NGC 3726

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May 19, 2023

NGC 3726 is a rather small galaxy (on the sky) located under the bowl of the Big Dipper, as shown in the finder chart I prepared.. It’s about 45 million light years from us and is about 85,000 light years across. The galaxy has a short central bar that contains a supermassive black hole about 3 million times the Sun’s mass. NGC 3726 is also reported to have a massive dark matter halo. You’ll also see some pinkish H-alpha knots within the spiral arms.

A few other galaxies share this field with NGC 3726; some are highlighted in this annotated image.

Acquisition, focusing, and control of Paramount MX mount with N.I.N.A., TheSkyX and PHD2. Focus with Optec DirectSync motor and controller. Equipment control with PrimaLuce Labs Eagle 3 (2021) and 4 (2023) Pro computer. All pre-processing and processing in PixInsight. Acquired from my SkyShed in Guelph. Average transparency and seeing. Data acquired April 7-10, 2023 in a nearly moonless sky.

Luminance: Sky-Watcher Esprit 150 f/7 refractor and QHY600M camera with Optolong UV/IR filter
Chrominance: Takahashi FSQ-106 ED IV @ f/5 and QHY367C Pro one-shot colour camera with Optolong UV/IR filter

106 x 5m Lum = 8hr50m
34 x 5m Ha = 2hr50m
131 x 5m chrominance = 10hr55m
Total: 22hr35m
Image scale 0.72 arcsec per pixel (based on Luminance)
Finder Chart and Annotated Image: 
Data Reduction and Linear Processing

Preprocessing: The WeightedBatchPreProcessing script was used to perform calibration, cosmetic correction, weighting, registration, local normalization and integration of all frames.

Alignment of Master Frames: DrizzleIntegration was applied to the OSC frames (within WeightedBatchPreProcessing), and the result was aligned to the Luminance master with StarAlignment. This yielded aligned Ha, Lum and Colour masters.

Gradient Removal: DynamicBackgroundExtraction was applied to the three masters.

Colour Calibration:  ColorCalibration was used to calibrate the OSC master.

Deconvolution: BlurXterminator was used on each master with an automatic PSF and star sharpening set to 0.10.

Linear Noise Reduction: NoiseXterminator was applied to each image with settings Amount=0.9 and Detail=.15 

Stretching:  HistogramTransformation was applied to each image to make a pleasing yet bright image.

Combining Luminance, Colour and H-alpha Images
Luminance addition: LRGBCombination was applied to replace the lightness of the RGB image with the Luminance master.

H-alpha Blending: PixelMath was used to blend Ha into the LRGB image using the following expression with parameter values a=1.15 and b=0.07.
          Red:      max($T[0], a*Ha)
          Green:  $T[1]
          Blue:      iif($T[0]<a*Ha, $T[2] + b * Ha, $T[2])

Additional Processing
Star Removal:  StarXterminator was used to remove the stars.

Nonlinear Noise Reduction: NoiseXterminator was used to reduce noise in the background areas of the image with settings Amount=0.9 and Detail=0.3. 

Contrast Enhancement: LocalHistogramEqualization was applied three times using an inverted lightness mask to protect the background and select the galaxy. A Contrast Limit of 1.5 and 1 iteration were used for each application (scale 40, strength 0.18; scale 80, strength 0.25; and scale 150, strength 0.13).

Sharpening: MultiscaleMedianTransform was used to sharpen Layers 1 – 4 with strengths of 0.01, 0.025, 0.03, and 0.03, respectively.

Star Restoration: Saturation was boosted in the stars-only image using Curves before the stars were added back into the starless image using the PixelMath expression combine(starless, stars_only, op_screen())

The star image was used as a mask to shrink the stars slightly using MorphologicalTransformation (morphological selection mode, selection=0.11, strength =0.4, star shape 3×3 round).

Final Steps: Background, galaxy and star brightness, contrast and saturation were adjusted in several iterations using CurvesTransformation with masks as required. SCNR was applied (Green; average neutral, 50%). ICCProfileTransformation (sRGB IEC61966-2.1; Relative Colorimetric with black point compensation) was applied prior to saving as a jpg.