AO Press Release: Large Binocular Telescope Achieves Major Breakthrough Using Adaptive Optics

Click here to read more about ARGOS