Angular Differential Imaging: A Powerful High‐Contrast Imaging Technique

Angular differential imaging is a high-contrast imaging technique thatreduces quasi-static speckle noise and facilitates the detection of nearbycompanions. A sequence of images is acquired with an altitude/azimuth telescopewhile the instrument field derotator is switched off. This keeps the instrumentand telescope optics aligned and allows the field of view to rotate withrespect to the instrument. For each image, a reference PSF is constructed fromother appropriately-selected images of the same sequence and subtracted toremove quasi-static PSF structure. All residual images are then rotated toalign the field and are combined. Observed performances are reported for GeminiNorth data. It is shown that quasi-static PSF noise can be reduced by a factor\~5 for each image subtraction. The combination of all residuals then providesan additional gain of the order of the square root of the total number ofacquired images. A total speckle noise attenuation of 20-50 is obtained forone-hour long observing sequences compared to a single 30s exposure. A PSFnoise attenuation of 100 was achieved for two-hour long sequences of images ofVega, reaching a 5-sigma contrast of 20 magnitudes for separations greater than8". For a 30-minute long sequence, ADI achieves 30 times better signal-to-noisethan a classical observation technique. The ADI technique can be used withcurrently available instruments to search for ~1MJup exoplanets with orbits ofradii between 50 and 300 AU around nearby young stars. The possibility ofcombining the technique with other high-contrast imaging methods is brieflydiscussed.Comment: 27 pages, 7 figures, accepted for publication in Ap