The acronym AOLI stands for “Adaptive Optics Lucky Imager”. This project aims at building a camera able to deliver diffraction limited images in the optical part of the spectrum. This instrument is designed for the 4.2-m William Herschel Telescope, in 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 very difficult to achieve from the ground considering. The atmospheric turbulence rapidly degrades the incoming wavefronts, 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.

Since the original work of Antoine Labeyrie (1970) and David L. Fried (1978), it was shown that very short exposures of a source result into an irregular “speckled” image which characterize the instantaneous atmospheric turbulence. Several methods (e.g. the lucky imaging technique, the speckle interferometry and holography) have been developed to extract, under certain conditions, the high angular information from speckles data.

For long, “speckle science” has been scientifically limited to the observation of bright sources mainly because of read-out noise limitations of the detectors. As significant progress has been accomplished in the field of visible CCD detectors, in particular with EMCCD arrays, this has triggered a renaissance of the “fast imaging techniques”. This is what is exploited in AOLI.


How AOLI will work ?

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. These “first generation” instruments have equipped both telescopes with and without AO (e.g. 2.5-m NOT, 2.2-m CAHA, WHT+NAOMI).

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 correction. A modest correction of the turbulence - for instance due to particularly 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 for instance Lucky Imaging is implemented in the visible range on large telescopes (> 4m) with no wavefront correction: the speckle cloud is largely spread out over the seeing scale, and despite the applied shift-and-add process a halo forms around the coherent PSF core on a size scale comparable to the seeing (note that other techniques such as speckle holography can overcome this problem specific to the shift-and-add approach).

This is the reason why Lucky Imaging becomes more efficient on mid-size telescope when assisted by AO low-order correction. If enough of the large turbulent scales are removed, then the resulting r0 (the “Fried parameter”) will become large enough compared to the telescope diameter so that we are back to a situation similar to what is obtained on a small 2.5-m telescope. And we will consequently become “luckier” to see flatter wavefronts entering the telescope.  Therefore AOLI will benefit from a low-order curvature wavefront-sensor that will be able to sense stars as faint as 16 mag in I. The science channel will be equipped with different filters. A particularity of the AOLI science channel is due to its large diffraction-limited field of view up to 1-2 arcminutes obtained with for EMCCD arrays in the focal plane.

AOLI will deliver a large amount of data cubes stored on external disks and later reduced to produce diffraction-limited images of the science targets.