def define_from_fits(cls, fitsobj, extnum=0):
        """Define class object from header information in FITS file.

        Parameters
        ----------
        fitsobj: file object
            FITS file whose header contains the DTU information
            needed to initialise the members of this class.
        extnum : int
            Extension number (first extension is 0)

        """

        # read input FITS file
        with fits.open(fitsobj) as hdulist:
            image_header = hdulist[extnum].header
            return cls.define_from_header(image_header)