NGC 3726


Click image for full size version

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.

Tekkies:
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.