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Stellar Eclipses/Occultations
By
Rob Robinson (ALS Member/IOTA Member)

Many people have observed lunar eclipses. Many people have also observed solar eclipses. Chances are there are a fewer who have observed stellar eclipses, or what is more commonly known as occultations. Occultations occur when any solar system body, passes between earth and a star. These bodies can be not only the moon, but also planets and asteroids.

For an observer on earth, just about any night the moon is visible, there is often an occultation of a star that can be seen in an amateur's telescope. Stars on the order of eighth magnitude and brighter are quite common. Most generally, the first time an observer watches the disappearance of a star behind the dark limb of the moon, they are quite shocked at how quick the event happens.

The International Occultation Timing Association (I.O.T.A.), is an organization of both amateur and professional astronomers, who on a regular basis, time the disappearance and reappearance of stars occulted by the moon. Timing occultations can be done with modest equipment, ranging from a 60mm refractor to a large Newtonian reflector; a shortwave radio to obtain WWV time signals; and a stopwatch. Many amateurs are now using video cameras to record occultations, which refine the accuracy of the observation from 1/10 of a second to 1/30 of a second, and allows a permanent record of the event. One can use a regular handheld camcorder for stars brighter than 3rd or 4th magnitude, or you can use a low light camera, such as the Supercircuits PC23A ($79) attached directly to the focuser to record fainter stars to ninth magnitude.

I.O.T.A. also records timings of stars by asteroids, commonly known as appulses. By strategically setting up observers along an "observing fence", perpendicular to the asteroid's motion, size and shape of the asteroid can be determined. It was in the early 1980's that David Dunham made the bold statement that asteroids had moons of their own---by observing the results of asteroidal timings---which later proved to be true by Hubble in the 1990's.

I.O.T.A. is a non-profit (501c) corporation. The administrative site (www.occultations.org), is maintained by Rex Easton, which gives information on membership, where to send information and officers. Another web site (www.lunar-occultations.com/iota) is maintained by Rob Robinson, and gives information on upcoming events, occultation predictions, asteroidal predictions, informative articles on "how to", software and related links. For more information, contact David Dunham (dunham@erols.com) or Rob Robinson (webmaster@lunar-occultations.com).

Value of Occultations and Grazes

I have often heard the comment, "why observe occultations and grazes as they have no relevance in the age of the recent probes we have sent to the moon. We know all about the moon". So how true is this statement?

A group of observers, on a graze expedition, travel sometimes fifty to hundred miles to a selected site, to see a four to five minute event, and then send the data in for reduction. Have they done this in vain? Have they already observed something that the lunar probes have shown us?

Mitsuru Soma, of the National Astronomical Observatory in Japan, gathers information relating to graze data. He states, "The Clementine laser did not probe the lunar polar regions, so visual grazing occultation observations, inherently more accurate in any case due to the grazing geometry (their accuracy depends more on a good knowledge of the geographical position of the observer than on the event timings), will continue to be valuable in the foreseeable future". He goes on further to state, "I think occultations can be used to analyze the errors of the Hipparcos proper motion system". Dr. Soma's analyses of Aldebaran grazes observed in 1979-1980, compared with those observed a Saros cycle later in 1997-1998 in the same part of the lunar profile, have confirmed that the FK5 proper motion of this star is more accurate than the Hipparcos ICRS proper motion.

Grazes are the most dramatic of all observed events, other than a total eclipse of the sun. Anyone who has observed a star graze comes away with a feeling of awe towards the mechanics of the earth-moon system. Since the lunar profile is constantly changing, each grazing occultation is different, since the Moon's longitude and latitude librations only repeat themselves every 18.5 years; therefore, each observed graze is different. Observing total occultations sometimes can bring notoriety to the observer when he detects duplicity in a star occulted by the Moon that had not been previously known to be a binary star system. This conclusion is made when a star can be seen to disappear/reappear in two distinct "steps" over a time-period of approximately 0.5-seconds or less. Since the Moon has no atmosphere, it acts as a "knife edge", bisecting EACH star in a Binary System.

Timing Total Occultations

Modest equipment is all that is needed to time total occultation predictions. By submitting your observations, you too can contribute to the science of astrometry. There are some things you need to know about equipment in order to successfully time total occultations.

Time signals are needed for timing occultations. Short-wave radio time signals, such as WWV and WWVH at 5, 10, and 15 megahertz are preferred. Radio Shack used to sell a convenient receiver, the "Weatheradio-Timekube", for these frequencies for about $40, but it is no longer available. A more expensive alternative is a digital SW-Radio sold by Radio Shack, part #20-229 for $100. It gives a much more reliable signal than similarly priced general-purpose short-wave receivers. If you buy a general-purpose short-wave radio, try to get one that at least covers 5 and 10 megahertz, the best nighttime frequencies.
Coordinates for your site are needed for predicting when an occultation/graze will occur in your area. Outside of buying USGS 7.5-minute survey maps, there are several on-line mapping sites which are good enough for total occultations. The "Maps On Us" mapping site (www.mapsonus.com) should be sufficient for total occultation predictions, but for grazing occultations, the 7.5 minute maps are preferred for accuracy.
You will need a stopwatch, which most observers find more convenient for making timings. Most stopwatches now are digital, although mechanical stopwatches can still be found. Methods that don't use a stopwatch include the Eye-and-Ear method and tape recording method. Personal preferences are to use stopwatch and the short-wave radio.
The choice of telescope for observing an occultation is predominantly limited to what the observer would normally use for observing. Whether you use a refractor, reflector or Schmidt-Cassegrain, you can observe occultations and grazes. Size does make a difference in the limiting magnitude of observations that you can make. Most generally, a six to ten inch primary will allow observations of stars to eight or ninth magnitude, which is usually the faintest that predictions will show. Smaller primaries will limit the observer to only the brighter stars.
If you have a camcorder, you can videotape the brighter star occultations (e.g. down to 3rd magnitude). With a video surveillance camera attached directly to the telescope, a 6" reflector will reach 9.5 magnitude. By using a video camera, with the time signal overlay, you have a permanent record of the event.
Predictions can be obtained by a variety of ways. If you are a member of I.O.T.A., you will automatically receive predictions from a regional coordinator. You can also generate your own predictions, by using David Herald's freeware program "Occult". If you are not a member, and do not wish to generate your own, this author will provide predictions for your site, free of charge (you must provide site coordinates and telescope size).
With the short-wave radio, stopwatch, predictions and telescope, you are now ready to start. The easiest way is to start the stopwatch on the event (when the star disappears). Then, stop the stopwatch on the next whole minute tone of the WWV receiver. Subtracting the stopwatch time from the whole minute time gives you the "raw" event time. Everyone has a "reaction time" which is normally around 3/10's of a second. Subtracting this from the "raw time" gives you the actual event time. This sounds complicated but it is actually very easy, and will become second nature after the first few timings.
More information on timing total occultations and equipment can be found on the I.O.T.A. web site (www.lunar-occultations.com/iota/) under "Resources and Other Information". The website also provides current information about upcoming occultations and grazes.

A Method of Making Occultation Observations

 

Much information can be read on the IOTA homepage for lunar and grazing occultations. The beginning observer should read the documents pertaining to the basics. These pages can be found at:
http://lunar-occultations.com/iota
For this article, I would like to explain my method of making observations. This is not the only method, but for me it is the easiest. Those who are members of IOTA will recall that this is referred to as the Taylor Method.
An observer, knowing his local coordinates in longitude and latitude to the nearest 50 feet, can use various computer programs to generate predictions for total occultations for his site. Two of the most common software packages are 1) Occult by David Herald; and 2) Lunar Occultation Workbench by the Dutch Astronomical Observers. To determine one's coordinates, the preferred method is by measuring your site location on a USGS 7.5-minute survey map. There is a software program, called MSDP on the IOTA homepage, where the freeware program can be downloaded. The program simplifies the procedure and eliminates having to do the tedious math, in order to triangulate your site coordinates. A GPS unit can also be used, but averaging over a period of time is necessary to get accurate coordinates. Methods for using a GPS unit for coordinate determination can be found on Scott Degenhardt's website, located at http://www.home.com/~dega/
There are various methods for timing occultations, but what has proven effective for me, using a stopwatch and WWW radio time signals, is described in the following steps:

1)    Be sure to set the scope up at least a hour before the event so that the it can acclimate to ambient temperature.

2)    I always try to locate the star at least 10-15 minutes

3)    Before the event. This also helps eliminate the problem of looking for the star in the wrong hemisphere of the moon,       when using higher magnifications.

4)    Use an eyepiece that will give a magnification between 75-100x for stars brighter than eighth magnitude. For fainter stars, higher magnifications can be used, though I rarely go over 125x.

5)    Whether you use a mechanical or electronic stopwatch, it should be checked periodically with WWV or CHU for accuracy. This can be done by starting the watch on a minute tone, letting it run 5-10 minutes and then stopping the watch on the minute tone. Do this 3-5 times, and then average the difference. Since there is a reaction time to starting and stopping a watch, you should subtract .3 sec when starting and .3 sec when stopping for a more accurate calibration.

6)    I usually start watching the star 2 minutes before it is to disappear or reappear. Any sooner than this, and the eye becomes tired and fatigued. Any later than this, and there is the possibility of missing the occultation.

7)    When the star disappears or reappears, I start the stopwatch. Then I go inside and turn on the WWV or shortwave radio. I watch the 1/10 sec marks, in relation to the clicks or tones from the radio. I then write down this 1/10-second number. The watch is then stopped on a whole minute tone. Write down the whole minute, and second shown on the stopwatch, and add the 1/10-second you recorded earlier.

8)    The time you wrote down in step number 6, is then subtracted from the time given at the whole minute tone. This is the "raw time" of the disappearance.

Example:
Star disappeared and stopwatch started.
Times clicks or tones, are heard when the stopwatch is at 7/10's of a second, each second.
Stopwatch is stopped on the whole minute tone, of 2:43:00 and stop watch shows 2m 21 sec and the tenths.
Ignore the tenths, since you already know that it is to be 7/10's.
Add the 7/10th of a second to stop watch time = 2m 21.7s.

To get raw disappearance time subtract:

2:42:60.0 - 2:21.7 = 2:40:38.3

1)    After you have calculated "raw time", your reaction time or "Personal Equation" (PE) needs to be subtracted from the raw time. For most individuals, this is 3/10 of a sec. Therefore: 2:40:38.3 - .3 = 2:40:38.0.

The 2:40:38.0 is then the actual observed event time. This is the time that is reported to the International Lunar Occultation Centre in Tokyo, Japan. More can be found on reporting your timings on the IOTA homepage. There is a special format, which can be used for email, which will make the reports easily imported into their computer program for data reduction.

By using a consistent and proven method of timing occultations, one gains confidence in his observations. Over the years, I have timed stars that were off the predicted time more than 4-6 seconds, which is actually quite a large difference. If I knew the actual observation was accurate, and the calculations were not in error, I send these observation times in to ILOC anyway, since the reason of the large difference, could have been due to incorrect positional information of the star.

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