Asteroid Occultation Strategy for New Observers

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Asteroid Occultation Strategy for New Observers 2015-08-14T15:33:29+00:00


If you want to organize a team of observers but have no experience in asteroid occultations, here are some ideas.  On a visit to Sarajevo, Bosnia-Hecegovina I introduced asteroid occultation observations.


In the photo above a group of students at the university under the direction of Nedim Mujic gathers to pack for the occultation expedition.  For the introduction of this topic into a new setting the following considerations must be made:

1. Determine the number of potential qualified observers. A qualified observer is someone who can operate a telescope and find the target star (based on the faintness of the star) on his/her own.

2. Determine the number of actual sites you can organize for the effort. This is based on the combination of the number of telescopes that are capable of detecting the star, the number of observers and the number of timing systems. In order to field one qualified site you must have: one telescope, one observer+one assistant, one timing system, and a digital watch that has been preset to a time signal. A timing system can mean simply a handheld battery powered tape recorder (wth fresh batteries and a tape rewound to the start) + a shortwave radio preset to a valid time signal station. If the time signals are received well the observer’s task is to watch continuously beginning 2 minutes before through and including 2 minutes after the predicted time of central occultation. The observer should look for one or more occultations lasting anywhere from a fraction of a second to the maximum number of predicted seconds. The observer watches through a telescope and has the radio and recorder placed near his/her mouth so both voice and ‘speaking clock’ record at the same level on the recorder.

If the radio fails, the digital watch can be used by the assistant to voice the 0th second of each minute into the recorder. Record continuously beginning 5 minutes before predicted time of occultation to 5 minutes after for this failure case only. Call out as the watch hits the 0th second at the -5 minute, -4 minute, -3 minute and -2 minute points. Then repeat at the +2, +3, +4 and +5 minute points. The assistant must place the recorder close to the mouth of the observer for the 4 minute window beginning at -2 minutes before central occultation time.

Another method is to use video with a GPS time inserter. Without time recorded, the data is of little use! CCD integrating video cameras should not be used; standard 30 frames/second video systems are preferred.

You can have more than one person at a site for security but please avoid clustering groups of people and telescopes in one place or within meters of one another, since that wastes potentially valuable resources and can be distracting if there is a lot of conversation.

As an example, if a group has 5 stations and the path is 100km wide, consider to place each site 20-25km apart perpendicular to the path. If you have only one station, then you should make the appropriate site choice. If the path is wider or narrower, adjust the spacing appropriately.

3. Determine the latitude, longitude and altitude of each observation site using Google Earth. Google Maps can be found for occultations at

4. Look at the map of the occultation path and attempt to determine how many sites you can field and whether you can bracket the width of the path out to the path error limits. If you do not have enough stations to cover the path width, determine what you can do and how far you can drive. Always take into account traffic patterns and road closures.

5. The key to success is mobility. If you are using fixed stations, there is a big chance for failure. Mobile stations allow you to circumvent weather problems. Be prepared to move at any time and watch the sky to see where clouds might be forming. It is better to be mobile and try to find another site if clouds threaten than to sit under a cloud and not be able to do anything about it!

6. Use animated satellite weather tools if they are available to you over the internet. Engage local weather forecasters to help you in advance. Forecasters have access to the best tools and are experienced in such advice.

7. Lay out your sites so that you can use the fastest roads. You do not want to use roads in poor condition where you cannot move quickly unless there is no other choice.

8. Choose safe, dark sites without obstructions in that part of the sky where the occultation is predicted to occur. Safety is the most important aspect of choosing a site! If your site is not safe, abandon it a nd find another. Do not pick sites near highways, railroad tracks, sources of bright lights, bars, national borders, prisons or places where police or criminals are likely to frequent. Carry a cell phone where possible and GPS and find a place where oncoming headlights will not illuminate your or the telescope.

9. Protect the telescope from dew. If there is even the slightest chance of dew forming use a dew shield or dew removal device. Bring towels for your equipment, preferably white ones so you can avoid losing parts in the dark. Towels can also be used to protect radio/tape recorder from dew. If equipment experiences dew and it can be brought into an automobile, use the automobile heater to blast hot air on it to help remove the dew as a last resort. Consider to build a hose (such as one that goes from a clothes dryer to the vent) that can go from the heater to the telescope from lens. Such a hose must be short enough so the benefit of heater air is achieved.

10. Time signal sources are generally short wave stations. Each station has a unique characteristic tone and sometimes voice announcements. It is vital to know how to recognize the time signals and to be sure (especially if there are no voice announcements) that you know the precise minute associated with each tone. Data collected without time annotation is almost impossible to utilize.

There are two types of time recording: a. GPS time inserter which can embed GPS time signals into a video stream; b. audio signals received on short wave radio that can be heard and recorded on a portable tape recorder.

11. TRAIN OBSERVERS BY GOING ONLINE: First time observers need to know what to expect. Assume your observers will watch through a telescope/eyepiece. If you have visual observers there will always be ‘personal equation’. This is the delay which each observer experiences in calling out the ‘D’ for disappearance and the ‘R’ for reappearance. For new observers this can be as long as 1 second. Such a time error is quite bad! The idea is to minimize the delay by preparing each observer ahead of time. Go to Google and find ASTEROID OCCULTATION VIDEO links on You Tube. You can download the video or else have each observer go online independently and watch using a tape recorder and radio; they should go through the actual experience of an asteroid occultation before performing the real one! This will help to shorten the reaction time.

12. Video is the best recording method since it eliminates personal equation. The video can be used for future training and it can be replayed over and over (unlike visual observation). Before attempting video recording be sure the telescope and video camera/recorder are capable of detecting the target star.

13. Finding the correct star is important. If you are focusing on the wrong star, the effort becomes a total loss. Find the star days ahead of the event date. To be certain, locate the star by confirming other stars around it using the various charts located at the URL associated with the occultation in question. For each even there are 5 levels of charts from wide field of view to narrow field of view.

14. Experienced observers should be used to train new observers. Even if experienced observers have no asteroid occultation experience themselves, the fact that they can set up a telescope and find a (faint) star using charts is a key skill that must be learned.

15. Dependence on GPS telescope mounts can be a problem as well as an asset. It takes time to perform an alignment. In case there are clouds in the sky this can delay locating the star, perhaps too long! The best telescope mounting to use is one with an equatorial wedge. If the GPS fails to align, you can always point the telescope manually and use tracking.

16. Tracking in the field is critical. You want the telescope to do the ‘work’ for you and tracking the star is very important. That way you can concentrate on making the observation instead of pushing the telescope around.

17. Finding the target star can be time consuming. But the 5 levels of star charts located at the link will allow you to make your way to the star step by step, using progressive charts with decreasing fields of view. The best way to find the star ahead of timeis to use a software program called GUIDE8. This allows you to create what are called ‘prepoint charts’. Prepoint charts helps those to watch the occultation who do not have a telescope with a motorized tracking capability such as a Dobsonian. These star charts permit you to set up on the correct sky position (elevation/azimuth) of the target star far ahead of time, assuming the occultation does not occur shortly after sunset. GUIDE8 produces a sky track along the same declination as the target star and annotates the times along the track. The idea is to find a bright star that is easiest to locate and to know that this star is at the key position at a certain time.

For example, would it not be easier for you to locate a 7.0 magnitude star instead of one that is 10.0? If your target star is 10.0 and your telescope is capable of letting you see it, then the smart thing to do is to find a brighter prepoint star in the hours ahead of the occultation that will be located at the same declination and sky position as the target will be in at occultation time.

In the figure below you can see such a track.


Let us pretend the occultation occurs at 6:19 GMT. In the figure you can see the star located at the end of the straight line. This is the target star. Note how the line passes by other stars which are annotated with time in GMT. At 6:01 the track passes by a very bright star. This is one star you can detect much more easily than the very faint target star. So the idea is to find this star and have it centered in your telescope field of view at exactly 6:01. Then turn off any tracking motor and let the telescope remain there until the time of occultation. At 6:19 the target star will be in the field and you can begin your observation at 6:17 assuming the field of view is large enough.

In the next figure, notice the star magnitude scale in the lower left. Here the track of the line is shown passing other bright stars prior to the occultation time. One bright one appears next to the track between 4:42 and 4:43 GMT. A second bright one appears between 4:54 and 4:55. This means if you have prepoint charts available to you even many hours before the occultation, you can set up the telescope, point it at a bright star (assuming there is one along this early track) and leave it alone until shortly before occultation time. Such a very early set up is only advisable in a very dry climate. Be cautious that if you are in a humid environment, dew can form and impact a telescope/eyepiece surfaces. Therefore you should cover all affected surfaces and keep a dew heater running or a hair dryer available to help evaporate any dew.


WARNING: The only negative about this process is that you should be sure to have a wide field eyepiece, as wide a field as possible. We suggest one that provides at least 0.5 degrees. Since the earth rotates 1 degree in about 4 minutes of time your window of observation needs to cover that interval. The star will likely be outside this field of view beginning 2 minutes before central occultation time. If you have a 1.0 degree wide field then, the star will be within the field of view and you should confirm it with the 5 levels of charts available at the link. If the occultation lasts more than about 10-15 seconds there is a chance the target star could drift out of the field of view during the occultation if the field of view is small. You must do all you can to prevent this especially if you are using a large aperture telescope with a tiny field of view. Should you have a small field of view and cannot increase it, we recommend you use a tracking mount and locate the target early and not rely on prepoint charts.

18. Reducing your data. Once you have recorded voice calls on a tape recorder (along with time signals) you must be able to reduce the data. A good way is to use an accurate stopwatch and to key off the first second of the minute marker closest to the occultation. If the event above occurred at 07:46:23, then you would want to key off 07:46:01. The question then is why not use 7:46:00? The reason is that most time signal generators have a major tone at the 0th second and it may take some time to recognize it. Start your stopwatch at the 1st second and let it run until you hear the call out for the star’s disappearance. Then immediately stop the watch. You will have to perform this action many times and create an average. Always report the time in GMT and not in local time. Perform the same reduction for the reappearance. Write down the two resultant averaged times. Then you must query the observer and see if he/she can tell you how long the delay was between the time the event was recognized and the time he/she called out. This should be done as soon as possible after the event. In fact, the observer should annotate into the tape recorder a few minutes after the observation was finished as to the estimated delay of each call out.

19. Always have a contingency plan prepared. This means think ahead of time what you would do if any piece of equipment were to fail or if your site became unusable. If you are not able to recover from a failure then this is a recipe for disaster. Things like extra eyepieces and batteries are vital.

20. Report form: use the report form located at

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