docs: 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)