import math


def delta(l, m, delta0):
    """Convert a coordinate in l, m into an coordinate in Dec

    Keyword arguments:
    l, m -- direction cosines, given by (offset in cells) x cellsi (radians)
    alpha_0, delta_0 -- centre of the field

    Return value:
    delta -- Dec in decimal degrees
    """
    return math.degrees(math.asin(m * math.cos(math.radians(delta0)) +
                                  (math.sqrt(1 - (l*l) - (m*m)) *
                                   math.sin(math.radians(delta0)))))