LBN 437 and Sh2-126

LBN 437Click image for full size version (8 MB)

October 3, 2021

LBN 437 is a complex nebula in Lacerta, the Lizard. In keeping with the reptilan theme, LBN 437 looks to me like a gecko seen from above. However, Gail says it is more amphibian in nature, resembling this petrified toad poo we found in our driveway. You be the judge; I’m going with gecko.

I had originally planned to shoot only broadband (“natural” colour), but when I saw the faint red glow of Sh2-126 in the first few frames, I decided to get some H-alpha data to better reveal the dim red structures in the region, including the edge of the “snout,” which looks like a shock wave. This structure was very clear in the H-alpha image. The acronym “LBN” refers to Lynd’s Catalogue of Bright Nebulae. This catalogue’s entries were identified by inspection of red and blue images from the National Geographic – Palomar Observatory Sky Atlas. 

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 4 Pro computer. All pre-processing and processing in PixInsight. Acquired from my SkyShed in Guelph. AVerage or better transparency and seeing. Data acquired September 6-24, 2021.

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

149 x 10m Lum = 24hr50m
  44 x 10m Ha    =  7hr20m
275 x 5m OSC    = 22hr55m
Total: 55hr05m
Image scale 0.72 arcsec per pixel (based on Luminance). 

Data Reduction and Initial Processing
Preprocessing: The WeightedBatchPreProcessing script was used to create Luminance and H-alpha master frames from the mono camera and an RGB master frame from the one-shot colour camera. DrizzleIntegration (2x CFA Drizzle) was applied to the RGB set. The RGB master was registered to the Luminance master using StarAlignment.

Noise Reduction: The MUREDenoise script was applied to the Luminance and Ha master frames (20 iterations using settings determined with MureDenoiseDetectorSettings script).

Gradient Removal: DBE was applied to each of the three masters using Subtraction.

Colour Balance:  Colour was balanced with PhotometricColorCalibration.

Linear Noise Reduction: MultiscaleLinearTransform was used to reduce noise in the background areas, using an internal mask to protect bright structures. Layer settings for threshold and strength: Layer 1: 5.0 0.95, 2 iterations; Layer 2: 4.0, 0.85, 2 iterations.

Stretching:  HistogramTransformation was applied to make a pleasing, bright image, with background set to an intensity of approximately 0.10.

Lightness and H-alpha

Deconvolution:  A star mask was made from the Luminance master to use as a Local Deringing Support Image. The GAME script was used to make a mask selecting only two galaxies and one other region for deconvolution. Deconvolution was applied (30 iterations, regularized Richardson-Lucy, ParametricPSF mode with default settings; Global dark deringing = 0.03; Global bright deringing 0.005).

Linear Noise Reduction:  MultiscaleLinearTransform was used to reduce noise in the background areas, using an internal mask to protect bright stars. Layer settings for threshold and strength: Layer 1: 3.0 0.85, 1 iterations; Layer 2: 2.0, 0.75, 1 iterations.

Stretching:  HistogramTransformation was applied to make a pleasing, bright image, with background set to an intensity of approximately 0.10.

Combining Lightness, H-alpha and Colour Images
LRGB Combination: The lightness image was applied to the RGB image using LRGBCombination with default settings.

H-alpha Blending: PixelMath was used to add H-alpha to the LRGB image (the target image, $T), using the following expressions for the R, G and B channels:
     R:  max($T[0], 1.2*Ha)
     G:  $T[1]
     B:   iif($T[0]<Ha, $T[2] + 0.06*Ha, $T[2])

Additional Processing

Nonlinear Noise Reduction: TGVDenoise was used in L*a*b* mode to reduce noise with a mask used to target the background areas and protect the stars and nebula (max. 1,000 iterations and convergence selected for both lightness and chrominance). 

Contrast Enhancement: LocalHistogramEqualization was applied using an inverted star mask to protect the stars (scale 50, max contrast 1.5, strength 0.18, 1 iteration followed by scale 150, max contrast 1.5, strength 0.25).

Sharpening: MultiscaleLinearTransform was used to sharpen Layers 2 and 3 with strengths of 0.1and 0.05, respectively. The mask used for deconvolution was used to limit sharpening to a few specific regions of the image.

Final Steps: Background, nebula and star brightness, contrast, and colour saturation were adjusted in several iterations using CurvesTransformation with masks as required. ICCProfileTransformation (sRGB IEC61966-2.1; Relative Colorimetric with black point compensation) was applied prior to saving in jpg format.