I just recently got around to finishing up processing a batch of images of Pluto that I captured on August 3rd. At the time of the imaging session, Pluto was just two weeks past opposition, with opposition having occurred on July 20th.
On August 3rd, Pluto was moving slowly against the starry background at 3.5 arcseconds per hour. Pluto’s apparent movement on that night was slow in comparison to other minor bodies in the solar system, but pretty fast in comparison to itself. For example, earlier in this month on October 2nd, Pluto was only moving at 0.5 arcseconds per hour. At this speed it would have taken two successive nights of imaging to show the same amount of movement I captured in just two hours on August 3rd.
Capturing the images for this animation was a bit of a challenge. On August 3rd, Pluto’s V magnitude was predicted to be 15.0 by the Minor Planet Center and Lowell Observatory’s Ephemeris Service (NASA’s Horizons service was an outlier, predicting a somewhat brighter magnitude 14.34).
A magnitude 15 object is a pretty faint target for my 8-inch telescope, especially since I was imaging from a bright, Bortle 7, backyard, looking straight into the washed out Oklahoma City light dome to the south. In any event, I was pleasantly surprised to see that after slewing the telescope to Pluto’s coordinates, the dwarf planet popped up in the field of view, albeit faintly, right where the Lowell Observatory’s Asteroid Finder chart showed it would be. I haven’t done any photometric measurements on these images, but by my eyeball estimation, Pluto looks more like magnitude 15 than 14.
Processing the image sequence into an animation also proved a bit of a challenge. But, ultimately, the challenge taught me a new processing technique. I normally process animations by opening the time-sequenced group of images as a set of layers in the image processing program GIMP. I then manually align the background stars in each layer with a base layer (usually the first in the time sequence) and export the entire aligned sequence as an animated GIF.
In this case, however, for some reason, I could not manually align the layers. There was always a stubborn half a pixel difference between layers that I could not wring out. This tiny misalignment of background stars between layers caused the stars in the animation to appear to bounce around. After posting the animation on the Cloudy Nights Forum, I received some helpful comments and suggestions that led me to a solution, a solution that made the animation process easier and produced a much better alignment of the background stars.
The solution was using a program called DeepSkyStacker (DSS). I already had DSS, and had some familiarity with using it. The trick was to load the time-sequenced images into DSS and let it produce a sequence of registered (star-aligned) images, but not have it take the final step of stacking the images into a single image. Instead of having DSS create a single stacked image, I opened the registered sequence of images it produced as layers in GIMP, added some stretch and red circle highlights to each layer, and then exported the layered image as an animated GIF. The result: near rock-steady background stars with the only discernable motion being Pluto itself and the natural twinkling appearance of the background stars.
But, there’s more. DSS also calibrated each image so the background, star brightness, and color levels of each image were more uniform. I usually do this manually in GIMP, but for some reason, this was also a vexing problem with this group of images. Thanks to DSS, when I moved the sequence to GIMP, I only had to apply a simple stretch to each layer to bring the background and foreground levels to an eye-pleasing state. And finally, there was another bonus, DSS is freeware.
This project challenged my image capture and processing skills. But, it showed that my modest 8-inch telescope and camera could capture a magnitude 15 target even under heavily light polluted conditions. And, it also taught me a faster, easier method for producing animations of the smaller members of our solar system. Thanks to this project, I’m looking forward to finding and capturing smaller and fainter objects.
Image Details:
August 3, 2022 04:31:02-06:31:13 UT
Two hour time-lapse animation.
North is up. East is left.
FOV: 16.9×12.1 arcmin Original scale: 1.49 arcsec/pixel
Seeing: Good
12 image animation sequence. Each image is stack of 10 frames at 30 seconds, gain 200.
Captured with SharpCap
Processed with DeepSkyStacker & GIMP
Telescope: Celestron C8 (203mm SCT f/10) + Celestron 0.63 focal reducer/flattener (f/6.3)
Camera: ZWO ASI482MC
Mount: Celestron CGEM
Vmag (MPC): 15.0
Motion (MPC): 3.5″/hr toward 256 degrees