I
took this series of twenty 40-second exposures on February 13
using iTelescope
T30 in Australia. The exposures covered about 15 minutes in
total. When made into a movie, they show the comet and its new
fragment moving across the sky.
Not
a Fragment of his Imagination
On
February 10th Thierry Noel circulated an image that revealed a new
fragment of comet 73P, apparently trailing behind the main body of
the comet. This new fragment was quickly confirmed and designated
73P-BT.
I
set about to observe it myself, with the idea in mind of checking
for other possible fragments, and this is how the movie above came
about. The seeing was poor that morning in Australia, there was
moonlight, and the flat field went awry (I'm still looking into
why), but the series of images served their purpose. As far as I
can tell from the movie, there are no other new fragments nearby. On
the right is a stack of the same images in the movie, but with the
stars trailed.
If you are interested in how I planned this observation, see
"Behind the Scenes" below.
A
Comet Known to Fragment
Comet
73P was discovered by Arnold Schwassmann and Arno Wachmann on May
2, 1930. Thus its full name is 73P/Schwassmann-Wachmann. It is in
a five-year orbit around the Sun, taking it out near the orbit of
Jupiter and then coming in as close as the orbit of the Earth.
Every three orbits it comes particularly close to the earth,
giving us a nice show.
During
the 1995 apparition it fragmented into four bright pieces (A, B,
C, D). In 2006 it put on a fantastic show, breaking into
many more fragments. Observations by the Hubble Space Telescope
revealed dozens of smaller pieces (right).
The
main component that we see now is the one designated as
"C" in 1995. As far as I know the other large fragments
have been lost. It would be an interesting project to try to
recover them later this month, and I would not be surprised if
some of them didn't turn up. Most have drifted away and are now spread out along a line that stretches over more than 15 degrees of
the sky.
Planning
for Visual Observation
The
fragment was discovered just as the moon began to interfere in the
pre-dawn sky, where the comet is located, so there are no visual
reports as yet. But from CCD observations it appears that the new
fragment us running approximately one magnitude brighter than 73P itself.
It can be problematic to compare CCD magnitudes with visual
magnitudes, but a rough estimate puts 73P and 73P-BT at magnitude
13.7 and 12.7 respectively.
Based
on this, SkyTools predicts that our best chance to observe this
comet pair visually will begin on the morning of February 19, just
as morning twilight is starting. This is true for all latitudes up
to ~50N, north of which it will likely be too difficult. From
other northern latitudes it will be less than 20o
above the horizon when best seen. Southern-hemisphere observers
will have a slightly better view at an altitude of ~30o.
The comets will be rising along with the sun, so the trick will be
to catch them before the twilight sky is too bright. An 8-inch (20
cm) or larger telescope will be required.
These
aren't ordinary predictions. Based on recent observations I compute magnitude, coma
diameter, and "degree of concentration" parameters.
These parameters are input into SkyTools and made available to SkyTools users when they
update their Current Comets observing list, which is always
kept up to date with the latest bright comets. Using these
parameters, SkyTools can use its scientific contrast model to accurately predict
whether or not the comet will be visible in a given telescope
under local conditions, and the time when you will have the best
chance to spot it.
Without
SkyTools, observers have only the magnitude to go on, but In fact, the magnitude of a comet is a poor predictor of
visibility in the eyepiece. This is because comets vary greatly in
size and how concentrated their light is. A large diffuse comet
will be much more difficult to spot than a small one, or one that
has a bright center, even though the magnitudes are the same. To
make things even more interesting, a bright twilight sky will
affect the visibility of the large or diffuse comet to a greater
degree than that of the small or concentrated one. It is at this
point that most people without SkyTools simply throw up their hands.
Behind
the Scenes—How I
Planned my Observation
When I first saw the post by
Thierry Noel, I first did a quick check to see if there were known asteroids or comets shadowing 73P, perhaps posing as a
fragment. This was a simple matter of opening the SkyTools Interactive
Atlas, centering 73P, and setting the UT date to that of his
image. As I suspected, there were no comets or minor planets near
the position of the fragment.
My
next thought was to act quickly to confirm his observation.
Through my iTelescope.net subscription I have access to 20
different telescopes located around the planet. But this
posed a problem that needed a quick solution: which of the 20
telescopes at my disposal was the best one to use?
With
typical Planetarium software I would need to set the view to
show the comet the next morning from each telescope location. Then
I'd have to vary the time to try to get a rough idea of how high above
the horizon the comet would be when twilight began. This would be
time consuming: changing locations, date/time, and looking up the
time twilight begins at each location. But the SkyTools 3 Pro Nightly Planner
puts all this into a simple graphic:
The
teal dashed line is the altitude of the moon. The red dashed line
is the altitude of the comet. The background is an accurate
depiction of the sky brightness throughout the night. But it is
the blue line that is most important. This is the relative imaging
quality. The higher the blue line the higher the quality of the
image you can obtain, based on the predicted signal-to-noise
ratio for a given telescope/camera and a model of the comet and
brightness of the sky. So based on what we see above, I can get
good images from about 4:30 - 5:30. Switching locations allowed me
to quickly compare them to each other, and I determined that the
Siding Springs location would be best. But I still needed to pick
a telescope, and that would mean comparing fields of view,
resolutions, and sensitivity. Once I picked a telescope, I'd still
have to calculate the motion of the comets, in pixels, to
determine the maximum exposure I could use before the comets began
to trail. That is still quite a bit of work.
"Ah,
but it turns out that I have an advantage over everyone else, and its a
big one."
I am currently the only person in the world with access
to the nearly completed SkyTools 4, and it has an awesome new tool
to help choose a suitable telescope quickly and easily:
Above
is a screen capture from the Compare Imaging Systems tab of
the SkyTools 4 Object Info, set for 73P on the Night of February
13/14. The first thing it does is to select the optimum image
scale for each Imaging System (telescope+camera) by varying
the available focal reducers/extenders and binning. Then it
selects the most appropriate filter. Maximum observing times,
image scales, and exposures are tabulated, and the list is sorted
so that the best systems are put at the top.
The
columns are as follows:
Observing
Time—the
total time that the comet is observable from the location of
the telescope. This is affected by the latitude of the
telescope and how low to the eastern horizon the telescope can
be used. T27 in Australia can observe the comet for 70
minutes, but T24 in California offers only 15.
Exposure
Time—This
column displays the total exposure time required to reach the
target Signal to Noise Ratio (SNR) which I had set to 80. It
allows us to compare exposure times between systems.
Scale—a
parameter that describes the quality of the image scale, from
0-100. Forty is considered good. As you can see, the comet has
a poor scale on all of these systems, but some are better than
others. Regardless, we aren't likely to detect features in the
coma or determine if the new fragment is really a cluster of
smaller ones.
Size—the
size of the coma of the comet in pixels on the image. This is
of course related to the Scale parameter.
Resolution—the
expected resolution of the image in arc seconds. This
considers the quality of the astronomical seeing (that I have
selected elsewhere as excellent) and the airmass of the
comet during the exposures.
Trail
Time—how
long you can expose before the comet starts to trail on the
image, or if we are tracking the comet, this is how long until
the stars begin to trail.
Flt—the
optimum filter to use. Once you have selected a system you can
decide to explore the use of other filters as well, perhaps
for a color LRGB image.
Focus—the
selection of focal reducer/extender (not variable for these
telescopes).
Bins—the
optimum binning, selected as a compromise between image scale
and exposure time.
The
available observing time for T24 was too short, and T27 wasn't
available. I ultimately chose T30 for my observation. For T30 the
coma would cover 24 pixels and it could reach an SNR of 80 in 5
minutes, which seemed like a good combination.
Have
a look at T07, which is located in Spain. This system would
require much longer to reach the same SNR, likely because there is
no Luminance filter available. With twenty telescopes, it is nice
not to have to remember things like that.
I
knew I wanted to make a movie, so I didn't want to track the comet
and trail the stars. Instead I wanted to take a series of shorter
exposures, limited by the time it would take for the comet to
trail from its motion across the sky. So the next thing I did was
to use the SkyTools 4 exposure calculator to make sure that the
SNR I could expect in a 40-second exposure, on that morning, with
T30, would be high enough, and it was.
To
complete my planning I created a SkyTools 4 Imaging Project that
defined what I wanted to do. Then I entered the project into the
Scheduler for the night of the 13/14th and it generated a plan for
ACP Planner, which is the control system used for the iTelescopes.
The observing time was scheduled and the plan was uploaded to the
telescope, and there you have it.
I
can't wait to share SkyTools 4 with everyone. I believe it will revolutionize
the way people plan their imaging, and not just for those who use
remote telescopes. What if you only have one telescope? Well,
consider this: the same calculations that made the imaging system
comparison tool possible can be used to answer other interesting
questions. For example, imagine you just bought an OIII filter and
wanted to observe an emission nebula. Which ones should you try
for? E.g. which nebulae are strong in the OIII and are suitable
for your telescope and location? SkyTools 4 has the answer
to that question.
Want
to be among the first to have SkyTools 4? For a limited time we
are offering a free copy of SkyTools 4 to everyone who purchases SkyTools
3.
Greg Crinklaw —
Astronomer and Developer of
SkyTools
On
February 10th Thierry Noel circulated an image that revealed a new
fragment of comet 73P, apparently trailing behind the main body of
the comet. This new fragment was quickly confirmed and designated
73P-BT. 
Comet
73P was discovered by Arnold Schwassmann and Arno Wachmann on May
2, 1930. Thus its full name is 73P/Schwassmann-Wachmann. It is in
a five-year orbit around the Sun, taking it out near the orbit of
Jupiter and then coming in as close as the orbit of the Earth.
Every three orbits it comes particularly close to the earth,
giving us a nice show. 
