The Cave Nebula – Hubble Palette

Sh2-155, the Cave NebulaClick image for full size version

August 21, 2021

The Cave Nebula lies in the constellation Cepheus about 2,400 light years away, pretty close in cosmic terms.  The “cave” has a bright rim ionized by the hot blue-white stars in the vicinity. The main “cave” structure is 10 light years across and is catalogued as Sharpless 155 (Sh2-155) and also as Caldwell 9 (the Caldwell catalogue was compiled by Sir Patrick Moore, and named for his mother’s maiden name). Near the lower left corner of the image is a blue reflection nebula, vdB 155. The Cave Nebula region makes a nice natural colour image, highlighting red emission nebula, blue reflection nebulae, dark nebulae and many colourful stars. I’ve also imaged the Cave Nebula before at higher resolution; that image was published in the April 2016 issue of Sky and Telescope.

Acquisition, focusing, and control of Paramount MX mount, with TheSkyX. Focus with Optec DirectSync motor and controller. Unguided. Automation with CCDCommander. Equipment control with PrimaLuce Labs Eagle 3 Pro computer. All pre-processing and processing in PixInsight. Acquired from my SkyShed in Guelph. Average transparency and seeing. Data acquired March 2-14, 2021 in a mostly moonless sky.

Sky-Watcher Esprit 150 f/7 refractor and QHY 16200-A camera with Optolong H-alpha, O(III) and S(II) filters

Ha: 49 x 10m = 8hr10m
O3: 47 x 10m = 7hr50m
S2: 45 x 10m = 7hr30m

Total: 23hr30m

Image scale 1.15 arcsec per pixel 

Data Reduction and Cleanup
The WeightedBatchPreProcessing script was used to perform calibration, cosmetic correction, weighting, registration and integration of all frames. 

Creation and cleanup:  The S2, Ha and O3 were combined to make an RGB image.  DynamicBackgroundExtraction and BackgroundNeutralization were applied to the image.

Linear Noise Reduction:  MultiscaleLinearTransform was used to reduce noise in the RGB image. Layer settings for threshold and strength: Layer 1: 5.0 0.85   Layer 2: 4, 0.75  Layer 3: 3, 0.64  Layer 4: 2.0, 0.2  Layer 5: 1, 0.1.

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

Creation of Synthetic Luminance:  ImageIntegration was used to make a noise-weighted average of the three narrowband masters. This was used as synthetic luminance.

Deconvolution:  A star mask was made to use as a Local Deringing Support image. A copy of the  luminance image was stretched to use as a range mask. Deconvolution was applied (80 iterations, regularized Richardson-Lucy, external PSF made using PSFImage script with 50 stars).

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: 4.0 0.9   Layer 2: 3.0, 0.75  Layer 3: 2.0, 0.6  Layer 4: 0.5, 0.2.

Stretching:  HistogramTransformation was applied to make a pleasing, bright image, with background set to an intensity of approximately 0.1.

Combining Luminance and Chrominance
LRGB Combination: The synthetic luminance was applied to the chrominance image using LRGBCombination with default settings.

Additional Processing
Nonlinear Noise Reduction: TGVDenoise was used in L*a*b* mode to reduce noise with a mask used to mainly target the background areas and protect the stars (max. 1000 iterations and convergence selected for both luminance and chrominance). 

Contrast Enhancement:  LocalHistogramEqualization was applied twice using a mask to select the bright parts of the nebula and protect stars and background. The first pass was with a scale of 50 (max contrast 1.5, strength 0.25, 1 iteration), followed by a scale of 150 (max contrast 1.5, strength 0.27, 1 iteration).

Sharpening: MultiscaleLinearTransform was used to sharpen Layers 2 and 3 with strengths of 0.27 and 0.1, respectively. A mask was used to limit sharpening to high-signal areas and to protect the stars and background regions.

Hue Adjustment: The green tones were transformed towards blue, and yellows towards red, using the CurvesTransformation Hue control. SCNR was applied with a strength of 0.20 through a Green mask generated with the ColorMask script.

Final Steps: MorphologicalTransformation was applied (morphological selection=0.11; strength =0.5; 3×3 kernel) through a star mask to slightly reduce the stars. Background, nebula and star brightness, contrast, saturation and hue were adjusted in several iterations using CurvesTransformation with masks as required. The DarkStructureEnhance script was applied with a strength of 0.15. ICCProfileTransformation (sRGB IEC61966-2.1; Relative Colorimetric with black point compensation) was applied prior to saving as a jpg.