Book Review: Occultation Observer’s Handbook, by Walt Robinson and Hal Povenmire, First Edition, 2006.
This paperback book is available for $25. Information on how to purchase is at http://www.lunar-occultations.com/rlo/shop.htm
For many years the International Occultation Timing Association (IOTA) has been encouraging observers to time occultations. The stated purpose of this book is to explain how to get started in doing this. In its 270 pages, it succeeds admirably in describing the equipment and techniques needed to successfully observe the various types of occultations. The basic organization is by the type of occultation to be observed. It starts out with a brief history of occultations, and then proceeds through total occultations (lunar), grazing occultations (lunar), planetary occultations and grazes, and asteroid occultations. That organization is good since it also represents going from more simple observing techniques to more sophisticated ones. At the end of the book there is a glossary, bibliography & reference section, and a charts, tables and picture section.
Total occultations (T.O.):
In this section the authors describe total lunar occultations as the moon passing in front of a star. They outline the methods and equipment used for timing them. The equipment can be fairly cheap. The minimum is a shortwave radio and a stopwatch. You also need to accurately know your location. USGS survey maps or a GPS is useful to determine this. There are also on line resources to find your location accurately. Any telescope can be used although longer focal lengths have advantages. For this work any location can be used. This type of occultation can be observed just about any night the moon is visible. IOTA provides software that can predict the occultations from your location. The book details how to read the output of several software packages. There are multiple occultations from most sites every night. In order to accurately time a T.O. you must take your “personal equation” into account. The personal equation is your personal reaction time. There is simulation software (one version written by Doug Kniffen, well known in the St. Louis area) that allows you to establish your personal reaction time so that you can subtract it from the timed event.
The International Lunar Occultation Center (ILOC) collects and maintains data from lunar occultation observations from all over the world. Your observations can be submitted in a standard format described in the book.
You can further extend the range and accuracy of your observations by means of video equipment. A low light security camera or a camcorder is capable of recording many T.O.s.
Most observers time the disappearance of a star on the moon’s dark limb. It is very difficult to time its reappearance on the bright edge. After full moon stars reappear on the dark edge and can be better observed. Since this occurs late at night, the number of reported observations of reappearances is much lower than that of disappearances. This means that the eastern edge of the moon is far better determined that the western edge.
Data obtained from this technique was more important before we left laser reflectors on the moon. Since the moon varies in distance and size as it travels its orbit, calculations based on this data was originally used to determine the moon’s orbit. While this data is less important now, it is the best way to learn the basic techniques for observing occultations, and how to properly time them. At least 0.1 second accuracy is required for useful data.
Grazing Occultations (G.O.):
The moon appears to move eastward by its own diameter each hour occulting the starry background. If an observer positions himself properly, he will see the occulted star appear to ‘graze’ the edge of the moon. In this instance, the star will be seen to wink in and out as it moves behind the mountains and valleys of the moon. This is a grazing occultation. As you might imagine, this occurs over a much smaller area than a total lunar occultation. I.O.T.A. provides free software to predict grazing occultations for your longitude and latitude. Often you have to travel to observe these since they are somewhat rare at a given location.
As you might expect, preparing for a G.O. is a bit more complex than for a total occultation. Timings are desired to be 1/30 of a second in this instance. Usually a video camera attached to a tracking telescope and the shortwave radio mentioned above are the equipment needed for useful observations. The ‘personal equation’ is eliminated by catching the time data on the audio portion of the video tape.
Since the moon is traveling eastward, grazes will appear near the north or south pole. The ‘cusp angle’ (angle between the terminator and the graze) is calculated and used to predict where the graze will occur. The difficulty of the observation is a function of how much the moon is illuminated and the cusp angle to that illumination. The book does a very good job of explaining how big a scope one needs to observe a given G.O. For example, a general rule is “A graze becomes marginal when a 7.0 magnitude star grazes a 70% illuminated moon 7 degrees from the cusp with a 7 inch F/7 telescope at 70x.” Given that, regular graze predictions are for stars brighter than 8.5 as things become quite difficult. They explain how to read the data output of the IOTA program so that one can be successful at observing these.
The authors say that the primary purpose of observing grazes is to refine the moon’s motions and position in ecliptical latitude. Total occultations can accurately determine the RA or longitude of the moon. An accurately time graze can pin down the declination of the moon to within an accuracy of 50 feet. It is hoped that within a few years the moon’s position will be known to within 50 feet at all times.
An interesting sidelight are the very close binaries that have been discovered by this technique. By using video cameras and analyzing the appearance and disappearance of the stars, many double stars with less than 0.1 arc seconds separation have been observed disappearing one at a time behind the mountains of the moon. These discoveries have been confirmed by spectroscopy since doubles that close cannot be observed visually.
A grazing occultation also gives us a very accurate position on the star that is being occulted. In fact, stellar diameters can be measured by this technique. This data can be used as a reference system for stellar movement over time. This data is also important over time in determining the rate of rotation of the earth.
The authors again give you everything you need to get started. Telescopes, appropriate video cameras, radios and such are all discussed in detail. It turns out that ebay can be a source for an inexpensive video camera for this type of work. Security cameras are also available that lend themselves to this work. For further help, IOTO members on line have two yahoo groups that discuss upcoming events and even have on line maps for many of the grazes.
Asteroid Occultations (A.O.)
The third type of occultation of interest to IOTA is the asteroid occultation. These have some characteristics in common with the Lunar grazes in that they occur over a very small area and one often has to travel to observe them. They are more difficult to observe than the grazes in that the occulting asteroid is much more difficult to find than the moon. Often, the asteroid is invisible, being too dim to observe. The way you observe the occultation is by observing the magnitude change when the (usually) dimmer asteroid passes in front of the (usually) brighter star. Often the magnitude change is large. This allows you to make accurate timings of these events.
The equipment needed is similar to that used for lunar grazes. In the case of the asteroid occultation, the most sensitive video camera is the best since both the stars and asteroids are quite dim. There are several reasonably priced security camera that can record magnitude 10-11 stars using a 8” SCT with field flattener. Add accurate timing by recording the short wave signal on the video tape and an accurate knowledge of your location with a GPS unit (or accurate mapping). These can also be done visually with a stopwatch and tape recorder as with the total occultations.
The information obtained from this process is quite valuable. Using the timing method above, each observer obtains a chord which represents the size of the asteroid measured from his location. By having multiple observers, the multiple chords give a representative cross section of the asteroid and the size and shape can be ascertained. In addition, there have been cases where a second occultation have been observed showing that the asteroid in question has a moon.
As in the other sections, the authors take you through the process stepwise and discuss the details of how to obtain the data. They discuss methods of reading the data provided by the Occult software provided by IOTA so that you can understand what you need to know to successfully make and report your observations.
Conclusion:
I felt this was an excellent “How To” book to get started in this area of astronomy. If you have been thinking about wanting to make a contribution to astronomy, but don’t quite know how to get started, this may be just the book you have been looking for. It is reasonably priced and has a easy-to-read style that makes it go quickly. There is a lot of historical information that is quite interesting and puts the observations of occultations into perspective. I have my stopwatch, shortwave, and video equipment and I am ready to get started!
John Duchek
Moon Hunters NASA’s Remarkable Expeditions to the Ends of the Solar System by Jeffrey Kluger, previously published as “Journey beyond Selene”. © 1999, First Touchstone edition, 2001. Published by Simon & Schuster. This paperback is available for $14 and has 315 pages. The center pages of the book contain high quality color photos of the 4 Galilean moons of Jupiter, Saturn’s Titan, Uranus’ Miranda, and Neptune’s Triton.
