Click image for full size version
August 14, 2018, Space.com Image of the Day September 12, 2018
M27 is a classic example of a “planetary nebula,” so named because they looked similar to planets to early observers. M27 is known as the Dumbbell Nebula because of its visual appearance through a telescope. It is found in the constellation Vulpecula (“the Little Fox”), which is well placed for observing shortly after dark at this time of year. The nebula’s colours in this image are due to gas thrown off by the dying star at its centre. This white dwarf emits radiation that causes the gas to glow in visible light. Hydrogen emits mostly red and oxygen mostly green-blue light. Like all planetary nebulae, the Dumbbell will gradually expand and dissipate, becoming fainter over time. I guess we’re lucky to be here at this particular time in history and able to see it. The dumbbell is about 1360 light years away, and is 10,000-15,000 years old.
I have shot this object several times with different equipment. The 2014 image was published several times, including Astronomy Magazine Picture of the Day on May 19, 2017. I also imaged it in 2016. It is interesting to compare the three images. The 2016 version shows more of the teal oxygen emission and the colours are more electric. The current version shows more of the faint hydrogen outer shell and has a more natural overall colour.
Sky-Watcher Esprit 150 f/7 refractor, QHY 16200-A camera, Optolong Ha, O3, L, R, G and B filters, Paramount MX. Acquisition with TheSkyX unguided. Focused with TheSkyX @Focus3. Automation with CCDCommander. All pre-processing and processing in PixInsight. Acquired from my SkyShed in Guelph. No Moon, average to excellent transparency and fair to above average seeing. Data acquired July 17-August 6, 2018.
18x10m L, 12x10m R, G and B, 31x20m Ha and 17x20m O3 (Total = 25hr).
Data Reduction and Cleanup
The BatchPreProcessing script was used to perform calibration, cosmetic correction and registration of all frames. ImageIntegration was used to make the Ha, O3, L, R, G and B masters. DynamicCrop was used to crop all the masters identically. DynamicBackgroundExtraction was applied to each master.
Creation and cleanup of SynthL: The L, R, G and B masters were combined using ImageIntegration (average, additive with scaling, noise evaluation, iterative K-sigma / biweight midvariance, no pixel rejection).
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 (50 iterations, external PSF made using DynamicPSF tool with about 30 stars).
Linear Noise Reduction: MultiscaleLinearTransform was used to reduce noise in the SynthL image. An internal mask was used, with layer settings for threshold and strength as follows: Layer 1: 3, 0.7 Layer 2: 2, 0.5 Layer 3: 2, 0.45.
Stretching: HistogramTransformation was applied to the SynthL to make a pleasing, bright image.
Creation and cleanup: ChannelCombination was used to make color image from the R, G and B masters. The RGB image was processed with PhotometricColorCalibration using a small preview of background sky as the background reference.
Linear Noise Reduction: MultiscaleLinearTransform was used to reduce noise in the RGB image. An internal mask was used, with layer settings for threshold and strength as follows: Layer 1: 3, 0.75 Layer 2: 2, 0.6 Layer 3: 2, 0.5.
Stretching: HistogramTransformation was applied to the RGB image to make a pleasing, bright image.
Ha and O3
Deconvolution of Ha: A star mask was used as a Local Deringing Support Image. A copy of the Ha image was stretched to use as a deconvolution mask. Deconvolution was applied (80 iterations, regularized Richardson-Lucy, external PSF made using DynamicPSF tool with about 20 stars).
Deconvolution of O3: The star mask from the previous step was used as the Local Deringing Support Image. A copy of the O3 image was stretched to use as a deconvolution mask. Deconvolution was applied (50 iterations, regularized Richardson-Lucy, external PSF made using DynamicPSF tool with about 20 stars).
Linear Noise Reduction: MultiscaleLinearTransform was used to reduce noise in the background areas of the Ha and O3 images. 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 Ha and O3 images with background brightness approximately matching the SynthL.
Combining SynthL, RGB, Ha and O3
SynthLRGB: The processed SynthL was applied to the RGB image using LRGBCombine.
SynthLHaO3RGB: PixelMath was used to add Ha to the red channel and O3 to the green and blue channels with the following expressions, using a mask to ensure smooth transitions.
Red: max($T, 1.2*Ha)
Green: max($T, 1.7*O3)
Blue: max($T, 1.7*O3)
Sharpening: Using a mask to protect stars, background and dim nebulosity, MultiscaleLinearTransform was applied to sharpen layers 2 and 3 (bias of 0.1 and 0.2, respectively).
Final Steps: Background, nebula and star brightness, contrast and saturation were adjusted in several iterations using Curves with masks as required.
Image scale is about 1.15 arcsec per pixel for this camera/telescope combination.