Plotting Asteroid Occultation Paths with Guide


I recommend that you use version 7 or later of Guide. Although version 7 works fine, Version 8 (when available) will save time because it already includes the Tycho-2 positions for stars. Version 8 also includes some additional features when plotting paths.

*** You must enable the "High Precision" mode for planets. Go to the "Display" main menu item and select "Data Shown". In the Data Shown dialog check the Full Precision check box in the Planet Display box in the lower right. While you are in the Data Shown dialog, set Asteroids to "auto" mode, check the box for "labels", and select you choice for Asteroid labels (in the upper right - I use number only for less clutter).

*** You will also need to setup a user dataset for entering the positions for occulted stars. Although Guide 7 will include positional data for an occulted star, Guide 7 does not use the latest Tycho-2 based data. In addition, you may need to supplant Guide 8's dataset in the future, so it's best to include the option for manually entering star positions in Guide. You can easily add new "stars" via Guide's user dataset feature. This is a fairly simple process but I recommend that you peruse Guide's documentation on user datasets before adding new stars. There are two steps to enabling a user dataset for entering your own star positions in Guide. Here is a quick description of adding a new star in Guide. First, you must place a data file (e.g. and a definition file (e.g. oneoff.tdf) in the Guide directory. Guide will read these two files whenever you start Guide (note that this means you must restart Guide after adding a new star to the dataset). Second, you must enable the display of your user dataset via the "toggle user datasets" dialog of the "Extras" main menu item. As an example, here is the definition file and sample data file from my Guide directory: oneoff.dat and oneoff.tdf (note: I setup my "oneoff" dataset so that I just enter the star position with proper motion already included - the same data presented in the asteroid occultation updates). In my user dataset, I use an abbreviated name for the star (e.g. the last set of digits for a GSC/Tycho star) to make it easy to see the new position when Guide displays the field of view.


  1. Enter the Star position:
    The IOTA asteroid occultation update should include the position of the star in J2000 coordinates, with proper motion to the time of the event. You must make sure that Guide is using the same position. As mentioned above, Guide's positional data will not always be the most accurate data (particularly for Guide 7). If Guide's position, with proper motion to the time of the event, matches the position provided in the update, you can use Guide's position. To check Guide's position against the position from the update, you must run Guide, set the date/time to the date/time of the event, GOTO the star in question, right-click on the star, open up the "more info" dialog, and read Guide's position with proper motion to the "current" time. If this position does not match the position from the update summary, you must exit Guide, enter the star in the user dataset as described above, re-run Guide, and check that the user-dataset star position is correct.
  2. Enter the asteroid elements:
    Under the main menu item "Extra", select the "Edit Comet data" dialog. At the end of the list of comets in this dialog, you will see an entry called "new asteroid". Select this entry and enter the data for the comet. Guide assumes that the epoch is TDT (instead of UT) which is fine, since the asteroid occultation updates will normally give the epoch in TDT. For the name of the asteroid I normally use the asteroid number inside parathensis: example - "(49)" for (49) Pales. Enter the data carefully here. If you make a mistake and save the wrong elements, the only way to change the data is to use a text editor to change the appropriate entry in Guide's "comet.dat" file. If you try to change it through the Edit Comet data dialog, Guide will start treating the object as a comet instead of an asteroid.
    You will note that Guide 7 does not ask for the asteroid diameter. Guide infers the asteroid diameter from the asteroid's absolute magnitude. To set the asteroid diameter and determine the width of the occultation path, you must choose the appropriate absolute magnitude. Bill Gray has provided the following information to make this task easier.

    "About resetting asteroid diameters: for its 'built-in' asteroids,Guide can rely on data from the MAGETAL.DAT file on the CD-ROM (a set of diameters determined by the IRAS satellite a while back). For the 'user-added' asteroids, though, such as the ones you would add for an occultation, the diameter is computed from the absolute magnitude based on the assumption of a 4% albedo. I just ran a little program to generate diameters, in kilometers, for absolute magnitudes (M) from 3.0 to 17.9."
    M Diameter M Diameter M Diameter M Diameter M Diameter
    3 1655.7 6 415.9 9 104.5 12 26.2 15 6.6
    3.1 1581.1 6.1 397.2 9.1 99.8 12.1 25.1 15.1 6.3
    3.2 1510 6.2 379.3 9.2 95.3 12.2 23.9 15.2 6
    3.3 1442 6.3 362.2 9.3 91 12.3 22.9 15.3 5.7
    3.4 1377.1 6.4 345.9 9.4 86.9 12.4 21.8 15.4 5.5
    3.5 1315.1 6.5 330.3 9.5 83 12.5 20.8 15.5 5.2
    3.6 1255.9 6.6 315.5 9.6 79.2 12.6 19.9 15.6 5
    3.7 1199.4 6.7 301.3 9.7 75.7 12.7 19 15.7 4.8
    3.8 1145.4 6.8 287.7 9.8 72.3 12.8 18.2 15.8 4.6
    3.9 1093.9 6.9 274.8 9.9 69 12.9 17.3 15.9 4.4
    4 1044.6 7 262.4 10 65.9 13 16.6 16 4.2
    4.1 997.6 7.1 250.6 10.1 62.9 13.1 15.8 16.1 4
    4.2 952.7 7.2 239.3 10.2 60.1 13.2 15.1 16.2 3.8
    4.3 909.9 7.3 228.5 10.3 57.4 13.3 14.4 16.3 3.6
    4.4 868.9 7.4 218.3 10.4 54.8 13.4 13.8 16.4 3.5
    4.5 829.8 7.5 208.4 10.5 52.4 13.5 13.2 16.5 3.3
    4.6 792.4 7.6 199.1 10.6 50 13.6 12.6 16.6 3.2
    4.7 756.8 7.7 190.1 10.7 47.7 13.7 12 16.7 3
    4.8 722.7 7.8 181.5 10.8 45.6 13.8 11.5 16.8 2.9
    4.9 690.2 7.9 173.4 10.9 43.5 13.9 10.9 16.9 2.7
    5 659.1 8 165.6 11 41.6 14 10.4 17 2.6
    5.1 629.5 8.1 158.1 11.1 39.7 14.1 10 17.1 2.5
    5.2 601.1 8.2 151 11.2 37.9 14.2 9.5 17.2 2.4
    5.3 574.1 8.3 144.2 11.3 36.2 14.3 9.1 17.3 2.3
    5.4 548.2 8.4 137.7 11.4 34.6 14.4 8.7 17.4 2.2
    5.5 523.6 8.5 131.5 11.5 33 14.5 8.3 17.5 2.1
    5.6 500 8.6 125.6 11.6 31.5 14.6 7.9 17.6 2
    5.7 477.5 8.7 119.9 11.7 30.1 14.7 7.6 17.7 1.9
    5.8 456 8.8 114.5 11.8 28.8 14.8 7.2 17.8 1.8
    5.9 435.5 8.9 109.4 11.9 27.5 14.9 6.9 17.9 1.7
  3. Set the Date/Time to the time of the Event
    Set Guide's date/time to the approximate date/time of the event (within the minute is good enough).
  4. Zoom to Level 15
    Set Guide's zoom level to level 15. Note: if you try to zoom closer than 15, the new asteroid position is not displayed.
  5. Go To the asteroid
    Under the "Go To" main menu item, select "Asteroid" and enter the number of the asteroid in this event. This will place the event asteroid in the center of the field of view and you should see at least three objects in the field of view: the star to be occulted positioned according to the data on the Guide CD, the position of the Asteroid according to Guide's database, and the new position of the asteroid according to the updated orbital elements. In addition, if you added an updated star position with the user dataset feature, you will see a fourth object - the star according to the updated star position. Using the example of the asteroid 49 Pales, Guide's original position for the asteroid 49 Pales will be labeled 49, the new position will be labeled (49) - assuming that you followed my suggestion for naming the "new" asteroid. The new star position should be labeled with the name you gave it in the user dataset file.
  6. Select the Asteroid and the Star
    You will now "select" the asteroid and the star. To select the asteroid, try to right click on the new, updated asteroid position (e.g. "(49)"). When you right click on the asteroid, Guide will display an info dialog with the name of the object selected and the following options: OK, NEXT, MORE INFO. Often, the original asteroid position and the updated position are so close that it is hard to select the new position. I have found that it helps to right click on the field of view just to side of the new position (opposite from the direction of the old position) - Guide selects the nearest object. If you accidentally select the old asteroid position, select NEXT in the dialog and it will move to another object in the field of view (usually it moves to the new asteroid position next). Once the dialog shows the name of the updated asteroid, click on the OK button.
    Now right click on the star to be occulted and hit the OK button on the info dialog to select the star. If you have entered an updated star position, via the user dataset feature, you must be sure to select the new star position. Try to use the trick of right-clicking on a spot just to the side of the new star position. If you "miss" the new star position and accidentally select the old star position, don't use the NEXT button. Just try clicking on the new position again. I have noticed that the Guide does better when you select one object and hit OK, then select the next object and hit OK - without a NEXT or any other object in between.
  7. Plot occultation path
    Under the "Extras" main menu item, select "Show eclipse path" and Guide will display the occultation path for the selected asteroid and star. If the "Show eclipse path" option is grayed out (not available), you should try again to select the asteroid and star. Once Guide displays the occultation path, you can use the mouse to zoom in on specific areas of the map. Guide also offers and option for saving the currently displayed map as a .bmp file (under the "Extras" main menu item). I have noticed that the .bmp file looks better if the display driver is set to true color mode (16 million colors) - the path is gray instead of yellow.


Page last updated: April 5, 2001.

Questions? contact Steve Preston