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Description
Lunar occultation light curves have been recorded since the mid-20th century using high-speed photomultipliers. Running at high cadence for high angular resolution, such recordings were usually made on large telescopes and limited to the brighter stars - and were not large in number. While a small number of video recordings of lunar and asteroidal occultations were made from about 1980, they became common from about the year 2000, when inexpensive low-light security cameras became available. As of 2016, almost all lunar and asteroidal occultation observations are recorded using video, with the video recording being measured using software packages such as Limovie [http://astro-limovie.info/limovie/limovie_en.html], and Tangra [http://www.hristopavlov.net/Tangra3/]. As a result, light curves are now routinely generated for almost all lunar and asteroidal occultation observations, especially those coordinated through the International Occultation Timing Association and related organisations around the world. This is resulting in large numbers of occultation light curves being obtained each year - albeit with some limitations on time resolution and signal-to-noise ratios. As of 2016, video recordings are mainly made using one or other of the two international video standards - NTSC, or PAL. Both NTSC and PAL use an interlaced video scan, whereby each frame of the video is comprised of two interlaced, time-sequential, fields. The frame rate of an NTSC system is 29.97 frames/sec (59.94 fields/sec), while that for PAL is 25 frames/sec ( 50 fields/sec). Consistent with broadcast television standards, the majority of video cameras used for recording occultations use 8-bit CCD's. However some video recordings are made using progressive scan, 12 to 16-bit digital video systems. For lunar occultations, the temporal resolution is governed by a combination of the frame (or field) rate of the video recording, and the rate of motion of the moon. The typical topocentric motion of the moon is between about 0.3"/sec and 0.4"/sec. The motion of the lunar limb in a direction normal to the star is reduced by the cosine of the difference between the direction of motion of the moon and the position angle of the star. As a result, the typical rate of motion of the lunar limb normal to the star is in the range 0.2 to 0.4 "/sec. At video frame rates this provides a spatial resolution of about 0.01" to 0.02" at frame rate, or 0.005" to 0.01" at field rate. In recent years it has been possible to accurately determine the orientation of the lunar limb at the point of an occultation, using data from the Japanese Kaguya satellite, and more recently the US Lunar Reconnaissance Orbiter - Lunar Orbiter Laser Altimeter (LRO-LOLA). The LRO-LOLA data allows the slope of the lunar limb to be reliably determined over circumferential distances of less than 0.2" in the sky plane. As a result, all data elements required to analyse a lunar occultation light curve are well determined - and are included in this archive. The motion of most asteroids is much less than the moon. As a result, the angular resolution attainable at video frame rate is much smaller than for a lunar occultation, and is commonly in the range 0.0001" to 0.001". However asteroidal occultations frequently involve fainter objects than for lunar occultations, and many observers use integrating video cameras to detect these fainter occultations; the resolution attainable with an integrating camera is reduced in proportion to the number of frames integrated. Unlike lunar occultations, the orientation of the occulting limb of an asteroid relative to the star is generally not well established. Furthermore it can generally be assumed that the limb of an asteroid is likely to have significant irregularities at scales greater than the potential angular resolution attainable, but smaller than the angular distance between adjacent observed occultation chords. There is also the issue of the rotational orientation of the asteroid differing for observers located at different points along the occultation path, placing a limit on the accuracy of the limb slope that can be derived from adjacent occultation chords. Accordingly, at this time the record does not attempt to specify the orientation of the limb of the asteroid at the occultation event.
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