Wide-Field Millimagnitude Photometry with the HAT: A Tool for Extrasolar Planet Detection

We discuss the system requirements for obtaining millimagnitude photometric precision over a wide field using small-aperture, short focal length telescope systems such as those being developed by a number of research groups to search for transiting extrasolar planets. We describe a Hungarian Automated Telescope (HAT) system, which attempts to meet these requirements. The attainable precision of HAT has been significantly improved by a technique in which the telescope is made to execute small pointing steps during each exposure so as to broaden the effective point-spread function (PSF) of the system to a value more compatible with the pixel size of our CCD detector. Experiments during a preliminary survey (spring 2003) of two star fields with the HAT-5 instrument allowed us to optimize the HAT photometric precision using this method of PSF broadening; in this way we have been able to achieve a precision as good as 2 mmag on brighter stars. We briefly describe development of a network of longitudinally spaced HAT telescopes (HATNet).