(c) Skyhound

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 ScenesHow 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 Timethe 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 TimeThis 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. 

  • Scalea 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.  

  • Sizethe size of the coma of the comet in pixels on the image. This is of course related to the Scale parameter. 

  • Resolutionthe 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 Timehow 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. 

  • Fltthe 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.

  • Focusthe selection of focal reducer/extender (not variable for these telescopes).

  • Binsthe 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 

SkyTools 3, because the astronomy matters.

Read more about SkyTools 3

Read about the SkyTools 3 exclusive features for visual comet observing