Interferometric apparatus and methods by which the local surface characteristics of photolithographic mirrors or the like may be interferometrically measured in-situ to provide correction signals for enhanced distance and angular measurement accuracy. In this way signals from a number of endpoints can be simply added to one another in order to provide a composite broadcast signal. The set of notification preferences may indicate a desired notification medium via which the notification message is to be sent, a notification addressee to which the notification message is to be sent, andor a notification timing preference indicating a date andor time during which the notification is to be sent.