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 foreseen for the 4.2-m William Herschel Telescope, in the island of La Palma (Canary Islands), but is also built in the perspective of equipping in the future the 10.4-m Gran Telescopio de Canarias (GTC).
Obtaining optical diffraction limited images is almost impossible to achieve from the ground considering the absence 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 a recent time, optical diffraction-limited images were only delivered by the HST, which operates in atmosphere-free conditions.
However, since the work of Antoine Labeyrie (1970) and David L. Fried (1978), it became evident that very short-exposure time images would almost catch up a given realization of the atmospheric turbulence. Several diffraction-limited PSFs (defined as “speckles”) could be retrieved from these short-exposure images and scientifically exploited to reconstruct a diffracion-limited optical image. This principle has been exploited in techniques such as speckle interferometry or lucky imaging.
For long, “speckle science” has been scientifically limited to the observation of bright sources mainly because of detectors strongly limited by the high readout noise. More recently, amazing amount of progress has been achieved in the field of visible CCD detectors, in particular with EMCCD and photon-counting sensors with almost no readout noise, and thus triggering a new era for fast readout/low readout noise imaging in the optical.
AOLI will be constructed on the heritage of smaller scale fast imaging projects like FastCam or LuckyCam. These instruments have been operated mainly in the V, R and I bands on telescopes ranging from 1.5 to 5-m.
Like for any ground-based corrected PSF, we can decompose the signal into a narrow coherent and diffraction-limited core surrounded by the incoherent “seeing” halo, which prominence depends on the quality of the turbulence compensation. A modest correction of the turbulence - for instance due to bad weather conditions - would lead to a significant fraction of the energy being rejected into the halo, unavoidably resulting in a poor dynamic range of the image. This is exactly what happens when Lucky Imaging is implemented in the visible range on large telescopes (> 4m) with nothing else than speckle re-centering. This is the reason why Lucky Imaging - or more precisely “frame selection” - becomes much more powerful if assisted by AO low-order compensation of the atmospheric turbulence correction of the first few orders.