Obtain EXPTIME

def get_exptime(self, img):
        """Obtain EXPTIME"""
        header = self.get_header(img)
        if 'EXPTIME' in header.keys():
            etime = header['EXPTIME']

        elif 'EXPOSED' in header.keys():
            etime = header['EXPOSED']
        else:
            etime = 1.0
        return etime