def get_expstart(header, primary_hdr):
    """shouldn't this just be defined in the instrument subclass of imageobject?"""
    if 'expstart' in primary_hdr:
        exphdr = primary_hdr
    else:
        exphdr = header
    if 'EXPSTART' in exphdr:
        expstart = float(exphdr['EXPSTART'])
        expend = float(exphdr['EXPEND'])
    else:
        expstart = 0.0
        expend = 0.0
    return expstart, expend
