def svo_filter_url(telescope, photofilter, zeropoint='AB'):  
    """
    Returns the URL where the filter transmission curve is hiding.

    Requires arguments:
        telescope:   SVO-like name of Telescope/Source of photometric system.
        photofilter: SVO-like name of photometric filter.
    Optional:
        zeropoint:   String. Either 'AB', 'Vega', or 'ST'.
    Output:
        url:   URL of the relevant file.
    """
    url = 'http://svo2.cab.inta-csic.es/theory/fps3/fps.php?' + \
          'PhotCalID=' + telescope + '/' + photofilter + '/' + zeropoint
    return url