def __get_cnpix(a) :
    """
    Gets CNPIX cards from HDU a, sets them to 1 if they don't exist
    """
    try:
        cnpix1=a.header['CNPIX1']
    except:
        a.header['CNPIX1']=1
    try:
        cnpix2=a.header['CNPIX2']
    except:
        a.header['CNPIX2']=1

    return a.header['CNPIX1'],a.header['CNPIX2']