def guess_band(filename):
    """ Guess the passband using the image's filename"""
    if '_B_' in filename:
        passband = 'B'
    elif '_V_' in filename:
        passband = 'V'
    elif '_R_' in filename:
        passband = 'R'
    elif '_I_' in filename:
        passband = 'I'
    else:
        passband = 'clear'
    return passband