Casitas de Gila, Gila, New Mexico
April 6 - 8, 2022

During my Spring trip to a dark sky site in New Mexico with galaxy season in all its glory, a pair of interacting galaxies made for an intriguing target.  For NGC 4490 and its companion, NGC 4485, I used my Celestron 9.25” SCT at prime focus (FL = 2350mm, f/10).  NGC 4490 was the first target of the night and I captured about 6 hours of data nightly using Hydrogen-a, Luminance, Red, Green, and Blue filters.  I imaged broadband data with 4 minute exposures and Ha with 10 minute subframes.  For these interacting galaxies, I was pleased with the results.  As compared to other objects imaged during the trip, NGC 4490 came out the best.  Was it the conditions, the equipment, image processing or exceptional piloting?  The equipment did not change, image processing was similar, and the piloting had its usual bumbling and share of mistakes.  However, the seeing and transparency improved over this time frame, and the moon was just beginning to cast more light across the sky.  Guiding performance varied from 0.6 to 0.8 arc-min, which higher than my best Losmandy G11 performance, but sufficient for the long OTA focal length.

I continued to have success with the NormalizeScaleGradient script applied to the aligned subframes, especially with changing background gradients from the light of the post crescent waxing moon.  I used StarNet2 in PixInsight to create starless images in the linear state.  Luminance and RGB starless and stars images, and the Ha starless image, were stretched using the GeneralisedHyperbolicStretch script followed by the application of the HistogramTransformation and CurvesTransformation tools to fine tune the appearance.  The Ha image was combined with the LRGB image using Shawn Nielsen’s PixelMath method (Visible Dark YouTube channel).  The addition of the Ha data enhanced both the red and blue color channels.  After background denoising with the TGVDenoise tool, the Ha+LRGB image colors were saturated and adjusted, and then the image was finished in Photoshop.  This was a galaxy where various sharpening tools, including HDRMultiscaleTransform, HistogramEqualization at three scales, and UnsharpMask, were used fairly aggressively, yielding acceptable results.

NGC 4490, also known as the Cocoon Galaxy, is a barred spiral galaxy type SB(c)d.  It is located in the constellation Canes Venatici.  It interacts with its smaller companion, NGC 4485, and as a result is classified as a starburst galaxy.  Both spiral galaxies have extreme distortions from their gravitational interaction.  It appears that the two galaxies are unlikely to merge.  Instead, they are moving rapidly past each other, leaving a trail of stars, gas and dust between them.  In the attached image, you can see a bright star forming region between the galaxies.  NGC 4490 and NGC 4485 are listed in the Atlas of Peculiar Galaxies as Arp 269.  They are between 25 and 30 Mly from Earth.  NGC 4490 has a diameter of 58.7 kly while NGC 4485 is 17.1 kly across. (Wikipedia, NASA and SkySafari Pro).

Imaging details:

Celestron 9.25" Edge HD SCT (FL = 2350mm, f/10)
Celestron off-axis guider with a ZWO ASI 174MM mini guide camera
Losmandy G11 mount with Gemini 2
ZWO ASI 2600MM Pro cooled monochrome camera (-10oC)
ZWO 36mm Hydrogen-a, Luminance, Red, Green, and Blue filters
RA: 187.60586, DEC: 41.63028o, Equatorial camera rotation: 340o 

Software:
      Sequence Generator Pro, ASTAP plate solving, PHD2 guiding,
      Losmandy Gemini ASCOM mount control and web client interface, 
      SharpCap Pro for polar alignment with the Polemaster camera,
      PixInsight 1.8.9 with StarXTerminator (AI version 10) and StarNet2,                     
      Photoshop CC 2022