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Comets, asteroids, and Astrometrica.

Since its introduction, this software has radically changed the way amateurs do astrometry of comets and minor planets.

In July 1995, when Alan Hale and Thomas Bopp stumbled upon the comet that now bears their names, one of the first things they did was determine its position in the sky. They did this by eyeballing as best as they could the comet's location relative to background stars and then plotting it on star charts to find the celestial coordinates. Their positions, which are good to about a few arcminutes, were sent to the Central Bureau for Astronomical Telegrams (CBAT) in Cambridge, Massachusetts, where the discovery was verified.

In order to compute the comet's orbit around the Sun, however, more precise positions were needed. This is where observations by other amateurs and professionals equipped with photographic and CCD cameras came in. Images could be measured with an accuracy of a few arc-seconds. Armed with such precise coordinates, Brian G. Marsden of CBAT was able to quickly derive a preliminary set of orbital elements for Comet Hale-Bopp, which, in turn, suggested we were in for a good show.

Astrometry, the science of making precise positional measurements of celestial bodies, has become much simpler and more straightforward with the advent of personal computers and commercial CCD cameras. All you need is a good-quality image of the object that includes three or more reference stars whose coordinates are accurately known. Astronomers measure the relative positions of the object and stars and, using simple mathematical formulas, convert them into the object's right ascension and declination (S&T: July 1990, page 71).

Several astrometry programs are currently available for personal computers. Among them is Astrometrica, developed by the Austrian computer engineer and amateur astronomer Herbert Raab. Astrometrica is a powerful, user-friendly program that works with reference stars in the CD-ROM version of the Hubble Space Telescope Guide Star Catalog. With Astrometrica, positional accuracy of 1 arcsecond or better is possible with just a few keystrokes.


Born in 1969 in Linz, Austria, Raab became interested in astronomy when he was 12 after receiving a small spotting scope from his parents. In 1995 he obtained a degree in computer science from the Johannes Kepler University. Last year he became president of Linzer Astronomische Gemeinschaft (Astronomical Society of Linz), the country's oldest organization founded by amateurs.

In addition to skywatching, Raab also pursues his other hobby, geology. "As a teenager, I was as much interested in geology as in astronomy," he says, "but apparently the latter came out the winner. My interest in geology explains my fascination with meteorites and impact craters, and this, in turn, accounts for my passion for asteroids and comets."

Just a decade ago astrometry was very complicated and laborious. Two of Raab's colleagues, Erich Meyer and Erwin Obermair, found this out in the early 1980s when they attempted to do astrometry of asteroids using photographs taken with their 12-inch f/5 reflector.

Meyer and Obermair determined relative positions on their negatives with a measuring engine. They identified reference stars with star atlases and checked their positions in voluminous printed catalogs. This was followed by many minutes of number crunching with hand calculators to reduce the data into usable celestial coordinates. Meyer and Obermair's initial enthusiasm was soon replaced by frustration, and before long they abandoned astrometry in favor of less-tedious pursuits such as deep-sky astrophotography.

Brian Marsden's article, "What Amateurs Should Be Doing," in the November 1988 Sky & Telescope, page 462, sparked renewed interest in photographic astrometry. By this time, powerful but modestly priced personal computers had entered the market and databases for reference-star positions became available in electronic form. This made astrometric determinations a lot easier and faster. By 1990 Raab and his team were regularly submitting astrometric observations of new comets and unusual asteroids to the Minor Planet Center (MPC) in Cambridge, Massachusetts, where they were published together with those made at professional observatories worldwide.

The Program

The early 1990s saw a dramatic increase in CCD-based astrometric observations by Japanese and Italian amateurs. In 1993 Raab and his associates decided to experiment with images obtained with the SBIG ST-4 autoguider that Obermair bought for his long-exposure astrophotography. Since no popular astrometry software existed, Raab decided to write his own program.

The early version of what would later become Astrometrica was very crude, but the results looked so promising that Meyer soon purchased an ST-6 camera. "Since then, we have done astrometry exclusively with digital images," says Raab. "The more we used the program, the more new features were added, and with the help of Tim Abbott and Olivier Hainaut [then with the European Southern Observatory], the data-reduction procedures were greatly improved."

Raab realized that other amateurs interested in astrometry might find Astrometrica useful. "By some lucky circumstance," he says, "Laurence Marschall, editor of the new CCD Astronomy magazine, came across my software, and it was announced in the Spring 1994 issue. Without doubt, this was a major factor why this software has become so popular."


Astrometrica is shareware, so licensed copies cost just $25 each. "I thought only a handful of amateurs would be interested in this kind of specialized software," says Raab, "but I was wrong; today there are nearly 300 registered users, mostly in the United States, Japan, and Germany." These include several professional observatories, colleges, and universities.

Although Astrometrica was originally developed to do astrometry on comets and asteroids, it has also been employed for novae, supernovae, and even meteors. Others use it for photometry of variable stars, novae, and supernovae, and even for hunting asteroids. "Apparently, it was the right software at the right time," notes Raab.

Awards and Recognition

Since switching from film to CCD images, Raab and his friends have submitted more than 2,000 astrometric positions to the MPC, and discovered almost 30 new asteroids. In 1994 Raab became a consultant to the International Astronomical Union's (IAU) Commission 20 on minor planets and comets during its 22nd General Assembly in The Hague, the Netherlands.

For his work in developing Astrometrica, the IAU honored him in May 1996 by naming asteroid 3184 Raab. Last February 4th, the Austrian government awarded Raab a gold service medal and Meyer and Obermair silver service crosses for their work on comets and asteroids. The group's asteroid discoveries were the country's first in 73 years.

Raab currently works for a company that develops business software. He lives with his wife, Agnes, in Leonding, near Linz. Raab plans to continue updating the software. He's currently working on a new release that will be compatible with many commercial CCD cameras and the new, improved astrometric catalog from the U.S. Naval Observatory. He also hopes to come out with a Windows-based version of Astrometrica.

"When I see the many observations done with Astrometrica each month in the Minor Planet Circular," says Raab, "when I think of the many new asteroids discovered, trans-Neptunian objects measured, light curves of asteroids derived, and all other endeavors where my software was involved, I know that it was worth all the effort."

For more information on Astrometrica, contact Herbert Raab at Schrammlstr. 8, A-4050 Traun, Austria; e-maik k3032e0@; World Wide Web: astrometrica.html.

RELATED ARTICLE: Astrometry Today

CCD cameras have revolutionized the way astronomers, especially amateurs, measure the positions of asteroids and comets. In the past, only a few dedicated amateurs practiced photographic astrometry. It was a process that required a lot of fiddling, and few had access to a precision measuring engine for extracting data from negatives. CCDs simplified the hardware needed for astrometry, but it wasn't until the emergence of easy-to-use software, such as Herbert Raab's Astrometrica, that the process became truly Streamlined and enabled observers to concentrate on more observing.

You need look only as far as the International Astronomical Union's Minor Planet Circulars (MPCs) to see how all this has changed the world of astrometry. The February 1990 MPCs contained minor-planet observations from 37 locations. Of these, 21 were amateur sites, almost exclusively in Japan. Only two professional observatories contributed data obtained with CCDs. Seven years later, the February 1997 MPCs contain observations from 87 sites, of which 61 are operated by amateurs, and all but 12 employ CCDs. Japanese amateurs remain very active, but they are joined by a growing number of observers in Italy, the United Kingdom, France, Germany, Australia, Portugal, Brazil, Mexico, and the United States.

So what will the role of amateur astrometrists be in the future? At present there are three areas where they can make important contributions: the follow-up of newly discovered comets and Earth-crossing asteroids; the monitoring of multiple-opposition asteroids awaiting enough observations to become numbered; and the discovery of new objects. More observers are needed. Anyone interested in beginning their own astrometric program can find out more on the Minor Planet Center's Web site at

- GARETH V. WILLIAMS, Associate Director, Minor Planet Center
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Title Annotation:includes related article on innovations in astrometry equipment; astronomical software
Author:Aguirre, Edwin L.
Publication:Sky & Telescope
Article Type:Product/Service Evaluation
Date:Aug 1, 1997
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