The Cave Nebula Region
December 19, 2017
The Cave Nebula region makes a pretty picture with red emission nebula, blue reflection nebulae, dark nebulae and many colourful stars. This region lies in the constellation Cepheus about 2,400 light years away, pretty close in cosmic terms. The Cave Nebula’s bright rim is ionized by the hot blue-white stars in the vicinity, primarily the bright star left of centre in this image. The main “cave” structure is 10 light years across. It 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 centre of the image is a blue reflection nebula, vdB 155, and the pretty, golden open cluster NGC7419 is at bottom centre, Sh2-154 is the large, diffuse red object in the lower part of the image. I’ve imaged the Cave Nebula before at higher resolution (but in a much narrower field). That image was published in the April 2016 issue of Sky and Telescope.
Moravian G3-16200 EC camera (on loan from O’Telescope), Optolong Ha, R, G and B filters, Takahashi FSQ-106 ED IV at f/3.6, Paramount MX, unguided. Acquisition with the SkyX, focused with FocusMax, scripted with CCD Commander. All pre-processing and processing in PixInsight. Acquired from my SkyShed in Guelph. Moderate moonlight for Ha, no moonlight for RGB, average transparency and seeing.
11x10m R, 11x10m G, 12x10m B and 24x10m Ha unbinned frames (total=9hr40m).
Creation and cleanup: The BatchPreProcessing script was used to perform calibration, cosmetic correction and registration of all frames. LocalNormalization was applied, and ImageIntegration and DrizzleIntegration were used to make the Ha, R, G and B masters using Local Normalization for scaling. The masters were cropped to remove edge artifacts. R, G and B were combined to make an RGB image which was processed with DBE, BackgroundNeutralization and PhotometricColorCalibration.
Combining Ha and RGB data: The NBRGBCombination script was run with default settings using Ha for red narrowband. PhotometricColorCalibration was applied.
Linear Noise Reduction: MultiscaleLinearTransform was used to reduce noise in the NB-RGB image. Layer settings for threshold and strength: Layer 1: 3.0 0.8 Layer 2: 2.5, 0.7 Layer 3: 2., 0.6 Layer 4: 1.0, 0.2 Layer 5: 0.5, 0.1.
Stretching: HistogramTransformation was applied to the NB-RGB image to make a pleasing, bright image. Curve was applied to boost contrast and saturation.
Creation and cleanup of SynthL: The four channel masters were processed with DBE and then integrated (average, noise weighted, no rejection) using ImageIntegration to create the SynthL.
Deconvolution: A star mask was made to use as a local deringing support image. A copy of the image was stretched to use as a range mask. Deconvolution was applied (50 iterations, regularized Richardson-Lucy, external PSF made using DynamicPSF tool with about 30 stars).
Linear Noise Reduction: MultiscaleLinearTransform was used to reduce noise in the background areas of the NB-RGB file. Layer settings for threshold and strength: Layer 1: 3.0 0.9 Layer 2: 2.0, 0.75 Layer 3: 1.0, 0.6 Layer 4: 0.5, 0.2.
Stretching: HistogramTransformation was applied to the SynthL to make a pleasing, bright image.
Noise Reduction and Re-Stretch: TGVDenoise was applied in Lab mode with 300 iterations with a range mask applied to protect high signal areas. This was followed by a HistogramTransformation to raise the black point (but with no clipping).
Local Contrast Adjustment: Bright areas of the nebula were processed with LocalHistrogramEqualization (max contrast 1.5, scale 50, strength 0.35) using a mask to protect background and stars.
Combining SynthL with NB-RGB:
The processed SynthL was applied to the NB-RGB image using LRGBCombine.
Final Steps: MultiscaleLinearTransform was used to sharpen the structures in the bright parts of the nebulae (layers 2-3 at 0.1). MorphologicalTransformation was used in selection mode (Selection 0.13, strength 0.6) to slightly reduce stars The DarkStructureEnhance script was used (8 layers, strength 0.2) to increase contrast between dark nebula and brighter regions. Background, nebula and star brightness, contrast and saturation were adjusted in several iterations using Curves with masks as required. The image was downsampled by a factor of two using IntegerResample to restore the original resolution.
Image scale is about 3.2 arcsec per pixel for this camera/telescope combination.