Regardless what your target is going to be, you must consider some issues common to any selection process.
There’s no point trying to work an 18th magnitude target with your 0.2m scope. You may be able to expose long enough to get it well enough for astrometry but down to 0.01 or 0.02 precision is going to be very difficult, if even possible. Here’s data that Arne Henden present to a meeting of amateurs and professionals in 1999 that can serve as a guide.
Telescope (m) Limiting Mag
0.2 13.5
0.4 15.0
1.0 17.0
2.0 18.5
The table assumes good optics, the use of a V filter, a 2-minute exposure, and a main-belt asteroid. That latter point – if you’re working asteroids – is important because you don’t want the object to trail too much. Otherwise, the light is spread over several pixels, which increases the noise. Also, it’s more difficult to accurately measure a trailed image. If the object is moving more slowly, e.g., a main belt asteroid near the stationary point or anything beyond the main belt, you can use a longer exposure. On the other hand, if the object is zipping across the sky, the exposures must be shorter and so your limiting magnitude is less.
There is a technique called “stack and track” that allows you take several shorter exposures and stack them on top of one another to create a single image that has the equivalent SNR of a much longer exposure. The technique also accounts for the motion of the object and keeps it as nearly circular as possible, which does cause the stars to trail. While it is harder to measure trailed objects, it’s easier to measure a brighter trail than a fainter one.
By this I mean the time you’ll be able to work the target on a given night. If it’s considerably shorter than the known or estimated period of the target, then you’ll get only a partial curve at best. If you’re trying to fill in gaps from previous runs, then it’s acceptable situation. However, if you don’t know the period at all, you want the longest possible run. Now we’re in a Catch-22.
A large majority of asteroids have periods less than 12 hours, with a significant portion of those in the 4 to 8 hour range. The figure above shows a chart of the number of asteroids versus the period in hours. Use that as your first best guess. For variable stars, you are probably working an already known star and so can figure more accurately how long you can follow the star on a given night.
Keep in mind that the target should be at least 30° above the horizon at all times. This rule can shorten a useful run considerably. In one case I was working an asteroid at about +7° declination. I’m at +39° latitude. This means the star never gets more than about 60° above the horizon. Naturally, I was trying to work this asteroid during the start of summer. The combination of the short night and the shortened time the asteroid was high enough allowed only 4 to 5 hours a night. I was fortunate; the period was between 5 and 6 hours so each run covered a good portion of the curve.
In short, use the dark time to determine which targets are better suited to
getting useful data. Of course, in some cases the circumstances dictate you get what you can when you can. If
the asteroid is at 14th magnitude for the first time in 25 years and won’t be that bright again until Jean Luc Picard is warping
about the galaxy, you should definitely concentrate your efforts on that asteroid in lieu of others.
What you’re trying to avoid is getting just enough data from a single run that when combined with a run several days later leads to a number of valid solutions. Petr Pravec of Ondrejov Observatory, Czech Republic, recommends that if you’re starting a new target that you try to do so such that you’ll be able to get two consecutive nights. Then, if you get data several days removed from the existing sets, you have a better chance of finding the true period.
Yes, it’s easy to say that one should stick to this rule, or at least try. Most
amateurs don’t have desert locations where runs of several successive nights are
more the rule than exception. Do the best you can but do check the weather forecast. If clouds are fairly certain the next night and you’re torn between
getting another night on a target that could use a little more data and starting
something new, get the additional data on the previously worked target. Then you
can hope for a brief run of at least two consecutive nights on the new target before it is out of reach for whatever reason.
You can find the CALL site at http://www.MinorPlanetObserver.com/astlc/default.htm. It has several services that are helpful for selecting targets.
This list is maintained by Alan Harris who periodically posts official updates. I make unofficial additions to the file to include results published in the Minor Planet Bulletin and on some Web sites, including the CALL site itself. This lists lets you know for which asteroids there are known lightcurve periods and amplitudes and the quality of the results. The rating system uses values of 1 to 4.
There are several caveats to this system. The most important is that a 4 rating means only that a pole position has been reported. The often assumption in this case is that the period must be very secure. That is not always true. In one instance, the asteroid had a rating of 4 but the data for the period justified only a 2. In fact, the posted period was eventually proved wrong.
Another assumption that’s often made is that if the asteroid has a 2 rating that it is not worth working. This is a bad assumption. Remember that a 2 rating means the period is in question, by a fair amount at times. I do recommend that if your goal is to determine lightcurves for which there is no previously reported results, you should tend to avoid those with a 2 rating and concentrate on those not on the list or with a 1 rating.
When reporting results, there is a tendency to be overly optimistic about the quality value. Be careful about that. With a few exceptions, many of the 3 postings I’ve seen on the CALL site really deserved a 2 when the lightcurve and/or data were reviewed.
Here’s the usual exception to the rule: a high school student looking to get started in asteroid lightcurve work selected an asteroid with a 3 rating. She reasoned that she could more easily concentrate on technique by working something with a well-established period. She could not get her data to match the published period. It turns out that period was wrong! I definitely do not encourage that you set out to verify the results of already published lightcurves but I do say that making assumptions can be very hazardous to one’s sanity.
This allows you let others know that you are working a given asteroid or that you need help getting a solution. Just because someone has an asteroid reserved doesn’t mean you can’t work it. However, it would make for a “good neighbor policy” to contact the person to see if they are still working the asteroid or would like some help.
The CALL site is where many amateurs post their results pending or post publication in something formal, such as the Minor Planet Bulletin. Again, because someone has posted a period doesn’t mean that additional work might not be useful. Of primary importance when posting results is to be honest about the quality rating of your results. Also keep in mind that when you post your results they fall into public domain.
There is a basic search utility that allows you to search for an asteroid in the reservations or results data by number, name, or both.
There are links to other popular Web sites featuring lightcurve results or observing programs. You should visit these to help assure that the asteroid you’ve selected hasn’t already been worked. Here are two of them, both related to shape modeling:
http://www.bitnik.com/mp/alpo/
If your goal is to work only those asteroids with no published data, you may have a hard time confirming that. You can visit the Astrophysical Data System (ADS) Web site and do a search for the asteroid. Its URL is
That may turn up something you didn’t find elsewhere. Still, unless something has been published and put into the ADS database, you’re not going to find it.
At some point, you have to concede you may not be the first but you can certainly be the latest and try to be the best.