Short exposure astronomical techniques for occultation detection

Image intensifiers coupled to video frame rate CCDs have the capability of observing much shorter duration events than conventional multi-channel optical astronomy detection techniques. This equipment is particularly suited to observations of unpredicted stellar occultations. An image intensified CCD system can offer a moderately sensitive multi-channel photometric search tool for occultation events on 500 or more simultaneous channels with effective exposure times of the order of 0.03 s. The increased noise of such short-duration image-intensified techniques demand coincidence approaches with two or more systems. Stellar occultations can result from many types of objects passing along the line of sight between the Earth and a star. These objects can include: asteroids, Kuiper Belt Objects (KBOs), or interstellar planets and planetesimals. Calculations suggest that in most cases occultations by these objects will be of very short-duration. The research described here uses a semi-automated coincidence method to search for short-duration stellar occultations. The image-intensified CCD detectors used can detect about 500 stars per 0.033 s exposure time in a 1200 square degree field of view. During 12 hours of observation from each of two cameras over 2.2 million images, consisting of nearly 1.1 billion stellar images, were analyzed for this study. Semi-automated software performed the initial stage of the detection in real-time. After coincidence processing between the two intensified CCD detectors there was one promising event (a 0.5 magnitude dimming of a +7.6 magnitude K-type star at 7:16:35 UT July 31, 2000). However, additional analysis of this event indicated that the occultation was probably caused by terrestrial phenomena so no confirmed occultation events were identified in this search. This technique is likely best suited to the search for small KBOs, where direct observation from reflected light is difficult or impossible. This paper provides guidance for those wishing to set up observational programs aimed at detection of transient astronomical systems.