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Occultations

Contributed by Guy Nason, Toronto Centre

How to Observe and Record an Asteroidal Occultation

There are four different methods that can be used to observe and make a record of an asteroidal occultation. Each has advantages and disadvantages and all have varying degrees of scientific value. They are:

If you are successful in making an observation then be sure to report your results.

Simple Observation

The simplest technique, which also presents the least value scientifically, but may be useful to get you started in the enterprise, is to simply observe the event and note whether the target star dimmed or did not dim at or near the predicted time. Just set up a telescope (or use binoculars if the star is brighter than ~6th magnitude), locate the target star and watch carefully for a sudden change in brightness.

Advantages:

  1. Only very basic equipment is required
  2. A report that the star was, or was not, occulted may be useful, depending on other reports.

Disadvantages:

  1. The exact times of the events cannot be determined with any useful accuracy
  2. A chord can only be plotted on a sky plane diagram in a very coarse manner, if at all, and thus is of limited value
  3. Observations cannot be re-examined if the data need to be confirmed, such as in the case of a possible discrepancy with other observations.

Stopwatch Method

There are a few different ways to do this, but my favourite is to use two stopwatches. It works like this. "Zero" both watches. Observe the target star for 5 or 6 minutes centred on the predicted time for your location. When the star dims or disappears, start one of the watches. When it returns to its normal brightness, start the other watch. As soon as you can, get to an accurate time source such as a CHU or WWV short-wave broadcast, or by telephone via the talking clock at the Institute for National Measurement Standards - National Research Council at 613-745-1576. Stop both watches exactly ten seconds after a known top-of-minute beep, counting the beeps and hitting "Stop" right on the 10-sec. beep. Subtract the watch times from the known time when the watches were stopped. The earlier reading is the unofficial Disappearance time; the later one is the unofficial Reappearance time ("D" and "R", respectively, in Occultation-Speak). Subtract your "personal equations" (P.E.) or reaction times to get your official D and R (more on P.E.'s below). Note that television time displays, such as those on news or weather stations, are not accurate enough because of delays endemic in broadcast, cable and satellite distribution methods.

You can also do this with a stopwatch capable of "split" times. Reset the watch. Press "Start" when the star dims and "Split" (or "Lap") when it reappears. Note the interval time - this is your "Duration" ("Dur" for short). As soon as possible, get to a known time signal (see above) and stop the watch 10 seconds after a noted top-of-minute tone. Subtract the displayed time from the known time to get the actual D. Add the Dur to D to get R.

Advantages:

  1. Minimum required equipment
  2. With practice, times accurate to about 0.2 seconds can be obtained
  3. Data can be confidently used in generating sky plane diagrams. See below.

Disadvantages:

  1. As with a simple observation, no reviewable record is obtained, which means that data cannot be re-examined later.
  2. Accuracy can be lost if there is a long interval between the occultation and the stopping of the watches.
  3. No provision for obtaining times of any subsequent events such a possible double asteroid or a "satelloid" (asteroid satellite).

Radio and Tape Recorder

This is the most common method - and one of the two most preferred methods - of timing occultations. Besides a telescope, the observer needs to have on-site a short-wave radio receiver and a portable audio tape recorder (loaded with tape, of course). Set up the telescope in the normal fashion. Tune the s/w receiver to CHU at 3.330 MHz, 7.335 MHz or 14.670 MHz; or WWV at 2.5 MHz, 5 MHz, 10 MHz, 15 MHz or 20 MHz - whichever gives the strongest and clearest signal. It is strongly recommended to use a radio equipped with a tuner that can accept several preset frequency settings so that you can switch frequencies quickly and accurately should one frequency suddenly suffer signal deterioration, an all-too-common occurrence. Set the receiver and the tape recorder close together and near your position so that the recorder can pick up both the radio time signal broadcast and your voice. Experiment with a variety of placements of the equipment and make a series of test recordings to get good clear recordings of both the time signal and your voice. Three minutes prior to the predicted D, press "Record" (and verify that the tape is rolling and recording). When the star dims or disappears, yell, "Gone". When it returns, yell, "Back". (You can actually use any words you like, but I prefer "Gone" and "Back" because of their explosive first consonants, which help in doing careful tape analyses later.) Continue watching for an additional 3 minutes, in case of a secondary event ("satelloid"?). Then stop tape, pack up your gear, determine your location if not previously known and go home.

When you get home, or as soon as possible afterwards, replay the tape with a stopwatch in hand to measure the D and R times relative to the recorded radio time signals. Do this several times until you get consistent results. Discard obviously inaccurate times - you'll know - and average your findings. Then subtract your P.E.'s and report the results.

Advantages:

  1. Times can be obtained to accuracies of ~0.2 seconds.
  2. A permanent, reviewable audio record of your observation is made
  3. Secondary events can be recorded just as accurately as the primary D and R.
  4. Required extra gear (radio and recorder) are compact, light and inexpensive

Disadvantages:

  1. Accurate timings of occultations shorter than about 0.3 second are difficult, or may be missed completely.
  2. No visual record of the events, so confirmation of factors such as being on the correct star cannot be made.

Video Recording

This technique removes the human component from the record. It requires more equipment and serious battery power in the field, but the results can be spectacular. As with all the above methods, this one starts with a telescope set up in its usual way. After the target is located and confirmed, the eyepiece is replaced with a special low-light camera, which is then connected to a video recorder and a monitor. If available, a time inserter device is installed in the queue between the camera and the recorder/monitor. This puts a visible digital clock on screen and on tape (or whatever record medium is used). It makes data reduction very fast, simple and accurate. A short-wave receiver is also set up and tuned to a good CHU or WWV signal, which is connected directly to the "Audio In" of the recorder, or (preferred) a microphone is placed near the receiver and the observer (you), as in the Radio and Tape Recorder Method above, and it is plugged into the recorder. Make a few test recordings to ensure all is operating correctly. Begin recording three minutes before the predicted time and continue to three minutes after the last event (or predicted time). Pack up, measure your location, leave your site as clean or cleaner than you found it, and go home.

At home, replay the tape frame by frame (or field by field, if you can) and note the D and R to the frame (or field). Report the results. (One frame = two fields.)

Advantages:

  1. An irrefutable record is obtained that can be reviewed ad infinitum
  2. No P.E. estimate is needed
  3. Very short events can be confirmed
  4. 0.03 sec. time resolution is possible

Disadvantages:

  1. The cost of all that extra equipment can add up
  2. The extra equipment requires more time to set up and dismantle
  3. Batteries, batteries and more batteries (unless you're near an AC source)

Note to CCD camera users: Normally, CCD cameras designed for astro-imaging (SBIG, etc.) are not well suited to this method because of their slower integration times. But there is a way. It's called the drift - scan method and is described in detail by Australia's John Broughton here.

If you can, try to use the Radio and Tape Recorder method or the video recording method. Whatever you choose, try to have more than one option available to you in the field. Murphy and his Law ("If something can go wrong, it will") just love occultations, so having a back-up system is always a good idea. If you choose to use the Radio-and-Recorder (R-and-R) Method, toss in a stopwatch or two just in case. Or you can start with stopwatches and later on move up to R-and-R or Video, but keep the watches in your kit. If you go whole hog to video, keep an audio tape recorder handy, just in case (you already have the s/w receiver, so falling back to R-and-R is pretty easy).

Reporting Results

If you have data concerning an asteroidal occultation, IOTA would like to hear from you. When you have calculated your results and determined your actual location, please send your report either by e-mail or snail mail as you prefer. More detailed information and options for reporting are found here.

Please file your report even if you observed a miss. The lack of an occultation from your location is valuable information that puts a constraint on the asteroid's size. But if you were clouded out or experienced some other circumstance that resulted in no observation at all, please do not file a report.

Using OccultWatcher

This extremely useful program allows occultation astronomers to coordinate their observations and identify upcoming events of interest. It is well-integrated with published occultation information and Google Maps to allow you to plan your observing program. You can download it from Hirsto Pavlov's website.


Personal Equation Determination

Doug Kniffen has written a nifty little occultation simulator that you can download here. With practice you can develop a consistent personal equation (reaction time). Remember that consistency, not speed, is the objective.

Sky Plane Plots

Click here for several examples of sky plane plots. A particularly good example is the (1263) Varsavia event of 2003 July 18, that was observed by 81 occultationists from BC to CA.