Hunting Asteroids From Your Backyard
A convergence of technology has opened the realm of asteroid discovery to virtually any backyard observer.
You can discover an asteroid tonight. Digital technology and the CCD revolution have given amateurs the ability to do it.
At first glance, these may not seem to be profound statements. After all, you might discover a nova, supernova, or a comet too; amateurs have proven they can do these things. But in the case of asteroids there's a major difference. If you decide to search for them on any clear, dark night, you can be virtually guaranteed of success in your quest, whereas with the other three the chance is rather slim.
That's the conclusion I reached in early 1995 after accidentally discovering eight asteroids with a CCD-equipped 11-inch telescope in the course of following known objects during the previous 12 months. A new object had turned up in one out of six CCD fields covering 12 by 16 arcminutes each. This was small-number statistics for sure, but I reasoned it would be an easy evening's project to shoot six random fields and find something new.
In October 1995 I decided to put my idea to the test, and on the night of the 12th I imaged five overlapping fields near the ecliptic in Pisces. Covering a patch of sky only 18 arcminutes wide by 50 high, I was prepared to blame failure on the nearly 90 percent illuminated Moon nearby. To my delight, however, three moving objects appeared in the very first field! One turned out to be a known asteroid somewhat off its predicted position, but the other two were new and I received credit for their discovery.
Lucky night? Not really. By the close of the year I had searched on another eight evenings, usually by imaging several fields adjacent to those containing known objects, and examined a total area of about nine square degrees near the ecliptic. On all but one night I was successful, chalking up an additional 21 confirmed discoveries 18th magnitude or brighter.
A convergence of digital technology that includes CCDs, computer software, e-mail communication, and CD-ROM star catalogs has opened the realm of asteroid discovery to virtually any backyard observer. Here's what you need to know.
You begin asteroid hunting by taking two or three images of the same field during the course of an hour or so. With appropriate software you then align the images and alternately display them in rapid succession on a computer monitor the cyberworld equivalent of a photographic blink comparator. Anything moving in the field stands out like the proverbial sore thumb. The once-tedious task of measuring precise positions is now replaced by a few minutes clicking a computer mouse.
Just as impressive is the speed with which objects can be identified as new. Via the Internet or a direct modem connection to computers at the Minor Planet Center in Cambridge, Massachusetts, in less than a minute you can get a listing of all known asteroids and comets in any selected region of the sky. If a moving object doesn't turn up on the list, there's a good chance it's new.
All that remains is to make a confirming sighting on a second night (multiple sightings on one night don't qualify). Then you put the positions you've measured in a standard format and send them electronically to the Minor Planet Center. Usually within a day (sometimes within hours) an e-mail reply will acknowledge the observations and, if the object is indeed new, assign a designation.
It's worth mentioning that all this work can be done with commercially available hardware and software, and the whole process goes amazingly fast. Some evenings I've sat down at the computer keyboard to image, blink, find an object, measure its position, check if it is known, and prepare the data for electronic submission (assuming confirmation on a second night) without ever getting up from the chair.
As easy as all this sounds, there are some basic protocols for recording and measuring the positions of faint asteroids, whether known or newly discovered. Let's consider them in the order in which they occur.
There are no hard and fast rules regarding the telescope or CCD camera needed for asteroid work. To be effective, the system should record stars as faint as 18th magnitude with a single, 4-minute exposure. Almost any CCD camera on an 8-inch telescope can do this under a clear, dark sky.
While common sense tells us that a large field of view is helpful in any search program, a practical limit for asteroid work is set by image scale, which should be around 2 arcseconds per pixel. Having a smaller scale (covering more sky per pixel) will limit the positional accuracy of astrometric measurements. If the scale becomes too large, not only does the field of view become unnecessarily small, but the system's sensitivity drops, especially for moving objects that expose a given pixel for only a limited time. (A typical main-belt asteroid near opposition moves about 0.5 arcsecond per minute.)
Under average amateur observing conditions, pixel scales around 2 arcseconds give nearly optimum detectability of faint stars. Furthermore, at this scale frames taken about 10 to 15 minutes apart will show obvious asteroid motion when blinked.
To be useful for astrometry, the central time of an exposure should be known to an accuracy of one second (0.00001 day) or better if the object in question is moving rapidly. Some cameras automatically log the time of exposures directly from the host computer's clock, so the clock should be checked against radio or telephone time signals at the beginning of each night. Furthermore, users should note whether a camera logs the beginning or ending time of an exposure. The midtime is what you need to report, and for a single exposure it is simply half of the exposure duration added to the starting time. In the case of multiple exposures stacked to create a longer effective integration, the calculation is more complicated and good record keeping is necessary.
You also need to know the longitude, latitude, and altitude of your observing site to better than one arcminute. This information is readily obtained from topographical maps or a global-positioning-system (GPS) receiver.
This is the key to finding objects, and there's a lot to be said for experience. A multitude of pitfalls await the unwary. Hot pixels, random cosmic-ray detections, and a host of other CCD artifacts can mimic moving asteroids and fool the novice. After blinking hundreds of images during the past two years I have a better appreciation for a story that circulates among comet hunters. The best of them seem to know instinctively when an object in the eyepiece is a comet and not some faint galaxy because "it looks like a comet!" Indeed, there's something about a real asteroid that just looks right when images are blinked.
A good way to rule out image artifacts is to have three or more images and blink them in various combinations. And for this it helps to have equal intervals between exposures. I was surprised how difficult it was to identify a faint object when blinking one set of images separated by 20 minutes and another pair separated by an hour. The eye-brain combination expects to see similar motion in both instances.
Even better than blinking pairs of images is to assemble sets of three or more into an animated movie loop. This way even the faintest objects appear to jump out of the screen. Axiom's MIRA software has a particularly effective routine for aligning images and creating animation loops.
Some computerized blinking routines "compress" several CCD pixels into a single screen pixel to get the image to fit a particular display window. Experience has shown that this compression tends to mask faint objects.
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
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.
While there's nothing to prevent someone from hunting asteroids the first night out, I believe there's a better way to begin for reasons that involve human nature and cold facts. Computers do a wonderful job cranking out data, but all those delicious-looking numbers following the decimal points don't mean the answers are indeed accurate! Learning to work with any new technology takes time.
I strongly suggest starting out by observing known asteroids. First of all it will give valuable practice pointing the telescope to a star field where the target looks just like a star! After three decades as an observer I found this to be more of a challenge than expected. Fortunately, today there are excellent computer programs for generating star charts down to 15th magnitude based on data in the Guide Star Catalog. Popular ones include Guide 7.0, Maris Multimedia's RedShift 4, SkyTools by CapellaSoft, SkyMap Software's SkyMap Pro 7, and The Sky by Software Bisque.
Observing known objects gives you valuable experience blinking and, especially, measuring positions. Numbered asteroids (those with the best-known orbits) rarely appear more than a few arcseconds from the location predicted by up-to-date orbital elements (available from the Minor Planet Center's computer) and serve as good targets for checking one's observing techniques. It's easy to compare measurements with predictions to verify that your astrometry is being done correctly before you actually submit anything.
After a few such tests you can turn to more significant astronomical quarry. The Minor Planet Center maintains lists of "unusual" and "critical" objects those in exotic orbits or rarely observed in recent years. Chasing them provides needed data and also builds your credibility as an observer.
The center's director, Brian Marsden, and his colleague, Gareth Williams, have spent untold hours helping amateurs who appear capable of contributing useful astrometric data. But neither of them suffers fools gladly; they can't afford to. Each month they process thousands of observations from around the world; it isn't possible for them to "handhold" every beginner.
However, if you proceed cautiously and test observing procedures, there's a good chance the first positions you submit will be accurate. The reward will be the issuance of a "station code" for your observing site, and your data will be published in the monthly Minor Planet Circulars along with those from major observatories. There's a certain amount of pride that goes with seeing observations from "817 Sudbury" appearing alongside those from "809 European Southern Observatory," "691 Steward Observatory (Spacewatch)," and "675 Palomar Mountain."
Here again there are no hard rules. Chances of success are better when you are hunting near the ecliptic, and most objects exhibit a slight boost in brightness near the opposition point (exactly opposite the Sun in the sky). Looking away from the ecliptic drastically decreases the odds of finding something, but there's a good chance any such nonconformists will be moving in unusual orbits. Those of us living at temperate latitudes find the ecliptic low in the sky during summer months, thus offering another reason to search high latitudes.
Hardware also plays a role in search efficiency. Locking onto one field and taking three or four exposures spaced over an hour is one approach, especially with a telescope that lacks motorized slewing. Several different fields can thus be covered during an evening, and statistics suggest that after a few nights something will turn up.
Another possibility is to take a sequence of images of adjacent fields, a process that is particularly easy with computer-slewed telescopes or those equipped with accurate digital setting circles. Bear in mind that blinking is most effective when the fields of view match almost perfectly. If the scope is moved for any reason, you must center it again on the same field for the second exposure in a set. While my experience is limited, I've found that several of today's computer-slewed telescopes are remarkably accurate at returning to the same sky location when the coordinates on the display panel match for each exposure. This repeatability seems independent of the telescope's absolute pointing accuracy (how well the displayed coordinates match the corresponding sky location).
There's also a search strategy that guarantees useful data choose a field known to contain an object on the Minor Planet Center's unusual or critical list. There's just as much chance of having something extra in this field as there is looking at some random part of the sky. You can further improve your chances by shooting one or two fields adjacent to the target in the time that needs to elapse between any pair of frames slated for blinking.
With efficient methods of moving a scope between exposures, it's easy to take four pairs of overlapping frames in an hour with matching frames separated by about 30 minutes. From October '95 through January '96 I have never failed finding a new object when observing this way for two hours on a given night there's a lot of stuff out there waiting to be discovered, and it doesn't take long to find it!