NGC3372, The Eta Carina Nebula
February 20, 2016
This image is a collaboration with my friend, Shawn Nielsen (visibledark.ca). Shawn collected the data remotely from an observatory in Australia, and we processed it together in PixInsight.
The Eta Carina Nebula, also known as the Great Nebula in Carina, is located approximately 6,500 to 10,000 light year away from Earth in the constellation of Carina. It is one of the largest diffuse nebulae in our night skies and is easily visible to the naked eye, much like M42, which dominates the bottom third of Orion’s sword. The stars at the centre of the nebula (at least two) have a combined luminosity 5 million times that of the Sun. The larger star at the centre of the nebula has a history of brightening and fading, and for a few days in March, 1843 was the second brightest star in the sky until it faded to the point that it was no longer visible to the naked eye. More recently, it’s brightness peaked again in 2014 (magnitude 4.5, easily visible to the naked eye). These fluctuations (referred to as the Great Eruption and Lesser Eruption) suggest that a supernova may occur here in the near future.
This shot is remarkable not only because of the subject, but because of the imaging involved. It contains only 30 minutes of data – 15m each of Hydrogen-Alpha and Oxygen-3 data. It was acquired from halfway around the world. There was a nearly full Moon in the sky, and there is NO blue data used in this image. This was Shawn’s first data acquisition with a remote telescope (in Siding Springs, Australia).
SBIG STL-11000M camera with Ha and O3 filters, 106mm f/5 refractor, Paramount ME mount. Focused with FocusMax. Acquisition, guiding and calibration with MaximDL. Image registration and all processing in PixInsight. Shot from Siding Springs, Australia. Gibbous moon. Good transparency and seeing.
3x5m Ha and 3x5m O3 (total 30m)
Creation and cleanup: An RGB image was made by assigning Ha to red and O3 to G and B, and the resulting RGB image was cropped and processed with DBE, followed by ColorCalibration.
Linear Noise Reduction: MultiscaleLinearTransform was used to reduce noise in the background areas of the RGB image. Layer settings for threshold and strength: Layer 1: 3.0, 0.5 Layer 2: 2.0, 0.39 Layer 3: 1.0, 0.25 Layer 4: 0.5, 0.1. A mask was used to protect high signal areas.
Stretching: HistogramTransformation was applied to make a pleasing yet bright RGB image.
Creation and cleanup: The O3 and Ha masters were combined 1:1 in PixelMath to create the SynthL channel. The image was cropped to match the RGB and DBE was applied.
Deconvolution: A copy of SynthL was stretched to use as a deconvolution mask. A star mask was made from unstretched SynthL to use as a local deringing support. Deconvolution was applied (100 iterations, regularized Richardson-Lucy, external PSF made using DynamicPSF tool with about 20 stars; local deringing at 70% and global dark deringing at 0.03).
Linear Noise Reduction: MultiscaleLinearTransform was applied to reduce the noise. Layer settings for threshold and strength: Layer 1: 3.0, 0.6 Layer 2: 2.0, 0.5 Layer 3: 1.0, 0.4 Layer 4: 0.5, 0.15
Stretching: HistogramTransformation was applied to make an image with similar brightness to the RGB image. TGVDenoise was applied and the image was re-stretched to reset the black point.
Combining SynthL with RGB:
The luminance channel of the RGB image was extracted, processed and then added back into the RGB image as follows:
1. Set RGB channels equal using RGBWorkingSpace.
2. Extract luminance from the RGB image.
3. Apply LinearFit using SynthL as the reference.
4. Use ChannelCombination in Lab mode to replace the RGB’s luminance with the fitted luminance from step 2.
5. LRGBCombine was then used to make a SynthLRGB image.
HDRMultiscaleTransform was applied at 6- and 4-pixel scales to the bright parts of the nebula, followed by LocalHistogramEqualization at scales of 50 and 150 to restore contrast.
UnsharpMask was applied to the bright areas of the nebula. Contrast, brightness and colour saturation were adjusted in several iterations with the Curves tool, including boosting the blue channel in the centre of the nebula.
Image scale is about 3.5 arcsec per pixel for this camera / telescope combination.