def snr(mag=20, itime=1., read=24.5, sky=8.43, npix=24., zero=26.44, dark=0.0):
    """Calculate SNR of a photometric CCD observation of given parameters.

    The default keyword parameters are for the CTIO Blanco MOSAIC
    imager, operating in V band.
    """
    # 2009-02-20 14:40 IJC: Initiated

    star = itime * 10**(0.4*(zero-mag))
    noise = npix * (itime*(sky+dark)+read**2)

    return star * (star+noise)**-0.5