M106 is a large spiral galaxy in the constellation Canes Venatici. A few smaller galaxies are also visible in the image. All subs were acquired at two locations in good old light-polluted Hampton, VA over the course of two nights with decent seeing. Post-processing is a mixture of techniques I learned from Alejandro Tombolini’s M106 tutorial and from Juan Carlos Moreno’s M109 tutorial which cites techniques by Manfred Schwarz and Gerald Wechselberger to reduce background noise (see references below). Thanks to those guys for posting such excellent information.

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Processing Workflow (PixInsight)

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Pre-processing

1. Calibrated light frames for all exposures averaged together using Linear Fit Clipping rejection to take into account variability in sky brightness between the two nights. (ImageIntegration)

Post-processing

1. Initial crop (Dynamic crop)

2. Reduce horizonal banding (CanonBandingReduction)

3. Reduce vertical banding (FastRotation, CanonBandingReduction, FastRotation)

4. Neutralize background (BackgroundNeutralization)

5. Reduce background gradient (AutomaticBackgroundExtraction; normalize, divide)

6. Reduce background gradient (AutomaticBackgroundExtraction; subtract)

7. Reduce background gradient (DynamicBackgroundExtraction; subtract)

8. Reduce background gradients (DynamicBackgroundExtraction; divide)

9. Neutralize background (BackgroundNeutralization)

10. Set white balance (ColorCalibration; use galaxy as white reference)

Luminance processing

11a. Extract luminance channel (ChannelExtraction)

11b. Reduce background noise (MultiscaleLinearTransform; target background)

11c. Reduce background noise (MultiscaleMedianTransform; target background)

11d. Reduce hot pixels (2x PixelMath [Schwarz script]; protect stars)

11e. Even out background (PixelMath [Wechselberger equation]; protect stars)

11f. Reduce dark pixels (3x PixelMath [Schwarz script]; protect stars)

11g. Deconvolution (Deconvolution; protect stars)

11h. Mild dynamic range stretch (HistogramTransformation)

11i. Stronger stretch (HistogramTransformation; protect bright stars)

11j. Lower black point (HistogramTransformation; protect bright stars)

11k. Compress dynamic range of galaxy (HDRMultiscaleTransform; target galaxy)

11l. Enhance local contrast in galaxy (LocalHistogramEqualization; target galaxy)

11m. Brighten galaxy (CurvesTransformation; protect stars)

11n. Reduce small scale noise in galaxy (MultiscaleMedianTransform; target galaxy)

11o. Reduce star sizes (MorphologicalTransformation; target stars)

11p. Slight noise reduction (MultiscaleLinearTransform; protect stars)

RGB processing

12a. Reduce background noise (MultiscaleLinearTransform; target background)

12b. Reduce background noise (MultiscaleMedianTransform; target background)

12c. Reduce hot pixels (2x PixelMath [Schwarz script]; protect stars)

12e. Even out background (PixelMath [Wechselberger equation]; protect stars)

12f. Reduce dark pixels (3x PixelMath [Schwarz script]; protect stars)

12g. Mild dynamic range stretch (HistogramTransformation)

12h. Stronger stretch (HistogramTransformation; protect bright stars)

12i. Lower black point (HistogramTransformation; protect bright stars)

12j. Strong color saturation (CurvesTransformation; protect stars)

12k. Chrominance noise reduction (ACDNR)

12l. More color saturation, including stars (CurvesTransformation; protect background)

12m. Reduce large scale chrominance noise in background (ATrousWaveletTransform; target chrominance; protect galaxy & stars)

12n. Reduce chrominance noise in background (2x InterChannelCurves; protect galaxy & stars)

12o. Increase star color saturation (CurvesTransformation; target stars)

LRGB processing

13. Combine RGB and luminance (LRGBCombination)

14. Remove green chrominance noise (SCNR)

15. Increase star color saturation (ColorSaturation; target stars)

16. Mild sharpening (MultiscaleMedianTransform; protect background)

17. Reduce background level and mild contrast enchance (CurvesTransformation)

References:

Alejandro Tombolini’s M106 tutorial:

http://www.pixinsight.com.ar/en/info/processing-examples/21/messier-106.html

Juan Carlos Moreno’s M109 tutorial:

http://astrofotourbana.blogspot.com.es/2014/04/m109-procesado-de-una-galaxia-en.html

Manfred Schwarz’ galaxy tutorial:

http://www.astrophoto.at/PixInsight/

Gerald Wechselberger’s PixInsight tutorials:

http://www.werbeagentur.org/oldwexi/PixInsight/PixInsight.html