Plotting Asteroid Occultation Paths
with Guide
Setup
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. oneoff.data) 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.
Instructions
- 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.
- 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."
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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 |
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- 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).
- 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.
- 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.
- 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.
- 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