def mag_zeropoint(filter_name):
    """
    Given a filter name, return the number of photons per square centimeter per second
    which a star of zero'th magnitude would produce above the atmosphere. We assume that the star
    has a spectrum like Vega's.

    The numbers are pre-calculated and we just pick the appropriate one for the given filter.

    :param filter_name: Filter name string
    :return: Number of photons per sq.cm. per sec from zero'th mag star

    >>> mag_zeropoint("none")
    4320000.0
    >>> mag_zeropoint("B")
    391000.0
    >>> get_extinct_coeff("other")
    Traceback (most recent call last):
        ...
    ValueError: Bad filter name: other
    """
    photons_per_sq_cm = {"none": 4.32e+06, "U": 5.50e+05, "B": 3.91e+05, "V": 8.66e+05,
                         "R": 1.10e+06, "I": 6.75e+05}
    try:
        return photons_per_sq_cm[filter_name]
    except KeyError:
        raise ValueError("Bad filter name: {filter_name}".format(filter_name=filter_name))