def star_finder(att, sky, scanner):
    """
    Finds times at which source transit CCD line of scanner and estimates their position in the BCRS frame.
    :param sky: object
    :param att:object
    :param scanner: object
    :return: scanner.star_positions, scanner.obs_times.
    """
    for star in sky.elements:
        scanner.reset_memory()
        scanner.intercept(att, star)
        if len(scanner.times_deep_scan) != 0:
            scanner.deep_scan(att, star)
