Active & Adaptive Optics

AS4100 Astrofisika Pengamatan Prodi Astronomi 2007/2008 B. Dermawan

Active & Adaptive Optics

Bely

Majewski

Active & Adaptive Optics

• Active optics

is typically the term used for the removal of global, low-order Zernike polynomial effects at low frequency (< few Hertz) For example, image wander effects from wind buffeting or optical misalignment or figure distortion from thermal or gravitational loading. Also, low order, tip-tilt from seeing Typically through single tip-tilt tertiary or wobbling secondary mirror Also, piston motion for secondary (e.g., corrects defocus) Can also control the actuators on the primary mirror

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Active & Adaptive Optics

• Wavefront errors measured using a bright star off-axis by several arcmin, so not to interfere with science target • Compensated by moving secondary and the primary actuators • The actuators typically fairly broadly spaced (~

D

/12) Bely • Reference star typically not in isoplanatic patch, in which case need to average over seeing effects by relatively long exposures (~seconds). Allows fairly faint stars to be used

Majewski

Active & Adaptive Optics

• Adaptive optics

is typically the term used for the correction of high frequency (few to 1000 Hz) wavefront disturbances by atmospheric turbulence Feedback removal of wavefront distortions by countering them with moveable/shapeable optics Use of deformable mirrors to counteract corregations in wavefront coupled in feedback loop with detailed/complex wavefront monitoring Bely

Majewski

History

Active & Adaptive Optics

• Horace Babcock (Carnegie Observatories) first suggested the idea of adaptive optics • In remarkably prescient paper (1953) he: describes the overall concept; suggests a way of measuring the atmospheric waverfront distortions; proposes a concept for an adaptive mirror (using an oil film for which the thickness was controlled by electrical charges); discusses the small relative size of the isoplanatic patch (few arcsec); discusses the need for high time resolution and consequent limitation of wavefront sensing to bright (

V

< 6.3) stars • Unfortunately, the technology was just not ready (for several decades) to implement these ideas

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Active & Adaptive Optics

History • Following Babcock, adaptive optics was pursued in parallel (but independently) by astronomers (starting early to mid-1970s) and U.S. Department of Defense (latter starting in 1973) • Imaging of and from satellites through atmosphere gives DOD same problem as astronomers • Budgets for astronomy MUCH less than for DOD, and this is VERY expensive technology. So DOD got there much faster (by 1977 had succeeded with). Astronomers did not get there until late 1980s (and in some cases with some borrowed technology from DOD) • First experiments were a bit discouraging: Showed how expensive this would be. Application to areas only around bright reference stars seemed rather limiting • But a resurgence in interest due to several factors: Idea of synthetic, laser reference stars proposed by military in 1982, prototypes in 1984, declassified in 1991

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History

Active & Adaptive Optics

• Laser guide stars independently proposed by Labeyrie and Foy in 1985 • Better detectors (CCDs), increasing sensitivity • Realization that many complexities and limitations of AO reduced or disappear in the infrared. Recall that

r

0  Means that number of adaptive elements decreases. Means that temporal control frequencies decrease. Array detectors in IR developed. Diffraction limit approachable at NIR wavelengths (Strehl ratios approaching 1) • Better seeing sites and reduction of local seeing decrease complexities even more systems  6/5 • Now many observatories implementing adaptive optics .

Majewski

Active & Adaptive Optics

http://138.238.143.191/astronomy/Chaisson/ AT405/HTML/AT40503.htm

Majewski

Wavefront Corrector • Tip-Tilt Mirrors:

Up to the fifth Zernike polynomial can be corrected out with only tip/tilt and piston motion of, e.g., secondary mirror Many secondaries on infrared telescopes already move for chopping observations Fine (or fast) steering mirrors are smaller, easier to control (smaller inertia) optics at the telescope output

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Wavefront Corrector • Deforming primary:

Beyond tip-tilt correction requires deforming an optical element Deforming the primary involves either applying forces or moments with actuators The more actuators, the more "correctable" the wavefront Bely

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Wavefront Corrector • Dedicated, deformable, phase correcting mirror:

For compensation of atmospheric turbulence, requires much faster response than can get from primary mirror actuators Deformable mirrors made of this plates actuated by piezo-electric mechanisms Two main types: Piston motion actuators by piezoelectric stacks, Bimorph mirrors which are made from a pair of piezoelectric wafers glued together, with localized control of each. Local bending by stretching one wafer and contracting other at same point Bely

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Wavefront Corrector

On MMT and LBT, the secondaries can be deformable (MMT a 24 inch secondary with 336 actuators, LBT a 36 inch secondary with 672 actuators)

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Wavefront Sensors

• The Shack-Hartmann sensor: an evolved version of Hartmann test • Telescope entrance pupil re-imaged onto a lenslet array (instead of Hartmann mask) • Each lenslet creates an image of the star http://www.cvs.rochester.edu/williamslab/ r_shackhartmann.html • The position of the centroid of each lenslet image yields the slope of the wavefront tilt at the pupil position represented by the lenslet • Wavefront tilts are generally achromatic, so can use photons of a broad wavelength range to improve

S

/

N

(often use optical sensing to correct even NIR images)

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Wavefront Sensors

Lenslet focus light down to a CCD array; Each square cluster of four CCD elements forms a detector; When the incident wave is planar, Airy image spots form at null points at the centers of each four-element detector; When the wavefront is distorted, Airy-image spots are shifted from the null positions Hecht http://www.cvs.rochester.edu/williamslab/r _shackhartmann.html

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Isoplanatic Angle



To correct a field of view on the sky, need to have a reference star in the same isoplanatic angle

A good approximation of the isoplanatic angle is  0 ~

k r

0 /

H

, where

r

0 is the Fried parameter,

H

is the height of the turbulent seeing layer, and

k

is a constant The value of

k

was 0.6 (but this depends on how one defines the level of isoplanaticity; Beckers adopts 0.3 for this constant) The temporal timescale on which changes need to be made in that isoplanatic angle are given approximately by  0

~ k r

0 /

V

wind , where

V

wind Recall that

r

0 is the wind velocity in the turbulent layer has a  6/5 dependence, and so is larger at longer wavelengths

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Isoplanatic Angle

Using these equations (and adopting the more strict

k

= 0.3 and

H

= 5 km), one finds the following variation in Fried parameter (

r

0 ), temporal timescale (  0 ) and isoplanatic angle  0 with wavelength Beckers 1993 Note how small the isoplanatic angle and temporal timescale is in the optical and how the idea of adaptive optics really is best applied at infrared wavelengths

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Natural Guide Stars



To undertake adaptive optics requires a reference star in the isoplanatic angle

The above table shows how quickly one needs to read out a detector in order to sample the changing wavefront (  det ) a few times per change This readout rate for a detector translates directly to a stellar magnitude limit, given a detector with a given readout noise and quantum efficiency and a desired

S

/

N

(as shown in table footnote) As may be seen, the brightness of a star that can be used for Shack-Hartmann sensing and adaptive optics becomes increasingly bright at shorter wavelengths The last column in the table above shows what fraction of the sky has a star available at this magnitude within an isoplanatic angle, and therefore, in principle, can be corrected with adaptive optics Obviously this is rather limiting for high resolution imaging of interesting sources

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Artificial Guide Stars

• The solution to the limiting sky coverage and improved correctability (i.e. brighter reference sources) is to create an artificial guide star wherever it is needed • The current method to do this is using laser guide stars • Two methods are used: Rayleigh scattering involves focusing a powerful laser to a point 10 20 km up, above most, but not all, turbulence. Only backscattered photons from the focused height contribute to the wavefront estimate Probably more common is to make a sodium laser star: Use the NaD line at 5890 Angstroms. There is an ~11.5 km thick, enhanced neutral sodium layer in the mesosphere at about 90 km up, well above the 10 km "jet stream" layer and most turbulence. The 90 km enhanced sodium layer likely comes from meteoritic dust. Optical thickness of the layer is about 0.05 at sodium line center

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Artificial Guide Stars

Shoot NaD laser at this layer. Those Na atoms excited by laser reemit by spontaneous emission or by stimulated emission. Those emitted back to telescope can be used for wavefront sensing Beckers 1993 A sodium dye laser beam pierces the sky over Lick Observatory on July 22, 2003. The laser is the final piece of the laser guide star adaptive optics system that allows twinkle-free viewing of the entire nighttime sky. The beam, which reaches 60 miles into the upper atmosphere, is visible in scattered light for several kilometers http://currents.ucsc.edu/ 03-04/03-01/images.html

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LGS – NGS

Comparison of laser vs. natural guide star corrected images of the Galactic center (in L band at 3.8 microns) taken with the Keck telescope. The LGS image is 8 mins long, the NGS image is 150 minutes long www.astro.ucla.edu/ ~jlu/gc/pictures/lgs.shtml

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Performance

An animation showing a set of images of a star observed through turbulent atmosphere without any correction, with only tip-tilt correction with a fast steering mirror, and

closed loop

adaptive optics with a deformable mirror www.mpia-hd.mpg.de/ AO/INTRO/AOWFSintro.html

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http://outreach.atnf.csiro.au/ education/senior/astrophysics /adaptive_optics.html

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Multi-Conjugate Adaptive Optics (MCAO)

Proposed by Beckers in 1988, uses both multiple wavefront sensors and multiple deformable mirrors The complex beam path of a proposed AO instrument for Gemini telescopes as of 1999, with three deformable mirrors to conjugate at 8, 4 and 0 km Gemini Newsletter, Issue #19, December, 1999

Majewski

Multi-Conjugate Adaptive Optics (MCAO)

Improving the inversion of the turbulence profile by sensing each layer and conjugating them each individually Equivalent to atmospheric tomography to get 3-D structure of turbulence layer For example, with two guide stars and sensors, MCAO like spectroscopy: Two wavefront sensors look at two different guide stars separated by an angle  ; The two beams have increasing shear,  h , with height, h A phase feature (marked by the "plus" feature) at height h 0 h 0 seen by the two sensors will be seen with a spatial shift  h 0 . With  known, can be derived for that phase feature Once column distribution of turbulence determined, can use multiple deformable mirrors to correct Gemini Newsletter, Issue #19, December, 1999

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Multi-Conjugate Adaptive Optics (MCAO)

MCAO removes cone effect given the multiple beams (as shown in the cone effect of the previous figure) Net effect is substantially larger corrected field of view (even 1-2 arcmin) with uniform PSF Gemini Newsletter, Issue #19, December, 1999