Measuring Positions
The Hubble Guide Star Catalog is available on CD-ROM and contains accurate positions of nearly 20 million stars covering the whole sky. Without this aid, CCD astrometry would be almost impossible, since a typical field would not contain the minimum requirement of three reference stars.
Sky & Telescope photo.
There are several programs currently available for making astrometric measurements with CCD images. Two of the most popular are
Astrometrica and
CCD Astrometry. The former was developed by Herbert Raab; the latter is by John Rogers. Both programs require reference-star data from the Hubble
Guide Star Catalog.
Don't be misled by programs that produce approximate positions of objects in CCD images. Astrometry requires positional accuracy to better than 1 arcsecond. Despite the rigorous mathematics involved, the Raab and Rogers programs do the work in the blink of an eye with a few mouse clicks. Advanced image-analysis packages such as MIRA also perform astrometric calculations, but Astrometrica and CCD Astrometry are truly a pleasure to use since they have been customized for asteroid and comet astrometry with amateur CCD cameras. They even assemble the data in the proper format for electronic submission, a great advantage when you consider the mess that could arise from transposing even a single pair of digits in the multitude of numbers associated with a typical measurement.
Checking an Identification
In addition to being tailor made for astrometric work with amateur CCD cameras, the two programs mentioned above automatically format data for electronic submission to the Minor Planet Center. The objects in this message include 1995 SL5 (coded as J95S05L), a new object found by Dennis di Cicco being reported with his sequentially numbered personal code and an asterisk denoting the discovery observation, and the numbered minor planet 3503 formated with a leading zero in anticipation of the day when numbered asteroids reach the 10,000 mark. Click on the image to see the complete message.
Courtesy Sky & Telescope.
Once an object is located and measured, the next step is to see if it's known. For this there is no better place to turn than the Minor Planet Center, which offers one-stop shopping for all the needs of asteroid and comet observers. It is both the international clearing-house for discoveries and the place to submit routine astrometric measurements. It also has interactive programs for generating ephemerides of new and known objects as well as a host of other services. There is no charge for reporting discoveries and astrometric measurements, but a modest subscription fee is required for other services. Everything you need to know is available at the center's
Web site or by writing the Minor Planet Center, Mail Stop 18, Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138 USA, or you can
e-mail the center.
While there are sky-simulation programs that plot the positions of known asteroids, the ones I'm familiar with include only the permanently numbered asteroids (some 7,000 objects). But there are thousands more in the Minor Planet Center's database, and these should be checked before reporting an object as a possible discovery.
What do you do if you find an unknown object? Most important is to get positions on a second night. With a few exceptions reserved for objects having highly unusual orbits, the Minor Planet Center does not publish or give discovery credit for a single night's observations. Even in the case of known objects, it is most helpful if you can measure positions on a second night. When you have enough data to submit electronically, astronomers at the center will analyze and add them to more than one million positions already made by professional and amateur observers around the world.
