Helper function to calculate location of image pixel in (l,m) coords.
    Assumes peak pixel, but input can be provided in pixel units.
    minmax defines whether to look for image maximum or minimum.

def calc_lm(d, im=[], pix=(), minmax='max'):
    """ Helper function to calculate location of image pixel in (l,m) coords.
    Assumes peak pixel, but input can be provided in pixel units.
    minmax defines whether to look for image maximum or minimum.
    """

    if len(pix) == 0:  # default is to get pixel from image
        if minmax == 'max':
            peakl, peakm = n.where(im == im.max())
        elif minmax == 'min':
            peakl, peakm = n.where(im == im.min())
        peakl = peakl[0]; peakm = peakm[0]
    elif len(pix) == 2:   # can also specify
        peakl, peakm = pix

    if len(im):
        npixx, npixy = im.shape
    else:
        npixx = d['npixx']
        npixy = d['npixy']

    l1 = (npixx/2. - peakl)/(npixx*d['uvres'])
    m1 = (npixy/2. - peakm)/(npixy*d['uvres'])

    return l1, m1