Monday, March 13, 2017

Using SkyTools 4 to Image the new Fragment of Comet 73P

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 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? 

Using SkyTools 3 I could switch between different locations in the Nightly Planner, looking for the one that gave the best view. 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 I have an advantage. 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.

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