def grism_res(pupil='GRISM', module='A'):
    """Grism information
    
    Based on the pupil input and module, return the spectral
    dispersion and resolution as a tuple (res, dw).
    
    Parameters
    ----------
    pupil : str
        'GRISM0' or 'GRISM90', otherwise assume res=1000 pix/um
    module : str
        'A' or 'B'
    """
    res = 1000.0
    if 'GRISM90' in pupil and module == 'A':
        res = 1003.12
    if 'GRISM0' in pupil and module == 'A':
        res = 996.48
    if 'GRISM90' in pupil and module == 'B':
        res = 1008.64
    if 'GRISM0' in pupil and module == 'B':
        res = 1009.13
    dw = 1.0 / res
    return res, dw
