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When Is Lucky Imaging Better Than Adaptive Optics Techniques?There is no doubt that Adaptive Optics techniques have been successful in certain areas. When the reference star is bright enough it has been possible to achieve very high Strehl ratios, and Adaptive Optics has been used extensively in the near infrared were we have not yet tried to apply Lucky Imaging techniques because of the lack of suitable detector systems. However there are a number of circumstances where the achievements of Lucky Imaging have exceeded those of Adaptive Optics. In order to understand the circumstances under which Lucky imaging excels we need to look at the way that Adaptive Optic systems generally work. Adaptive Optics works by breaking up the telescope aperture into cells of size of the order of r0 and detecting the reference star in each cell. This is most often done with the Shack Hartmann sensor:
The images produced show an array of image is of a star, one from each of the lenslets in the Shack Hartmann sensor.
The relative motions and positions of the star within each cell are then used to work out what the phase errors in the wavefront are at any instant and a computer controlled flexible mirror is distorted to compensate for these phase errors. A schematic of such a system is shown below, where the blue light from the star is used for the wavefront sensor to give an image like those shown above, and the deduced wavefront errors drive a wavefront corrector (here a flexible mirror) to remove the errors in the input wavefront, and therefore pass a corrected (and ideally diffraction limited) wavefront on to the science instrument.
If the reference star is very bright then it may be possible to work out what the phase errors are and to correct them before they change (and remember that they are changing very rapidly on timescales of the order of milliseconds). The reference star has to be very bright anyway because it must be detected with good signal-to-noise in each of the cells of the sensor rather than over the whole aperture of the telescope as is the case with the Lucky Imaging technique. Typically perhaps 20 cells in the sensor would be used with a 2.5 metre telescope. In practice it means that there is a very small probability that a reference star will be found close enough to the object of scientific interest for adaptive optics to be usable whereas with Lucky Imaging we are able to work with very much fainter reference stars. We find that we therefore have a much higher probability of finding a reference star within our field of view. For more information on reference star magnitudes and availability click here. Isoplanatic Patch SizeThe other problem which greatly affects the application of Adaptive Optics is the limited isoplanatic patch. There are a few cases in astronomy when we are happy simply to resolve two objects. We may wish to look at a very close pair of stars so that we can separate the components and look at their relative motions. However virtually all astronomy depends on comparing the brightness of the object under study with others in the field so that we can measure positions and brightnesses with useful accuracy. The problem with adaptive optics is that the shape of the star images changes very rapidly with the distance of an object from the reference star. This arises because adaptive optics tries to compensate for the phase fluctuations in the atmosphere at every instant, including when they are particularly bad. The poorer these conditions are the more rapidly the image shape changes with distance from the reference star. With Lucky Imaging we discard images formed when the phase fluctuations are bad and only use those which are least affected. This gives us star image profiles that vary much more slowly across the image. Not only does this mean that we get images that are much easier to work with for astronomers but we are also able to find reference stars over a much larger area of sky than is possible with adaptive optics. This larger area to search for reference stars means that we have a much higher probability of finding one. The mean size of the isoplanatic patch measured at Paranal, the site of the European Southern Observatory VLT, is only about 2.6 arc seconds in V band (equivalent to about 4.5 arcsec in I-band at 850nm) whereas our measurements given isoplanatic patch approaching one arc minute in diameter. For more information on why Lucky imaging gives an isoplanatic patch so much larger than does Adaptive Optics click here. Atmospheric Turbulence Model ProblemsOne final problem which is only becoming clear now that Adaptive Optics
systems are being commissioned and found to be less good than expected is due to
the fact that although atmospheric turbulence has a power spectrum very similar
to that predicted by models based on Kolmogorov turbulence theory, the
turbulence actually found in practice is significantly different in a way that
makes the construction of Adaptive Optic systems very much harder. For more
information on the complexities of atmospheric turbulence
click here. |
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Institute of Astronomy & Cavendish Laboratory, University of Cambridge,
Madingley Road, Cambridge, UK |
(Image from Gordon Love, Durham)