Contains: M 31, Great Nebula in Andromeda, NGC 224, M 32, NGC 221, M 110, NGC 205
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Andromeda Galaxy, M31

Technical card

Resolution: 9006x5964

Dates: Oct. 26, 2014

Frames: 36x300" ISO800

Integration: 3.0 hours

Darks: ~30

Flats: ~30

Bias: ~50

Avg. Moon age: 2.56 days

Avg. Moon phase: 7.22%

Astrometry.net job: 415251

RA center: 10.695 degrees

DEC center: 41.288 degrees

Orientation: -84.399 degrees

Field radius: 1.388 degrees

Locations: Staunton River State Park, Red Oak, VA, United States

Description

This image was published in the March 2015 issue of Reflector!

The Andromeda Galaxy is the nearest galaxy to the Milky Way. At a distance of 2.5 million light years, it is one of the farthest objects visible to the naked eye and spans an area six times the diameter of the full moon in our night sky.

This is a four-panel mosaic of the Andromeda Galaxy captured at the Staunton River Star Party in late October. Despite only 45 minutes of exposure time per panel, the image came out quite nice owing to the dark skies (blue zone) and cool temperatures. I’m so used to imaging in urban environments that I was stunned to find that light pollution gradients did not exist in any of the panels, so I skipped DynamicBackgroundExtraction all together. I must say that making the mosaic with the PixInsight StarAlignment procedure was trivial and worked splendidly. Most folks recommend using GradientMergeMosaic to heal the seams between panels, but I didn’t find it necessary. There is at least one star split by a seam where GradientMergeMosaic would have helped, but you’d have to look for a while to find it.

With this image I’m wrapping up shop for a several months while we welcome a new youngster to our family. Thank you to all the folks who have liked and commented on my images during this past year - I sincerely appreciate your encouragement. I look forward to returning to the night sky in 2015. In the meantime, I’ll keep watching all of your great work.

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Processing Workflow (PixInsight)
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Pre-processing
1. Calibrate light frames using darks, bias, flats
2. Integrate each panel using sigma clipping rejection.
3. Create mosaic (StarAlignment).

Post-processing
1. Initial crop (Dynamic crop) .
2. Neutralize background (BackgroundNeutralization).
3. Set white balance (ColorCalibration; use entire image including galaxy).
4. Non-linear stretch (HistogramTransformation, lower midtones slider aggressively).
5. Non-linear stretch (HistogramTransformation, raise blackpoint slider).
6. Set luminance coefficients to 0.333333 for RGB channels (RGBWorkingSpace).
7. Increase contrast (CurvesTransformation “s” curve).
8. Compress dynamic range. (HDRMultiscaleTranform, scale: 8).
9. Restore colors and smooth star profiles to the heavily stretched stars using the method described by Vincent Peris (see Peris reference below).
10. Distribute color into star cores by duplicating image, blurring the duplicate with Convolution, increasing color saturation with CurvesTransformation and then blending the result into the original image through a star mask with the PixelMath expression: $T*0.4 + 0.6*M31_blurred.
11. Increase local contrast slightly (LocalHistogramEqualization; kernal radius: 128, w/ mask selecting galaxy).
12. Reduce luminance noise (ACDNR to luminance with luminance mask).
13. Reduce chrominance noise (ACDNR to chrominance with strong luminance mask).
14. Small contrast curve (CurvesTransformation).
15. Reduce green (SCNR to green).
16. Increase galaxy color saturation (CurvesTransformation w/mask selecting galaxy only; applied twice).
17. Sharpen galaxy details (MultiscaleMedianTransform w/mask selecting galaxy only).
18. Mild chrominance noise reduction, particularly in the galaxy where saturation brought up some noise (ACDNR to chrominance w/ luminance mask).
19. Reduce star sizes (MorphologicalTransformation, morphological selection w/star mask selecting stars).
20. Lower background to median value of 0.1 using a contrast curve (CurvesTransformation).
21. Another small hit of color saturation (CurvesTransformation w/ mask selecting galaxy only).
22. At the very center of the core was a pink spot where the image was saturated. The right way to fix this would have been to choose a shorter exposure in acquisition, so I’m left with plan B here: use InterChannelCurves (reference channel: green, target channel: CIE c) to reduce the pink color to neutral grey w/mask selecting pink spot. Brighten the neutral grey spot to a white that blends in with the rest of the core. This worked pretty well and is not all that noticable. Not optimal, but did the trick :)
23 Rotate 180 degrees (FastRotation).
24. Set ICC profile to sRGB for web publishing (ICCProfileTransformation).

References
“Dynamic Range and Local Contrast” software tutorial by Vincent Peris:
http://pixinsight.com/tutorials/NGC7023-HDR/index.html

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Jason Tackett
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Andromeda Galaxy, M31, Jason Tackett

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