Brian D. Warner
Palmer Divide Observatory
17995 Bakers Farm Rd.
Colorado Springs, CO 80908
The asteroids, or minor planets, of our solar system were once considered the
“vermin of the sky” since they often spoiled otherwise useful photographs with
their slightly trailed images, usually right over the item of interest in the photograph. The picture is now reversed. In recent years, the role of asteroids
has dramatically changed as new discoveries and theories emerge. We’ve gone from
a handful of large minor planets in a region between Mars and Jupiter (the “Main
Belt”) to a range of groups: Near Earth, Mars-crossers, Trojans, Centaurs,
Qbewannos, TNOs, KBOs, and a host of others in between. Learning how each set evolved and how each set is similar and different from any other set has dramatically changed our view of the solar system and lead to many a heated debate.
In recent years, asteroids have played the key role in development of the YORP effect theories, which states that the re-radiation of accumulated heat by an asteroid can significantly affect the orientation of the asteroid’s spin axis and even its spin rate. Much of the proof for these emerging theories will come from amateurs providing lightcurves of high and consistent quality over the span of many years. For example, it took one researcher and his team of graduate students nearly ten years to gather enough data to establish one of the first solid foundations of proof for the YORP effect and, even more profound, to establish that the YORP effect maybe the significant force in the evolution of spin axis orientation and rate of smaller asteroids.
The following sections will give a brief overview of some the many reasons that asteroid lightcurve work is important, how amateurs can go about taking part in this important field, and some examples of work done at the Palmer Divide Observatory.
Much of what’s in this article is extracted from my book, “A Practical Guide to Lightcurve Photometry and Analysis.” This book can be ordered through Bdw Publishing/Palmer Divide Observatory.
Unfortunately, at the time of this writing there weren’t any on-line articles of great detail on the YORP effect. If you visit the ADS abstract search page, you can search for “YORP” in the title and abstract keywords and find references to several articles. If you have access to the journals, you can do further reading. YORP is also covered in some depth in Asteroids III, which is available from www.amazon.com (it’s not cheap!)
There is much more than the rotation rate of an asteroid to be determined by getting its lightcurve. With that data, you can help reveal not only the secrets of the asteroid but also the formation of the solar system. Click here for just some of the things that can be done with or learned from asteroid lightcurves.
There are several sites dedicated to obtaining and working with lightcurve data. The most prominent are
Mikko Kaasalainen Shape Modeling Program
Steve Slivan’s Koronis Family Program
ALPO NEO Photometry and Modeling Program
Planned Radar Observations
If you have a scope of 20cm or more and a good CCD camera, you can get started with asteroid lightcurve work. There have been some people experimenting with uncooled web cameras. It appears some work can be done but if you want to participate in the more critical studies, you need a better camera so that you can do work on the order of 0.01-0.02m precision.
Click here for some of the considerations you should keep in mind.
Are filtered observations required? It depends on what you’re trying to accomplish. If you’re taking the most straightforward path and trying to determine just the period and amplitude of a lightcurve, filters are not usually needed. The exception (there’s always at least one) is if you are working an asteroid where you have to get many sessions over a stretch of time, say several weeks or even months. In this case, it helps to convert the magnitudes to a standard system so that you can more easily merge the data sets.
In any event, you must take into account the fact that the asteroid gets fainter or brighter due to changing Earth-Sun distances and phase angles. This can be done “ad hoc” by getting the data points from different runs to match by arbitrarily adjusting the data for one session up or down on a plot. However, it’s never a bad idea to put a little more science into such “fiddling with the data”, which is what reducing to standard magnitudes can do.
There are many excellent packages for controlling your camera and/or telescope, whether or not you are automating your system. A very small listing includes:
MPO Connections (Bdw Publishing)
TheSky (Software Bisque)
Astronomer’s Control Panel (ACP - DC3 Dreams)
Starry Night Pro (Space Software)
MPO Connections (Bdw Publishing)
CCDSoft (Software Bisque)
MaximDL (Diffraction Limited)
AstroArt (MSB Software)
Some programs provide the basics while others go all out and include scripting capabilities or can be scripted using Visual Basic or COM programming. The program you’re running and how you want to run it will dictate which software is right for you.
There are some very good and affordable photometry programs available to amateurs today. Most of the programs do much more than photometry, so you’ll get a lot more for your money than just being able to get date/magnitude pairs. Here is a partial list of what’s available; my apologies to those not included.
MPO Canopus (Bdw Publishing)
MaximDL (Diffraction Limited)
Mira (Axiom Research)
IRAF (IRAF programming group - UNIX based)
AstroArt (MSB Software)
There are tens of thousands of asteroids out there. Which ones are good targets for your observing program? The answer is another question: What are your goals? You need to define those to at least some degree and then start the process of selecting a target. One of the things you should do is visit newsgroups or Web sites to see what you might do to provide the most immediate help. Some group may need some timely extra observations so as to get important data while it’s still available. A passing Near Earth asteroid that is suspected of being a binary is just one example.
Those in for the long run usually define one or two general goals and then peck away a little bit at a time. This allows for occasional excursions from one area into another from time to time and so keep one’s interest going. I found the diversions to eclipsing binaries very helpful for my asteroid lightcurve work. Asteroids can be very reluctant in giving up their secrets and that is sometimes frustrating. A couple of nights on a shorter period eclipsing binary with its regular, symmetrical curve refreshes the spirit.
Click here for more on selecting targets.
It really is worth the effort to understand what you’re going to be doing before you do it. This way if things don’t go perfectly, e.g., the equipment malfunctions, you can concentrate on those issues and not worry about shooting the right target at the right time. In fact, if you’re well organized and the equipment is cooperating, you can go read a book, watch TV, or even talk to your family; build up that pool of Spousal Permission Units any way you can!
Click here for an observing run outline. This is, of course, just one way of many ways to go about your work. As time goes one and you learn more about what you’re doing, you may come up with your own methods and approach.
How you go about measuring your images depends on the photometry software package you use. If nothing else, develop and use a routine that assures you’re getting the best possible results from your images. This includes merging flats and darks as needed as well as any settings within the software that affect photometry. Once you’ve measured your images, you should have data in one form or another. Often it’s a simple text file with at least date/magnitude pairs that can be used for data analysis.
There are three general pitfalls when doing period analysis using data from several nights. They are:
Overstating the precision of the period of the lightcurve
Overstating the amplitude of the lightcurve
Being fooled by alias periods
I’ll leave it up to you to determine if the Palmer Divide Observatory represents the “real world.” Here are some links to sites from highly skilled observers who have done some important work in asteroid lightcurves.
Brian D. Warner Palmer Divide Observatory
Robert D. Stephens Santana Obseravtory
Raoul Behrend et al Geneva Observatory
Larry Robinson Sunflower Observatory
Petr Pravec Ondrejov Observatory
Among the first four alone – all amateurs, more than 200 lightcurves have been reported. For Warner and Stephens, most of the results have been published in the Minor Planet Bulletin. This is the place for amateurs to turn when they want to publish their results. Do not presume that putting the results on your web site is enough. Pages come and go and are hard to find. The articles in the MPB are indexed by the Astronomical Data Services (ADS) and are considered juried.
I recommend that you become a part of an organization that appeals to your interests. If variable stars are your game, then the AAVSO and/or its counterpart in your country is the place to start. Don’t just join, but take part by visiting the newsgroups supported by the organization or developed by others that act in parallel to the official groups. In particular, try to attend one of the annual meetings. It’s a great way to put a face to the email names you encounter, to discuss old and new ideas, and to build on what you’ve learned.
For asteroid workers, there is the Minor Planets Section of the Association of Lunar and Planetary Observers (ALPO). The section is responsible for publishing the Minor Planet Bulletin, which is the place to publish your asteroid lightcurve results. ALPO holds annual meetings that you should consider if in your general area.
The Astronomical League, the world’s largest federation of amateur astronomers, also holds annual meetings and has an Asteroid Observing club. This is mostly for visual observations but that doesn’t mean those involved don’t know their asteroids. The AL annual meetings are big affairs where you can meet a number of fellow amateurs.
While general meetings are often informative, sessions that concentrate on specific parts of the hobby and that include workshops are often the most rewarding in terms of picking up new ideas. The one I recommend most is the annual meeting of the Society of Astronomical Sciences. Formerly known as IAPPP-West, this group has staged its workshop in the hills above Los Angeles for many years. For the past few years the workshop has concentrated on getting the most science out of one’s equipment. The topics have ranged from improving photometry technique, spectroscopy, lightcurve work, searching for variable stars and extrasolar planets, to the building of an observatory complex for a college. You can find information about SAS at http://www.socastrosci.org.
The MPML has as regular visitors some of the leading asteroid researchers in the world. Anyone with an interest in asteroids is welcome and all related questions are welcome. The URL is http://groups.yahoo.com/group/mpml.
It seems the Internet offers no end to the number of newsgroups. You’ll find a number of those related to CCD imaging, asteroids, telescopes, photometry, and any other range of related topics. Search them out and use them. I can’t count the times I’ve been stuck with a problem or question of some sort and had it solved or answered in short order by posting a message to a news group. It is amazing the wealth of knowledge that has been accumulated by amateurs in recent times. It really does make one realize that the line between amateur and professional is drawn with a thinner pencil every day and that the label of “amateur” has less now than ever to do with the level of knowledge and experience.
I was fortunate to have two mentors over the years. Consider being one yourself. It’s one thing to share all that you’ve learned with fellow amateurs but it’s another thing entirely to help bring someone along who wasn’t necessarily interested to start. I’ve heard stories of everyday people who became top-notch asteroid observers simply because someone sat with them one night and showed them how to go about taking an image and measuring the position of an asteroid.
Some of them took on second lives, practically living at the observatory during every free minute. I remember the story of one person’s project to get high school kids involved in asteroid astrometry. In that program, one student went from nearly dropping out to being among the best in his class because someone took time to show him that all those things he was learning could be applied to things he did at the observatory.
Whatever and however much you do with lightcurve work, keep in mind that it should be fun to at least some degree. If you find it’s getting a little frustrating, try taking a visual or quick imaging tour of the Universe for a diversion. The asteroids and variable stars will still be there.
Enter the forum and ask your questions to the author!
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