M16, The Eagle Nebula
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February 22, 2015
The Eagle Nebula, M16, is in Serpens. It includes a star cluster just 1-2 million years old as well as the intricate emission nebula that fills most of the image. Like many of the Messier objects, Messier didn’t discover the Eagle. It was first observed by Jean-Philippe de Cheseaux in 1745-46. It lies about 7,000 light years away. The structures at the centre are known as “the Pillars of Creation,” made famous by the Hubble Space Telescope’s amazing photo. The Pillars of Creation are about 9.5 light years tall (90 trillion km).
This object lies about as far south I can shoot from my home observatory. This limitation is due to trees, the wall of my observatory and light pollution from the Guelph Auto Mall to my south. Nevertheless, I was able to capture this image of the heart of the Eagle Nebula over 5 nights in July 2012 and processed it in February 2015. I shot red, green and blue first, then shot through a clear filter on a couple of very transparent nights, and finally through an Ha filter on a night where light pollution was bad due to haze.
SBIG STL-11000M camera, Baader LHaRGB filters, 10″ f/6.8 ASA astrograph, MI-250 mount. Guided with STL-11000’s external guider and a 500mm f.l. Lumicon guide scope. Acquistion, guiding, calibration, registration and integration all done using Maxim-DL. Focusing with FocusMax. All processing in PixInsight. Shot from my SkyShed in Guelph, Ontario. Shot over five nights under variable conditions. L was shot with no moon and good transparency. Ha was shot with no moon and average transparency. R,G,B was shot with waxing gibbous moon and good transparency. Seeing was average on all nights.
9x5m of R and B, 8x5m G, 9x10m L and 6x10m Ha, (total 4 hr 45m).
Creation and cleanup: The L, R, G ,B and Ha masters were combined using the ImageIntegration tool (average, additive with scaling, noise evaluation, iterative K-sigma / biweight midvariance, no pixel rejection). DBE was applied to neutralize the background.
Deconvolution: A star mask was made to use as a local deringing support. A copy of the image was stretched to use as a range mask. Regularized Richardson-Lucy Deconvolution was applied (75 iterations, external PSF made using DynamicPSF tool with about 30 stars).
Stretching: HistogramTransformation was applied to create a nice looking image.
Ha, R, G and B masters were cropped to remove edge artifacts from stacking. The R, G and B channels were combined to make an RGB image. Ha and RGB were processed with DBE, ColourCalibration was applied, and the Ha and RGB were combined with the NB-RGB script. Then 10% Ha was added to the Blue channel using (NewBlue=Blue+0.1*Ha) without rescaling. This was applied in the nebula only using a mask to protect stars and background. HistogramTransformation was applied using settings from the luminance stretch above.
The luminance was extracted from the HaRGB image, processed and then added back into the HaRGB image as follows:
1. Extract luminance from the HaRGB image.
2. Apply LinearFit using the SynthL channel as a reference.
3. Use ChannelCombination in the Lab mode to replace the luminance of the HaRGB with the fitted luminance from step 2.
4. Use LRGBCombine to apply SynthL to the HaRGB image.
HDR: A new mask was made with PixelMath by subtracting the deringing support from the deconvolution mask. The new mask was used to protect bright stars and background from HDRMultiscaleTransform, which was applied at 6 and 4 wavelet scales.
Noise Reduction and Re-Stretch: TGVDenoise was applied in Lab mode with 300 iterations with a range mask used to protect nebula and stars. This was followed by a HistogramTransformation to raise the black point (but with no clipping).
LHE: LocalHistogramEqualization was applied to the same area as HDRMultiscaleTransform above to restore contrast.
Dark Structures: DarkStructureEnhance script was applied with a strength of 0.25.
Star Reduction and Colour Adjustment: Morphological transformation (3×3, 4 iterations, strength 0.15) was applied using a star mask to protect background and nebula. The same mask was used with ColourSaturation to boost star colours.
Final Steps: Curves was applied to adjust contrast. The luminance was extracted and used as a mask for ColourSaturation. The same mask was used with UnsharpMask (radius 2.0 and strength 0.5).
Image scale is about 1.1 arcsec per pixel for this camera / telescope combination.