Optics & Photonics News - November 2009

When we mounted our Lucky camera behind the low-order adaptive optics
system on the Palomar 5-m telescope, we generated the highest-resolution
images ever obtained in the visible on any telescope.

on Earth. For example, with the VLT 8-m telescopes, we would expect to reach close to 20 mas resolution and about 16 mas resolution on the Keck 10-m telescopes. In addition, we have been try- ing to understand what limits the isoplanatic patch size to around one arcminute in diameter. The patch size is defined as the radius at which the the brightness of the core of the image is reduced by a factor of 1/e. We see that the core of the image progres- sively becomes attenuated as we measure objects further from the reference star. The cores of stars further from the reference star become elongated in a direction toward the reference star, and that is what is responsible for the reduction in apparent sharpness.

The elongation is, in turn, caused by the fact that the tip-tilt errors are not constant across a wide field but are slightly different for various areas of the image. As we saw before, the tip-tilt error changes relatively slowly, so we have the opportunity (at least in principle) to look at individual frames and measure the positions of objects in those frames.

We then construct a rubber sheet distortion pattern for each image, so that each may be transformed onto a proper rectangular grid before it is combined into the summed images. We can use even fainter objects in the field to calibrate these distortions, since they are small and slowly varying and in this way substantially increase the isoplanatic patch size.

Our simulations suggest that we should expect an isoplanatic patch size as large as 3-5 arcminutes in diameter. We have made very recent observations using an array of four electron-multiplying CCDs configured optically to give a contiguous image size of about 4,000 3 1,000 pixels. Early results suggest our model predictions are correct.

European Southern Observatory

Electron-multiplying CCDs are at the heart of Lucky Cam technologies. Noiseless internal amplification allows for the readout noise to be effectively reduced. The Lucky Imaging camera typically runs with a gain of about 2,000, giving a signal-to-noise on an individual photon of about 20: 1. Recent developments of this technology—funded by the European Southern Observatory— have produced a device of 240 3 240 pixels. With eight electron-multiplying outputs working in parallel, the device can operate at up to 1,500 frames per second in photon counting mode.

The same techniques may be used just as easily in the near-infrared. At longer wavelengths, both r0 and t0 are larger so that these methods become progressively easier to implement. The readout rates required in the infrared must be reasonably fast (the wind speed does not change), but photon counting is much less important since photon flux rates are generally much higher in the infrared.

The application of these methods as a way of feeding high-resolution visible spectrographs should be relatively straightforward. Many modern spectrographs now use integral field units. These consist of an array of micro-lenses in the telescope image plane. The light striking each micro-lens is fed into a fiber and the bundle of fibers taken to the spectrograph. The output of the many fibers are placed in a line to look like the slit of a conventional spectrograph. Combining these with a similar fiber bundle at a nearby reference star, close enough so

that the phase distortions are also well
corrected, provides the guidance as to
the alignment of the target object on the
integral field unit array of lenses. It also
indicates which moments provide the
highest resolution.
Many of these techniques have
now been demonstrated success-
fully. By accepting the loss inevi-
table when a significant fraction
of the images are rejected, scientists can
achieve much higher resolutions than
are possible by any other means. All the
techniques required to build high-resolu-
tion systems are now in place. The recent
success of Lucky Imaging has been
largely due to the introduction of new
technologies, particularly electron mul-
tiplying CCDs, which allow fast-frame-
rate cameras to detect photons with
excellent signal-to-noise. Astronomers
hope that they will be available for gen-
eral use before too long. Undoubtedly,
Lucky Imaging will allow astronomers
to study yet fainter and more compact
objects both near and far. It offers the
exciting promise of a further leap in our
understanding of the universe. t

Craig Mackay ( cdm@ast.cam.ac.uk) is with the Institute of Astronomy, University of Cambridge, United Kingdom.

[ References and Resources ]

>> D.L. Fried. “Probability of getting a lucky short-exposure image through turbulence,” J. Opt. Soc. Am. A 68, 1651 (1978).

>> O. Guyon et al. “Improving the Sensitivity of Astronomical Curvature Wavefront Sensor Using Dual Stroke Curvature,” Publ. Astron. Soc. Pac. 120, 655 (2008).