Occultation light curves Dave Herald, Murrumbateman, Australia Derek Breit (USA), David Dunham (USA), Eric Frappa (France), Dave Gault (Australia), Tony George (USA), Tsutomu Hayamizu (Japan), Brian Loader (New Zealand), Jan Manek (Czech Rep.), Kazuhisa Miyashita (Japan), Hristo Pavlov (Australia), Steve Preston (USA), Mitsuru Soma (Japan), John Talbot (New Zealand), Brad Timerson (USA) ************************** * Introduction * ************************** 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. This catalogue is a repository of such light curves. The objective is to make these occultation light curves readily available for use when investigating a star. ****************************** * Nature of the light curves * ****************************** The light curves in this catalogue fall into three distinct categories * A lunar occultation Disappearance, or Reappearance. The light curve is a single transition between full light and zero light - albeit with the potential for one or more intermediate steps caused by companion stars. The duration of the light curve is generally only several seconds, but may be longer if a known companion star is present. * A lunar grazing occultation. These events involve the star clipping along the lunar limb, potentially disappearing and reappearing many times over a period of several minutes. For these events the motion of the star in a direction normal to the lunar limb is small, whilst the motion parallel to the lunar limb is large. The full duration of the event may be many minutes; however the light curve may be split into shorter segments so that the parameters relating to the lunar limb can be better specified. * An asteroidal occultation. These light curves typically involve a drop in the light curve, followed some time later (typically several seconds, but occasionally over a minute) by a rise in the light curve. Intermediate light levels occur when a star is a double star. Binary asteroids can result in more than one drop and rise in the light curve. This can also occur when the observer is close to the edge of the occultation path, and the topography of the asteroid is suitably rough (a 'grazing' occultation). Apart from intermediate light levels attributable to multiple stars, the light curves can exhibit the effects of Fresnel diffraction, and of stellar diameter. ************************** * Recording equipment * ************************** 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 fainter objects (in particular, with asteroidal occultations), many observers use an 'integrating' video camera. All integrating cameras used by observers sum the data from a series of video frames before outputting that data - with the output from the video camera being constant for a number of frames equal to the integration period. The light curve generated using an integrating video camera should normally have only one data point per integration interval. While video cameras using a 'running integration' scheme are available on the market, their use is discouraged and they are rarely used. Each light curve includes the UTC corresponding to the time at the start of the exposure of the first data point in the record. Generally that time has been established using the 1PPS output of a GPS receiver - typically linked to the video recording via a video inserter. However steps have not been taken to ensure absolute accuracy of the start time as specified in this archive. Accordingly the start time should be treated as uncertain by an amount equivalent to 1.5 data points. For some light curves involving asteroidal occultations, the time linking has relied on a clock internal to a recording device, with that clock being calibrated before & after the event. This primarily occurs with observations that have been made using unattended equipment, with the calibrations occurring up to an hour or more before and after the occultation event. The absolute accuracy of the time for the start of such a light curve is likely to be as much as a significant fraction of a second; however the relative accuracy of the time during the duration of the light curve is essentially unaffected. Such situations are identifiable by the presence of several light curves of the same star by the same observer from different locations on the same date. It can be assumed that all light curves are 'white light'; observers do not routinely insert filters into the optical path. As a result, the optical pass band for the light curves is that of typical CCD's - good sensitivity in the red, and poor sensitivity in the blue. It can generally be assumed that the light curves have been generated under circumstances where the camera response is linear; however the possibility of non-linear response arising from saturation cannot be excluded in the case of bright stars. ************************** * Resolution issues * ************************** 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 10 to 20 mas at frame rate, or 5 to 10 mas 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. The light curves in the archive may show the presence of stellar duplicity, and occassionally stellar diameter effects and/or Fresnel diffraction. Light curves not showing these effects place limits (within the confines of the noise in the light curves) on the separation and/or brightness of any possible companion to that star on the date of observation, and on the diameter of that star. ************************** * Basis of the catalogue * ************************** This catalogue contains light curves measured from lunar and asteroidal occultations, with the majority coming from lunar occultations. In the main, the light curves have been obtained using telescopes having an aperture in the range 5 to 40 cm, using 8-bit NTSC or PAL video cameras. No attempt has been made to limit the light curves to 'the best' of those available. All submitted light curves are included, provided they (i) show clear evidence of an occultation event, and (ii) relevant associated information has been provided. As a result, there can be multiple light curves for a single star: - submitted by the same or different observers on different nights; - submitted by different observers on the same night, each observing at different sites; or - submitted by the same observer on the same night, using unattended observing techniques at different sites. Those light curves can have greatly different signal-to-noise values - reflecting widely different observational circumstances and equipment used to make the recording. For lunar occultations, the stars fall within a band 13d 20' wide centered on the ecliptic. The magnitude limit is highly dependant upon the size of telescope used, and the phase of the moon at the time of an observation. The majority of stars are likely to be brighter than mag 10. Stars fainter than mag 12.0 are unlikely to be present. Asteroidal occultations can occur anywhere in the sky - although the majority are within 15 degrees of the ecliptic. While the majority of observed occultations involve stars brighter than mag 12, much fainter stars may be present. ************************** * Record content * ************************** The information included with each light curve depends on whether it is a lunar or asteroidal occultation. The information provided is: * Date Date of the event. UTC corresponding to the first point in the light curve (h m s.ss). Duration of the light curve (seconds). Number of data points in the light curve. * Star identification, in each of: Hipparcos (I/239), Tycho2 (I/259), SAO (I/131A), XZ80Q (I/291), ZC (Robertson's Zodiacal catalogue, US Nautical Almanac, 1940USNAO..10..169R), UCAC2 (I/289), UCAC4 (I/322A), K2 EPIC ID. * Observer's longitude (d m s), latitude (d m s), altitude (m), and name. * Circumstances of the event - For Lunar occultations: Axis angle of the event (the position angle referred to the lunar north pole). Libration in longitude (deg). Libration in latitude (deg).. Slope of the lunar limb, derived from LRO-LOLA. Positive values correspond to an increase in the PA of the normal compared to the mean limb (deg). Normal rate of motion of the star relative to the mean limb ("/sec). Contact angle - the angle between the normal to the mean limb, and the direction of motion of the star (deg). Range is between -90 & +90 for disappearance events, and between 90 to 180, and -90 to -180. Positive values apply when the star is northwards of the direction of motion. Moon size. The ratio of apparent radius of the moon to its mean radius Position angle of the event (deg). Cusp angle. The angular distance of the event from the nearer cusp (deg). The relevant cusp is most commonly indicated as N or S (North or South), but near full moon the cusp can be E or W (East or West). Positive values are against the dark limb while negative values are against the bright limb of the moon. Illumination. The percent illumination of the moon. Moon altitude. The altitude of the star at the time of the event (deg). - For asteroidal occultations: Asteroid number. Asteroid name. * Data points in the light curve - with the values being normalised to be within the range +/- 9999. ************************** * References * ************************** The lunar occultation observations (but not light curves) are archived in Catalogue VI/132B Lunar Occultation Archive. That catalogue provides full details of the observation, and is updated on an irregular basis. Asteroidal occultation observations (but not light curves) are archived in NASA Planetary Data System, Small Bodies Node, Asteroid/Dust Archive, as Asteroid Occultations. The identifier is EAR-A-3-RDR-OCCULTATIONS-Vxx.x, and is located at http://sbn.psi.edu/pds/resource/occ.html That catalogue provides full details for all asteroidal occultation events, and is updated annually.