Above: Sample data panel and map of an occultation path. The solid black lines enclose the predicted path. The dotted lines enclose the area of uncertainty where an occultation might reasonably be expected to appear.
Sometimes an asteroid "occults" a star by passing in front of it and hiding it from the view of a well-placed observer on Earth. In such a case an observer in the right location would see the star disappear for a few seconds then brighten again. If the asteroid is bright enough, or if the star is a very close double, the star will seem to suddenly dim, leaving only the asteroid or the companion star visible for a short time. The occultation path across the Earth is roughly the width of the asteroid. It is impressive to see one star in the field of view blink out then reappear as predicted because of a solar system rock many millions of kilometers away. Occultations of first or second magnitude stars are more rare than total solar eclipses, but occultations of stars visible with backyard telescopes – magnitude 12 or brighter – are fairly common. It is not difficult for amateur astronomers to get scientifically useful information by timing occultations. Observers with common and inexpensive equipment can time these events to get information to help astronomers learn more about the asteroid as well as the target star.
On the home page of this web site we reguarly post a chart, listing upcoming occultations predicted to be visible within a two-hour drive of Toronto in the next few weeks. The chart lists the date, local time, magnitude of the target star, proximity to Toronto and a link to the web page for the event maintained by the International Occultation Timing Association (IOTA). These websites have links to North American maps showing the predicted paths.
The exact location of stars and asteroids is not certain, so the maps give a central path where an occultation is likely, plus a region on either side of the path where the occultation is still reasonably possible.
It’s worthwhile to try from your home in the “possible” area because the predicted path might be incorrect or a satellite of an asteroid might be observed. A report that the star did not dim is useful because it may limit the size of the asteroid. The “Location” column in the chart gives the approximate straight line distance and direction from downtown Toronto to the nearer occultation edge.
How are the times useful?
Accurate timings of asteroidal occultations can help refine the exact size and orbit of an asteroid, and even the exact position of the occulted star itself. Also, stars have been discovered to be members of previously unknown double star systems using these techniques; and in a few cases a satellite of the asteroid has been found or, if previously known, its size and orbit about the parent asteroid have been refined.
If several observers record disappearance and reappearance times, chords of timings can be combined to determine the shape of the asteroid such as shown below. This is information that cannot be found using the largest telescopes in the world, either on the ground or in space. Those telescopes lack the resolving capability to see details of an asteroid only a few dozen kilometres in diameter beyond the orbit of Mars. The only way to find these details is by timing asteroidal occultations, or by visiting the asteroid with a robotic space craft. Ground-based occultation observations are MUCH cheaper!
How to time occultations
There are 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.
1. Simple Observation
The simplest technique, which also presents the least value scientifically, may be useful to get you started in the enterprise. Just set up a telescope (or use binoculars if the star is brighter than ~7th magnitude), locate the target star and watch carefully for a sudden drop in brightness within 3 or 4 minutes of the predicted occultation time.
2. Stopwatch Method
There are a few different ways to use stopwatches, but a preferred way is to use two. “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 radio broadcast, (frequencies are listed in the "Radio and Recorder" section below) 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 reaction times to get your official D and R. See "Personal Equation" 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. This is a common feature of digital wrist watches. 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 D. Add the Dur to D to get R.
3. Cell phone or tablet app
Use either "Emerald Timestamp" (Apple) or "Time The Sat" (Android) to obtain very accurate times. For best timing results you must be in WiFi or cellular service range. Set up your telescope in the usual way and locate the target star. Select the appropriate app on your device. Begin watching about three minutes prior to the predicted time. If you see a drop in brightness, in the case of the iPhone or iPad, simply tap the on-screen button and again when it returns to normal. Android users press the volume button. Keep watching for 3 more minutes or so! You might see a second D and R if the asteroid has a companion. If so, tap as before. The device will record all your timings. Be sure to use one of the above apps. Do not rely on the standard clock on your device. It is not accurate enough. Apple users: label each time and email all timings to yourself to preserve an accurate record. Read the < Help> section to learn how to do this. Android users: do likewise as instructed in the Time The Sat app. Note the times and subtract your reaction time from those times. See "Personal Equation" below. Note also the coordinates of your location, using a GPS unit or a topographical map, to the nearest tenth of an arcsecond, if possible. Include your corrected times and your location in your report.
4. Radio and Recorder
This is one of the three most common – and preferred methods – of timing occultations. (cell phone app, above; and video recording, below, are the others.) Besides a telescope, the observer needs to have a short-wave radio receiver and a portable audio recorder. Set up the telescope in the normal fashion. Tune the s/w receiver to CHU at 3.330 MHz, 7.850 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 pre-set 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 recorder close together and near your position so that the recorder can pick up both the radio time signal broadcast and your voice. Important: Be sure that the recorder is set to continuously record and is not in voice-activated mode! 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 we prefer “Gone” and “Back” because of their explosive first consonants, which help in doing careful tape analyses later.) Continue watching and recording for an additional 3 minutes, in case of a secondary event (“satelloid”?). Then stop tape, pack up your gear, note the coordinates of your location using a GPS unit or a topographical map, to the nearest tenth of an arcsecond, if possible 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. See “Reporting your results” below.
5. Video Recording
This technique removes the human component from the record. It requires more equipment and more 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-sensitive video camera, such as the Supercircuits PC 164-CEX2 - http://www.supercircuits.com/security-cameras/indoor-security-cameras/super-low-light-b-w-video-security-camera-pc164cex-2 - which is then connected to a video recorder and a monitor. A time inserter device such as the IOTA-VTI (www.videotimers.com/home) is installed in the queue between the camera and the recorder/monitor. This puts an accurate visible digital clock on the screen and on the recording. It makes data reduction very fast, simple and accurate. For back-up, a short-wave receiver can be set up and tuned to a good CHU or WWV signal, and 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.
Note: Because of timing and sensitivity problems caused by compression, recording directly to a laptop or similar device is not recommended. Instead, most IOTA "occultationists" use digital videotape camcorders that are equipped with "A/V In" capability. This is a feature that only seems to have been included with some digital videotape camcorders, such as some Canon ZR series camcorders. They are out of production, but regularly show up on E-Bay, Kijiji and Craig's List at very reasonable prices.
(Back to the task at hand) 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). You can also generate a light curve by downloading the video in avi format and analysing it using Limovie http://www005.upp.so-net.ne.jp/k_miyash/occ02/limovie_en.html or Tangra (http://www.hristopavlov.net/Tangra3/) software. Report the results. (One frame = two fields.)
Note to CCD camera users: Normally, CCD cameras designed for astro-imaging (SBIG, etc.) are not well suited to occultation timing because of their slower integration times. But there is a way. It’s called the drift method and is described in detail by Australia’s John Broughton at http://www.users.bigpond.com/reedycrk/driftscantiming.htm.
If you can, try to use Method 3, 4 or 5. Method 3 (cell phone apps) is probably the easiest to get started with; methods 4 and 5 can come later. 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 apps method, toss in a stopwatch or two just in case. If you go whole hog to video, take your cell phone or tablet along just in case.
Reporting your results
If you time an occultation, or even if you observe no change in the target star's brightness, it is important to report your data to the International Occultation Timing Association. IOTA gathers such information from observers around the world, reduces the data, generates asteroid profiles and passes all gathered and calculated materials to the Minor Planet Center of the International Astronomical Association.
To report your information, go to IOTA's reporting centre at http://www.asteroidoccultation.com/observations/Results/ . In a box near the upper left corner of the web site, you'll find "Report Form Templates". Use it. You'll also see an article title: "What Information Should I Submit after Observing an Occultation?". Read it and follow its advice closely. Fill out the form and e-mail it to the address provided on the form.
Your personal equation (P.E.) is your reaction time, the time interval between the sighting of an event and the action taken as a result of it. This time lag is important to know if one expects to submit accurate occultation timings made by non-video recording techniques. A good way to determine your own P.E. is by using the Emerald Timestamp (iPhone, iPad) or Time The Sat (Android) app. See "Method 3" above. Watch the running clock and when you see a "zero second" (00, 10, 20, etc.) press the button. The extra tenths of a second displayed in your result is your P.E. Be careful not to anticipate the appearance of the zero second; wait until you actually see it. Repeat the process many times until you get consistent results. Remember that the important thing is not to be as fast as you can, but be as consistent as you can.
IOTA published a detailed occultation manual, Chasing the Shadow: The IOTA Occultation Observer's Manual which is downloadable free of charge at http://www.poyntsource.com/IOTAmanual/Preview.htm
We highly recommend this ebook.
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