def fun_wv(xchannel, crpix1, crval1, cdelt1):
    """Compute wavelengths from channels.

    The wavelength calibration is provided through the usual parameters
    CRPIX1, CRVAL1 and CDELT1.

    Parameters
    ----------
    xchannel : numpy array
        Input channels where the wavelengths will be evaluated.
    crpix1: float
        CRPIX1 keyword.
    crval1: float
        CRVAL1 keyword.
    cdelt1: float
        CDELT1 keyword.

    Returns
    -------
    wv : numpy array
        Computed wavelengths

    """
    wv = crval1 + (xchannel - crpix1) * cdelt1
    return wv
