One of the things that really impressed me when I was imaging Comet C/2025 A6 Lemmon on October 26, 2025 was that the tail changed from frame to frame.  There appeared to be a “knot” that moved along the tail away from the comet head.

It’s taken me a while to figure out how to make a movie of this motion.  One challenge was that since it was just after sunset and the comet was near the horizon, the background level changed from frame to frame.  I realized that I could use Local Normalization and generate normalized files that helped with the varying background a lot.  I also started with separate red, green, blue data that had to be aligned using CometAlignment to make RGB images and then re-aligned with CometAlignment with the original green positions to show the comet motion with respect to the stars.  I ended up making two sets of images: one aligned to the green frames at each time step to show the comet motion relative to the stars, and one with all the frames aligned to the comet to show the comet tail changes. 

This movie was made with 15 frames of data using 1 minute each of red, green, and blue data.  The comet showed this much motion over about 45 minutes!  

Cool!

Camera geek info – telescope:

• William Optics Zenith Star 73 III APO telescope
• William Optics Flat 73A
• ZWO 2” Electronic Filter Wheel
• Antila RGB filters
• ZWO ASI183MM-Pro-Mono camera
• William Optics Uniguide 32MM F/3.75
• ZWO ASI220MM-mini
• ZWO ASiair Plus
• iOptron CEM40
• Sargent, Texas Bortle 4 skies

Frames – telescope:

• October 26, 2025
  • 16 60 second Gain 150 R lights 
  • 30 0.05 second Gain 150 R flats
  • 15 60 second Gain 150 G lights 
  • 30 0.02 second Gain 150 G flats
  • 17 60 second Gain 150 B lights (16 for stars)
  • 30 0.02 second Gain 150 B flats
  • Matching darks and flat darks from library