Over the years, I have done many facets of amateur astronomy
including visual observing, film photography, CCD imaging, variable star
observing, and occultation and graze timings. In recent years, I have developed
an interest in asteroid research including astrometry - the recording of
positions to determine orbits; and photometry - the recording of brightness to
determine rotational periods.
What got me started in asteroid work is an article written by Dennis di Cicco of Sky and Telescope
magazine on just how easy this is to do from the city. Dennis related that fact
that he could make new discoveries every night. Things have changed since
that article was written several years ago. With the alarming realization that
Near Earth Asteroids pass close to the Earth on a regular basis, several
professional search programs have come on line in order to identify all of
these close asteroids over the next ten years. These automated
professional programs regularly sweep the sky on every clear night and have
found most of the brighter asteroids that amateur astronomers used to find.
Now, unless you are away from city lights and/or operate a larger telescope,
you are not likely to find any 'New' asteroids (I say that knowing that
amateurs still find new asteroids every day).
There is more work than ever to be done. There are now more
than 60,000 numbered asteroids, those with a well-known orbit. Rotational
periods have only been determined for about a thousand of these asteroids. A
brief description of the necessity for measuring asteroid light curves by Dr.
Alan W. Harris can be found here.
I subscribe to the Minor
Planet Mailing List which gives a lot of good ongoing information on
the subject of measuring asteroids. The loose group of professionals and
amateurs that subscribe to this list, also host an
annual workshop.
A workshop on photometry (which I am on the organizing committee) is hosted by
the Society for Astronomical Sciences
(formerly the IAPPP Western Wing).
Normally installed in the observatory is a C11 on a Losmandy mount controlled by an Astrometrics
GOTO System. Currently, a C14 on a
The main reason for configuring the instrument, as it exists,
is to allow for automated data gathering. The telescope takes an image of
an object, slews to a different section of the sky, takes and image of another
object, and so on. The data can then be reduced while the telescope is
operating, or the following day. The automatic nature of the equipment allows
for much greater data being gathered than ever before. On a typical
night, 80 to 150 CCD images are gathered of various asteroids and comparison
stars. The software also controls the Optec filter slider.
Obtaining asteroid light curves is done by obtaining
observations of an asteroid over several nights or weeks and charting the
change in brightness on an X-Y chart. By charting where the peaks and the
valleys on the light curve lay, you can find a repeating pattern that allows
you to determine how fast the asteroid rotates.
The average main belt asteroid looks something like a potato.
Therefore, during each rotational period, it shows two elongated sides to us,
and two shortened ends towards us. During those times, the light curve
will show two bright peaks, and two dim troughs. This average asteroid will
rotate once in an eight-hour period, allowing you to see most of the curve with
one night's data. Others, can be much more difficult.
Some of these pesky asteroids will take several days to rotate just once.
That means you need night after night of data to build the lightcurve. Even worse, is when the asteroid’s
rotational period is close to 24 hours. Here the asteroid will show nearly the same
face to you every evening making it difficult to complete the entire
lightcurve. In this case, a
collaborative effort with observatories spaced around the Earth is
advantageous. I often collaborate with Glenn
Malcolm, Bob Koff, Stephen Brincat or Brian Warner.
The target
Asteroid is first chosen determining which targets are within range of the
telescope and are favorably placed for a long, overnight run. Asteroids with
well-known lightcurves are eliminated by referring to Minor Planet Lightcurves
Parameters list posted at the Brian Warner's CALL Site. I tend to pick
dimmer asteroids, in the 14th magnitude range, to work on, even those the
curves may have a bit more scatter than the brighter objects.
The observatory
is in heavily light polluted, suburban skies. Exposures of 90 to 180
seconds will result in an adequate signal-to-noise ratio on asteroids down to
15th magnitude. A longer exposure requires guiding, and adequate guide
stars are not always available.
Accurately measuring the
asteroid brightness is a tricky task. You have to compare its brightness to
that of accurately measured stars. The distribution of those accurately
measured stars is much sparser than you might think. In a ten-degree square
section of the sky, there might be a couple of stars that have had accurate
photometry done on them. Furthermore, they certainly don’t lie in the field of
view of your target asteroid, which then has the nasty habit of moving from
field to field, night after night. Also, those stars might not be in the
range of brightness of your asteroid. All of the rest of those stars
plotted in your star charts are only good to within ½ magnitude
– if you're lucky!
The reduction program I use, Canopus,
by BDW Publishing, requires an initial measurement of high accuracy stars to
set the Magnitude/Intensity Relationship. This measures the performance
of the system and allows for proper interpolation of the measured
results. I slew the scope to a field close to the asteroid with stars of
medium to high accuracy magnitudes. These fields are taken from the LONEOS
catalog produced by Brian Skiff at Lowell Observatory, Flagstaff,
AZ. The system loads an image of the LONEOS field, aligns the image to a
catalog of stars, and measures every known star in the field. Then it
discards measurements that are substantially off the predicted magnitude.
The result is a sequence of measurements of between 4 and 30 stars in the red
magnitude band. From this, a linear relation of measured intensity vs.
magnitude is computed and applied to subsequent measurement.
The level of accuracy you must obtain is quite stringent. The
typical asteroid has an amplitude (change in brightness) of just a few tenths
of a magnitude. The professionals like to therefore measure to 1/100th
of a magnitude in order to generate a smooth, repeatable lightcurve. The
amateurs who endeavor to do this work, although seeking to obtain the Holy
Grail of ‘1% Photometry’, are usually satisfied in getting a 2% - 3% of a
magnitude accuracy.
The C11 is
controlled using MPO Connections, a telescope/camera control
and scripting program written by Brian Warner Publishing. The C14 and
Finding a match on the rotational period is also very tricky.
I use Brian Warner's Canopus
software as it is the only Windows based software I am aware of that will do
this function. Since the asteroid moves across different fields over the course
of the study, different comparison stars are used from night to night. As
I discussed, these comparison stars do not have well-established
magnitudes.
The rotational period determination is accomplished using a
routine developed by Dr. Alan Harris at JPL. This performs a Fourier Analysis
on the data, allowing different parameters such as number of harmonics, period,
size of period steps, etc. to be held constant while others are varied.
Finally, it does a least squares fit to determine the most likely period.
Now, you must apply judgment in picking the best fit. As I
previously mentioned, most asteroids show a classic two-peaked light curve,
which is symmetrical in shape. Often, the program will try to fit a three or
four peaked lightcurve, lopsided to one side. While this is not impossible, it
is highly unlikely. In an instance such as this, try an incremental harmonic
order scan to reduce the possibilities. Perhaps you just need more data.
email:
rstephens@foxandstephens.com