Angular differential imaging is a high-contrast imaging technique that reduces quasistatic speckle noise and facilitates the detection of nearby companions. A sequence of images is acquired with an altitude/azimuth telescope while the instrument field derotator is switched off. This keeps the instrument and telescope optics aligned and allows the field of view to rotate with respect to the instrument. For each image, a reference point-spread function (PSF ) is constructed from other appropriately selected images of the same sequence and subtracted to remove quasistatic PSF structure. All residual images are then rotated to align the field and are combined. Observed performances are reported for Gemini North data. It is shown that quasistatic PSF noise can be reduced by a factor $5 for each image subtraction. The combination of all residuals then provides an additional gain of the order of the square root of the total number of acquired images. A total speckle noise attenuation of 20-50 is obtained for a 1 hr long observing sequence compared to a single 30 s exposure. A PSF noise attenuation of 100 was achieved for a 2 hr long sequence of images of Vega, reaching a 5 contrast of 20 mag for separations greater than 8 00 . For a 30 minute long sequence, ADI achieves signal-to-noise ratios 30 times better than a classical observation technique. The ADI technique can be used with currently available instruments to search for $1M Jup exoplanets with orbits of radii between 50 and 300 AU around nearby young stars. The possibility of combining the technique with other high-contrast imaging methods is briefly discussed.

Direct imaging of exoplanets is limited by bright quasi-static speckles in the point spread function (PSF) of the central star. This limitation can be reduced by subtraction of reference PSF images. We have developed an algorithm to construct an optimized reference PSF image from a set of reference images. This image is built as a linear combination of the reference images available and the coefficients of the combination are optimized inside multiple subsections of the image independently to minimize the residual noise within each subsection. The algorithm developed can be used with many high-contrast imaging observing strategies relying on PSF subtraction, such as angular differential imaging (ADI), roll subtraction, spectral differential imaging, reference star observations, etc. The performance of the algorithm is demonstrated for ADI data. It is shown that for this type of data the new algorithm provides a gain in sensitivity by up to a factor 3 at small separation over the algorithm used in Marois et al. (2006).

We present the results of the Gemini Deep Planet Survey, a near-infrared adaptive optics search for giant planets and brown dwarfs around nearby young stars. The observations were obtained with the Altair adaptive optics system at the Gemini North telescope and angular differential imaging was used to suppress the speckle noise of the central star. Detection limits for the 85 stars observed are presented, along with a list of all faint point sources detected around them. Typically, the observations are sensitive to angular separations beyond 0.5 ′′ with 5σ contrast sensitivities in magnitude difference at 1.6 µm of 9.5 at 0.5 ′′ , 12.9 at 1 ′′ , 15.0 at 2 ′′ , and 16.5 at 5 ′′ . For the typical target of the survey, a 100 Myr old K0 star located 22 pc from the Sun, the observations are sensitive enough to detect planets more massive than 2 M Jup with a projected separation in the range 40-200 AU. Depending on the age, spectral type, and distance of the target stars, the detection limit can be as low as ∼1 M Jup . Second epoch observations of 48 stars with candidates (out of 54) have confirmed that all candidates are unrelated background stars. A detailed statistical analysis of the survey results, yielding upper limits on the fractions of stars with giant planet or low mass brown dwarf companions, is presented. Assuming a planet mass distribution dn/dm ∝ m −1.2 and a semi-major axis distribution dn/da ∝ a −1 , the 95% credible upper limits on the fraction of stars with at least one planet of mass 0.5-13 M Jup are 0.28 for the range 10-25 AU, 0.13 for 25-50 AU, and 0.093 for 50-250 AU; this result is weakly dependent on the semi-major axis distribution power-law index. The 95% credible interval for the fraction of stars with at least one brown dwarf companion having a semi-major axis in the range 25-250 AU is 0.019 +0.083 −0.015 , irrespective of any assumption on the mass and semi-major axis distributions. The observations made as part of this survey have resolved the stars HD 14802, HD 166181, and HD 213845 into binaries for the first time.

ABSTRACT. Speckles dominate shot noise within the halo of adaptively corrected bright star images and, consequently, impose severe limits on ground-based attempts to directly detect planets around nearby stars. The e †ect is orders of magnitude greater than conventional photon noise. It depends on the dwell time of the speckle pattern, the brightness of the star, and the fraction (1 [ S) of residual light in the halo (S being the Strehl ratio of the image). These predictions agree well with limits found using the Canada-France-Hawaii Telescope adaptive optics bonnette. The limiting brightness for detection is proportional to (1 [ S)/S, emphasizing the need for large Strehl ratios. Strategies to reduce speckle noise are proposed ; the encouraging results of a test are presented.