Lucky Imaging is a remarkably effective technique for delivering near-diffraction-limited imaging on ground-based telescopes. The basic principle is that the atmospheric turbulence that normally limits the resolution of ground-based observations is a statistical process. If images are taken fast enough to freeze the motion caused by the turbulence we find that a significant number of frames are very sharp indeed where the statistical fluctuations are minimal. By combining these sharp images we can produce a much better one than is normally possible from the ground. We have routinely taken Hubble resolution images (0.15 arcsec resolution) on the Hubble sized telescope is (~2.5 m). More recently we have used the same techniques behind a low order adaptive optics system in order to give even higher resolution on telescopes that are too big to have a significant chance of conventional lucky imaging without adaptive optics assistance.

Lucky imaging is not a new idea. The name "Lucky Imaging" came from Fried (1978) though the first calculations of the Lucky Imaging probabilities were first carried out by Hufnagel in 1966 (see reference pages (click here) for copies of the Hufnagel papers that are otherwise difficult to find) and these principles have been used really quite extensively by the amateur astronomy community who have been able to take very high quality images of bright objects such as Mars and the other planets. There is more information about Amateur Lucky Imaging here..

AOLI: Adaptive Optics Lucky Imager for the WHT 4.2 M and GTC 10.5 Meter Telescopes

AOLI Is a major collaboration between the University of Cambridge, Institute of Astronomy, the IAC in La Laguna, Tenerife, the ING in La Palma and the Universities of Cartegena and Cologne.  The acronym AOLI stands for “Adaptive Optics Lucky Imager”. This project aims at building a camera able to deliver diffraction limited images in the visible range. This instrument is first designed for the 4.2-m William Herschel Telescope, in the island of La Palma (Canary Islands), but later intended for use on the 10.4-m Gran Telescopio de Canarias (GTC).

Obtaining optical diffraction limited images is almost impossible to achieve from the ground because of the lack of efficient adaptive-optics systems for wavelength below 1.2-1.6 microns. The atmospheric turbulence rapidly degrades the wavefronts entering the telescope, which results in seeing-limited images with no spatial information below 0.8-1” resolution. Until recently, optical diffraction-limited images were only delivered by the Hubble Space Telescope

By combining Lucky Imaging techniques with low order adaptive optics it is possible to obtain diffraction limited images in the visible on ground-based telescopes.  AOLI is intended to give a resolution of approximately 3 times that of the Hubble Space Telescope from the ground on the WHT 4.2 m telescope and approximately 8 times that Of the Hubble Space Telescope on the 10.5 m GTC telescope

More details about AOLI works may be found (click here).

Observing and Results

A recent observing trip (July 2007) to the Palomar 200 inch telescope has been extremely successful. The images we obtained are the highest resolution direct images, about 35 milliarcsec FWHM, ever obtained either from the ground or from space in the visible at about twice the resolution of the Hubble Space Telescope.

M13

Images shown below are of the core of globular cluster M 13. The blinking image shows what the telescope delivers on its own, followed by what it delivers with the adaptive optic system and LuckyCam.

The images below show a direct comparison between the conventional image taken under conditions of good seeing (0.65 arcsec), the Hubble image from the ACS (centre) and our Lucky/AO image (right). The Hubble picture goes fainter because the exposure is longer and the wavelength shorter (where CCDs have a much higher sensitivity). The ACS image has been "drizzled" to improve its appearance. The Lucky image is as taken. The markedly better resolution of the Lucky image is clear. This is exactly what is predicted purely because the Palomar 5.1 m telescope is twice the size of the 2.5m Hubble.

Cat's Eye Nebula (NGC 6543)

A planetary nebula is formed when the central star evolves from a red giant to its final white dwarf phase. A relatively short time in the life of the star, possibly 10,000 years in total, gas is ejected from the surface of the dying star. We can look at the expansion velocity of these filaments and sure that the age of the bright inner shells is probably only about 1000 years. The nebula is about 3000 light years from Earth.

The image below shows the Cat’s Eye Nebula (NGC 6543) as imaged conventionally by the Palomar 200 inch telescope. The green light is oxygen emission, the red is hydrogen emission, and the blue is near-infrared radiation, again followed by the Cat’s Eye Nebula (NGC7543) as imaged with the Lucky Camera behind an adaptive optics system on the Palomar 200 inch telescope. The resolution in the Lucky image is lower than Hubble as the image covers four times the area of the M13 images above, but it is still a good demonstration of what can be done from the ground. These images are all slightly lower resolution than those of the globular cluster but nevertheless show the considerable improvement over conventional ground-based imaging that the AO system produces with LuckyCam.