def _subtractSky(image,skyValue,memmap=False):
    """
    subtract the given sky value from each the data array
    that has been passed. image is a fits object that
    contains the data and header for one image extension
    """
    try:
        np.subtract(image.data,skyValue,image.data)

    except IOError:
        print("Unable to perform sky subtraction on data array")
        raise IOError