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 ofcomet 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.
How I
Planned my Observation using SkyTools 4
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?
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.