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July 1, 2017, Sky & Telescope October 2017
IC 1396 is a large cloud of faintly glowing gas, composed mainly of hydrogen. It is a huge object in the sky, with a diameter more than six times that of the Moon. That’s because it is both relatively close to us (2,400 light years) and big (more than 100 light years across). The energy that makes the nebula’s gas glow comes from the bright star at the centre of the nebula, called HD 206267. Jutting in from the 9 o’clock position is the well known Elephant’s Trunk Nebula, which features in another image I’ve taken of this object. The Elephant’s Trunk is a star forming region, and there are a number of very young stars — less than 100,000 years old — in its vicinity. IC 1396 is textured throughout with many dark nebula. It’s hard not to notice Mu Cephei, the brightest star in the image. It is about 38,000 times as bright as the Sun. Bring your sunscreen.
Many people prefer these nebulae displayed in greyscale (black & white) images. I usually prepare both. You can find the greyscale image here.
Moravian G3-16200 EC camera (on loan from O’Telescope), Optolong RGB and 7nm H-alpha filters, Takahashi FSQ-106 ED IV at f/3.6, Paramount MX. Acquisition with the SkyX unguided. Focused with FocusMax. All pre-processing and processing in PixInsight. Acquired from my SkyShed in Guelph. Nearly full moon, average transparency and poor seeing.
8x10m R,G and B and 11x20m Ha unbinned frames (total=7hr40m).
Creation and cleanup: The BatchPreProcessing script was used to perform calibration, cosmetic correction and registration of all frames. DrizzleIntegration was used to make the channel masters. The R, G, B and Ha 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 ColorCalibration.
Combining Ha and RGB data: The NBRGBCombination script was run with default settings using Ha for red narrowband (scale 1.2) and blue (scale 0.01).
Linear Noise Reduction: MultiscaleLinearTransform was used to reduce noise in the NBRGB image. Layer settings for threshold and strength: Layer 1: 5.5, 1.0 Layer 2: 3.5, 0.84 Layer 3: 2.5, 0.71 Layer 4: 1.0, 0.55 Layer 5: 1.5, 0.28 Layer 6: 1.0, 0.16.
Stretching: HistogramTransformation was applied to the NBRGB image to make a pleasing, bright image.
Creation and cleanup of SynthL: The linear Ha, R, G and B masters were combined using the ImageIntegration tool (average, additive with scaling, noise evaluation, iterative K-sigma / biweight midvariance, no pixel rejection). DBE was applied.
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 (80 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 NBRGB file. Layer settings for threshold and strength: Layer 1: 5.5, 1.0 Layer 2: 3.5, 0.84 Layer 3: 2.5, 0.71 Layer 4: 1.0, 0.55 Layer 5: 1.5, 0.28 Layer 6: 1.0, 0.16.
Stretching: HistogramTransformation was applied to the SynthL to make a pleasing, bright image.
Noise Reduction and Re-Stretch: SCNR was applied (average neutral, 0.5). TGVDenoise was applied in Lab mode with 300 iterations with a range mask used to protect high signal areas. This was followed by a HistogramTransformation to raise the black point (but with no clipping).
Combining SynthL with NBRGB:
The processed SynthL was applied to the NBRGB image using LRGBCombine with default settings.
Contrast Enhancement: The contrast was boosted with 2 passes of LocalHistogramEqualization with max contrast of 1.5 for both passes (scale 50, strength 0.5 and scale 150, strength 0.15) using a mask to protect background and stars.
Final Steps: MultiscaleLinearTransform was used to sharpen the structures in the bright parts of the nebula (layers 2-4 at 0.05). Background, nebula and star brightness, contrast and saturation were adjusted in several iterations using Curves with masks as required. . The DarkStructureEnhance script was applied (8 layers, strength 0.15) to a copy of the image, which was blended back to the “flatter” parts of the image to accentuate dark features in the nebulae. The ColorMask script was used to make a blue/purple star halo mask. This mask was used with the Curve hue tool to correct bluish-purple haloes around medium sized stars.
Image scale is about 1.6 arcsec per pixel for this camera / telescope / combination and Drizzle integration.