Adaptive Optics: An Introduction and Overview

Transcription

1 Adaptive Optics: An Introduction and Overview Mike Hein PH464 Applied Optics Dr. Andres LaRosa Portland State University Winter 2005

2 Abstract: This paper presents a look at the technology and techniques of correcting optical aberrations using adaptive optical components. Introduction: The twinkling of the stars has provided inspiration to poets, musicians and artists of all sorts. However, to the astronomer that same twinkling has been a hindrance to their work and research. In 1730, Isaac Newton advised: Long telescopes may cause objects to appear brighter and larger than short ones can do, but they cannot be so formed as to take away that confusion of the rays which arises from the tremors of the atmosphere. The only remedy is a most serene and quiet air, such as may perhaps be found on the tops of the highest mountains above the grosser clouds. Newton s words have guided astronomers to the highest and most remote peaks in the world to seek better seeing conditions for their telescopes and although this has resulted in improved observations, the problem still exists. In fact, the very large telescopes typically used in observatories make the aberrations more noticeable. The primary goal of these large telescopes has been to seek out and observe fainter astronomical objects, e.g., nebulae and other galaxies. The same atmospheric effects that cause the stars to twinkle cause these fainter objects to appear as indistinct blurs against the dark background of the sky. In 1953, Horace W. Babcock, an astronomer at the Mount Wilson Observatory on California, proposed a system to compensate for the effects of atmospheric turbulence using a deformable optical element, but the technology to make his idea a reality didn t exist at the time and would not for another twenty years. In the 1970 s the Defense Advanced Research Projects Agency (DARPA) engaged the Itek Company to create a system of adaptive optics to allow the U.S. military to shoot down satellites. The project required an optical system to overcome the turbulent effects of the atmosphere to allow visual identification of the potential targets and a method to reduce the spread of a LASER beam in the atmosphere to ensure the delivery of the greatest possible energy on the target. Much of this project is still secret; however, 1

3 the basic technologies developed are being used in observatories to clear up telescopic images. Figure 1: Perfect image versus distorted image due to atmospheric turbulence (Image from Scientific American, June 1994) Content: Because even the nearest stars (excluding the Sun) are so far from the Earth, we are safe in thinking of them as point sources rather than extended objects. If we treat the light emitted by a star over time as a series of expanding concentric spheres, then 2

4 because of the tremendous distances involved, when that light reaches us on earth it is essentially a series of plane waves. As these plane waves interact with the atmosphere they become distorted by swirling currents in the air. These currents are called eddies and the effect they produce is directly related to their size. Eddies in the atmosphere can be imagined as columns of air extending upward from the ground and in constant, essentially random motion. The effects of atmospheric turbulence fall in to three categories: scintillation, beam wander and spreading. Scintillation is caused by light passing through eddy currents whose diameter is proportional to the square root of the product of the wavelength of light and the length of the path through the atmosphere. The effect of scintillation is to randomly change the intensity of the light. Beam wander results from eddies with a diameter larger than the aperture of the telescope and, as the descriptive name implies, these cause the image to move around in the field of view. Beam spreading is a smearing out of the image and is caused when the light passes through eddies less than the beam size. The combined effect of the turbulence is illustrated in Figure 1. Given the effects and the unpredictable nature of the atmosphere one might wonder if a method of correction is even possible. The answer, of course, is adaptive optics. All adaptive optic systems must perform two general tasks: wavefront sensing and wavefront correction. Wavefront sensing is the process of rapidly analyzing the shape of the incoming wave to determine the effect the atmosphere has imposed upon it. In astronomical AO systems, this is normally done on the order of one-thousand times per second (1 khz). The incoming wave is analyzed by a wavefront sensor, most commonly of the Shack-Hartmann type. Figure 2: Shack-Hartmann Wavefront Sensor 3

5 Figure 2 shows a schematic diagram for a Shack-Hartmann sensor. The sensor consists of an array of small spherical lenses and a charge-coupled device (CCD). The lenses (lenslets in the diagram) focus the incoming wave onto the CCD. If the wave is planar, the images formed by the lenses will hit the null spots between the CCD detectors and no signal is sent to the corrective optics. However, if the wavefront is distorted when it arrives at the detector the lenses will focus their image spots onto the CCD sensors. The output signals from the CCD are then analyzed by computer and the appropriate signals are sent to the corrective optics. The corrective optics system consists of a deformable mirror and a high-speed tiptilt mirror. When the signal from the wavefront sensor is analyzed it may be found that there exists an overall tilt across the entire wavefront, if so it is countered by the tip-tilt mirror. This is necessary because the amount of correction offered by the deformable mirror is limited to a few thousandths of a millimeter (a few microns). Deformable mirrors are of two basic types: monolithic and segmented. A monolithic mirror has a single solid reflective surface made of a single silicon crystal that is polished and aluminized and bonded to a network of actuators. The actuators may be piezoelectric, mechanical or a combination of the two, and are arranged in a regular array with a spacing of about seven millimeters between adjacent actuators being typical. The segmented mirror is made up of several smaller mirrors, each mounted on a single actuator. The segments can be square, triangular or hexagonal and for a particular mirror will all be of the same shape and size. The segments are closely fitted to leave as little gap between them as possible. Figure 3: From left to right, nineteen segment mirror, monolithic mirror actuator array and monolithic mirror surface. 4

6 The segmented mirror is capable of a wider range of correction than the monolithic type because the travel of the segments is not limited by strain on the mirror surface as with a monolithic mirror. The major drawbacks of segmented mirrors are that they require frequent adjustment and calibration to keep the segments correctly aligned and the gaps between the segments induce diffraction of the image. It is for these reasons that the monolithic type is more frequently used. Figure 3a: A complete AO system 5

7 Although the system we ve built up so far is a good one, it does have some limitations: to reliably correct for atmospheric effects requires a relatively bright star to use as a reference or guide star. In visible wavelengths ( nm) the guide star has to have an apparent magnitude a measure of the relative brightness of an object as seen from Earth - of 10 or brighter. The area around the guide star which can be reliably corrected is called the isoplanatic patch and is measured by the isoplanatic angle, for visible light the isoplanatic angle is about two arcseconds. The result is that in visible light, the AO system is capable of correcting less than one part in onehundred thousand of the sky. This figure is improved significantly by observing in infrared light; at 2200nm, the magnitude of the required guide star is 14, which expands the total portion of the sky that can be reliably corrected to one part in onethousand. Despite the improvement in sky coverage, the figure is still too small to be very useful. Fortunately, there exists a method of improving things even further: the artificial guide star. An artificial guide star is created using a laser to exploit certain characteristics in the atmosphere in producing a bright spot in the sky to use as a reference. The first method is called a Rayleigh beacon and uses a fairly powerful (~100 Watt) ultraviolet laser to create an artificial guide star about 2 arc seconds across at an altitude of 10-15km. The guide star is a product of Rayleigh scattering of the laser in the atmosphere. Because the altitude of the artificial guide star is fairly low, the incoming wavefronts can t be treated as planar over a large aperture. This imposes a limit on the use of 6

8 Rayleigh beacons to small (less than 2m) aperture telescopes. The second method uses a 10 watt laser at 589nm to excite a layer of sodium vapor in the Earth s atmosphere. This sodium layer exists at about 90km so that the wavefronts from the artificial guide star are more nearly planar over large apertures. These artificial guide stars don t negate the need for natural guide stars, but they do reduce the limiting magnitude to about 20. This combination allows nearly 100% sky coverage at infrared wavelengths. These two images are from the Lick observatory and show the effectiveness of an adaptive optics system in use. The images show the same star in infrared, the one on the left is the uncorrected image and the one on the right is with adaptive optics (this frame is taken from a movie file available at: Max/speckle8ao.mpg). Conclusions: As an amateur astronomer, I have spent many nights with telescopes, binoculars and just my eyes looking at the night sky, always wishing I could see more detail in the objects I observed. Although adaptive optics aren t practical for the typical backyard astronomer, they are allowing professional astronomers to gather more detailed and useful information about the universe using ground based telescopes. However, the uses of this technology are not limited to astronomy. Adaptive optics are currently being used in conjunction with other methods of imaging to improve the ability of vision specialists to study the retina and gain insights into diseases of the eye in people. By 7

9 correcting for distortions induced by the cornea and the lens of the eye, these doctors are able to image the living human retina in unprecedented detail. Even if these are the only uses this technology ever finds, the development will have been completely worthwhile. References: Carroll, Joseph, Daniel C. Gray, Austin Roorda, David R. Williams, Recent Advances in Retinal Imaging with Adaptive Optics, Optics and Photonics News, Jan 2005: p Chaisson, Eric and Steve McMillan, Astronomy Today 4th Edition, Prentice Hall, Upper Saddle River, NJ, 2002 Florence, Ronald, The Perfect Machine, Harper Collins, New York, NY, 1994 Hardy, John W., Adaptive Optics for Astronomical Telescopes, Oxford University Press, New York, NY, 1998 Hardy, John W., Adaptive Optics, Scientific American, June 1994: p Hecht, Eugene, Optics 4th Edition, Addison-Wesley, Reading, MA, 2002 Tyson, Robert K., Principles of Adaptive Optics 2nd Edition, Academic Press, San Diego, CA, 1998 AdaptiveOptics.com, 2002, Adaptive Optics Associates Inc., Center for Adaptive Optics, 2005, University of California Santa Cruz, 8