I was looking through my comet posts after my post yesterday, and I discovered I hadn’t posted this image of Comet PANSTARRS C/2021 S3 with two globular clusters that I made last spring.  Enjoy!

I hadn’t planned on imaging comets when we were in Dell City last spring, but when I saw this combination of two globular clusters and comet PANSTARRS C/2021 S3, I knew I had to try it.

Globular cluster M9, the brighter one to the left, is 25,800 light years away from us.  It’s 90 light years across, giving it an apparent size of 12 arcminutes.  Globular cluster NGC6356, the smaller one to the right, is 49,200 light years away from us.  Its apparent diameter is 8 arcminutes, giving it a diameter of 115 light years across.  Globular clusters are mind-bogglingly old parts of our galaxy and can be used to infer the age of the universe.  There are some interesting open questions about them, including their exact ages and whether they formed as part of our galaxy or were accreted later (probably a mix of both).  In the paper I found giving the ages for these two globular clusters, it shows that M9 is 14.60 ± 0.22 billion years old with one model, 14.12 ± 0.26 billion years old with a second model, and 12 billion years old in the literature.  It shows that NGC6356 is 11.35 ± 0.41 billion years old with one model, 13.14 ± 0.64 billion years old with a second model, and 10 billion years old in the literature.  No matter which age ends up being correct, ~10 billion years old is amazingly OLD!

Comet PANSTARRS C/2021 S3 was discovered by the Panoramic Survey Telescope and Rapid Response System located at Haleakala Observatory, Hawaii on images taken on September 24, 2021.  It reached perihelion (its closest point to the sun) on February 14, 2024 (the day after this image was taken) at 1.32 AU distance.  Its orbital eccentricity is higher than 1, meaning it’s on a parabolic trajectory and isn’t coming back.

I feel very fortunate that my trip out to the dark skies was timed so I could image this comet with two ancient globular clusters.  I also feel fortunate that I imaged it in a time when so many processing tools are being developed to make processing the image so much easier!  The tools I have this year are so much more powerful than the ones I had last year.

Camera geek info:

• Williams Optics Zenith Star 73 III APO telescope
• Williams Optics Flat 73A
• ZWO 2” Electronic Filter Wheel
• Antila LRGB filters
• ZWO ASI183MM-Pro-Mono camera
• ZWO ASiair Plus
• iOptron CEM40
• Dell City, Texas Bortle 2-3 dark skies

Frames:

• February 13, 2024
  • Interleaved LRGB lights
    • 5 60 second Gain 150 L lights (only used for the comet)
    • 5 60 second Gain 150 R lights
    • 5 60 second Gain 150 G lights
    • 4 60 second Gain 150 B lights
  • 30 0.05 second LRGB flats
  • 30 0.05 second darks
  • 30 60 second darks

Processing geek info:

• PixInsight
• BlurXterminator
• NoiseXterminator
• StarXTerminator
• ImageBlend