def air2vac(w_air):
    """
	Air to vacuum conversion formula derived by N. Piskunov
	IAU standard:
	http://www.astro.uu.se/valdwiki/Air-to-vacuum%20conversion

	Parameters
	----------
	w_air : array-like
		Array of air wavelengths assumed to be in Angstroms

	Returns
	-------
	w_vac : array-like
		Array of vacuum wavelengths converted from w_air
	"""
    s = 10 ** 4 / w_air
    n = 1 + 8.336624212083e-05 + 0.02408926869968 / (130.1065924522 - s ** 2
        ) + 0.0001599740894897 / (38.92568793293 - s ** 2)
    w_vac = w_air * n
    return w_vac
