View Occultation Screen

• The View Occultation Screen is invoked by the Search for Occultation when an event is found, or alternatively invoked by Browse Occultation Screen for deferred viewing of occultations. The screen is also invoked when in ephemeris mode when tracking the position of an asteroid along its orbit when no occultation target is specified.

• In the example shown, an occultation was found to occur on May 12 at 5h24m UT between 74 Galatea and a star from the user defined (UOB) auxiliary star database. Note under the Orbit Section of the Screen that a Refined orbit is available, only 0.7 day from its epoch. The other two orbit options are Automatic and Reference, the former being an mathematical approximation to the desired perturbed orbit used for enhanced speed in searching, and the latter is the reference orbit of the original source database. The [Refine] button allows the user to update the orbit to the epoch of occultation if the Refined orbit is judged to be too far out of date, i.e. 30 or so days.

• Other sections show detailed positional data for the asteroid for the date of occultation, the position of the asteroid shadow relative to the earth, i.e. noted in units of earth radii, thus values less than 1.0 indicate that the shadow falls somewhere on earth.; and if on the earth, the exact location of the shadow in earth coordinates and the nearest site to that location. The conditions at that shadow location are shown including an indication of darkness, the altitude of the event above the local horizon, shadow duration and shadow speed.

• Four controls allow the stepping of the asteroid in time. One, [Step Path] moves the asteroid in adjustable steps in time from entrance of its shadow onto the earth to exit from the earth showing the location of the shadow as it progresses.
  

• [Save Path] is similar and writes a text file of each path step into an external file for later examination. A Target Site is definable so that should the shadow come within the boundary of the site, the time stepping is paused. A user can select to output information about the centerline of the path or either/both of the two edges of the shadow path. The program accounts for the flattening of the earth to compute path locations.

• The two other controls,[Earth Plot] and [SkyPlot], show the path of the asteroid shadow across a graphical picture of the earth and the path of the asteroid across the sky, respectively. The graphics require a VGA display capable of 640x480x16 resolution/colors.
  

• Earth Plot shows the path of the shadow of an asteroid across the earth. Options are available for varying the time step size, the limits of the darkness level along the path, the limits of the acceptable altitude above the horizon, the orientation of the continents. The user can choose to display the centerline or the actual path width. Options are provided to introduce errors into the asteroid or star position to assess the impact on the predicted path. The altitude of the event above the horizon is optionally depicted on the path using a slanted line drawn from each point on the path, the length of which is proportional to the altitude. Lightness/Darkness conditions are shown by drawing a series of terminators on the map to indicate the location of sunrise/set through end of astronomical twilight. In addition a graphical output file (PCX) can be written for later printing.

• In this example based on the 74 Galatea search earlier, the shadow of the occultation is shown beginning in the equatorial Atlantic and sweeping across Florida, USA into western British Columbia, Canada. The number of path steps can be varied as well as parameters to control the display of steps depending upon local darkness and/or altitude of the event above the horizon. The path is a series of circles that approximate the diameter of the asteroid along the centerline or a series of lines perpendicular to the path representing the exact cross section of the shadow path for each location. The color of the circles or diameter lines are color coded to indicate the visibility conditions for each of the path locations.

• SkyPlot shows the path of the asteroid relative to the occulted star. The path length can be varied from a two month orbit segment down to several minutes for an extreme close up of the occultation with sub-arcsecond resolution.
  

• A circle is displayed showing the zone around the star such that when the asteroid path crosses within it, the shadow will strike the earth somewhere. Optionally, the earth locations of the shadow are superimposed on the SkyPlot. In this example, also from the Galatea occultation, the path crosses the 7 arcsecond diameter zone from left to right and the locations of the path from Atlantic to Alaska are listed on the right side of the display. From this view one can zoom out in increasing length orbit segments with full control of magnitude of stars shown, star labels, and the ability to click on a star to see its identification. The Grid can be toggled on/off. In addition, the display can be written to a PCX file for later viewing or printing.