def get_exptime(n, t_tot, overhead=10):
    """
    Function that calculates the exposure time
    per image

    ---
    Parameters

    n int or array
        number of fields
    t_tot float
        total ovservation time (s)
    overhead float
        overhead time per exposure (s)

    ---
    Returns

    exptime float
        exposure time per image
    """

    exptime = (t_tot - n*overhead)/n

    return exptime