def add(data,r=0.0,i=0.0,c=0.0):
    """
    Add Constant

    Parameters:
    
    * data  Array of spectral data.
    * r     Constant to add to read channel.
    * i     Constant to add to imaginary channel.
    * c     Constant to add to both channels.
    
    """
    data.real = data.real + r + c
    data.imag = data.imag + i + c
    return data