def get_ad_sublist(adlist, names):
    """
    Select a sublist of AstroData instances from the input list using a list
    of filename strings. Any filenames that don't exist in the AstroData list
    are just ignored.
    """
    outlist = []
    for ad in adlist:
        if ad.filename in names:
            outlist.append(ad)
    return outlist