M97, The Owl Nebula 

M97, The Owl Nebula

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June 8, 2015

I never understood this object’s monicker, The Owl Nebula, until I researched to prepare this post.  The name stuck after the 3rd Earle of Rosse, William Parsons, sketched it in 1848 with an appearance resembling an owl’s head.   The nebula formed about 8000 years ago from the gas thrown off by the evolving central star.  That star has now reached the white dwarf stage, and is currently much brighter and hotter than the Sun.  Its energy is causing the nebula’s hydrogen, oxygen and other gases to glow.

M97 contains a little more than 1/10th the Sun’s mass, but is spread out so thinly that there are fewer than 100 particles per cubic centimetre.  It is currently a little less than a light year wide, and is expanding at a rate of 27-39 km per second.  It will eventually become so diffuse that its feeble glow will no longer be visible.

If you look to the left of the nebula in the 10 o’clock position, you will see a small reflection nebula to the left of a prominent star.  At a similar distance from the nebula in the 3 o’clock position is an edge on spiral galaxy that looks like a small streak.  There are many more galaxies throughout this field.  All of them are millions of light years in the distance, while the Owl Nebula is a cosmic stone’s throw away at just 2,600 light years.

Tekkies:
SBIG STL-11000M camera, Baader Ha and RGB filters, 10″ f/6.8 ASA astrograph, Paramount MX.  Guided with STL-11000’s external guider and 80 mm f/6 Stellar-Vue refractor.  Acquistion, guiding and calibration done using Maxim-DL.  Focusing with FocusMax.  Automation with CCDCommander.  Registration, integration and all processing in PixInsight.  Shot from my SkyShed in Guelph, Ontario.  moderate moonlight for RGB, full moon for Ha.

10x10m R, G and B, and 4x20m Ha unbinned frames (total=6hr20m).

Synthetic Luminance:
Creation and cleanup: The Ha, R,G and B masters were cropped and 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. Deconvolution was applied (100 iterations, regularized Richardson-Lucy, external PSF made using DynamicPSF tool with about 20 stars).

Stretching: HistogramTransformation was applied to make a pleasing yet bright image.

RGB:
Creation and cleanup: The cropped R, G and B were combined to make an RGB image which was processed with DBE, BackgroundNeutralization and ColourCalibration.  The cropped Ha image was also processed with DBE and the NB-RGB Combine script was applied to blend the Ha into the RGB.  The RGB and NB-RGB images were combined 95:05.

Stretching: HistogramTransformation was applied to make a pleasing yet bright image.

Additional steps:  TGVDenoise was applied to the CIE-L only.  ChannelMatch was used to fix slight misalignment of red channel (about 0.15 px).  Colour saturation was boosted on stars and nebula. 

Combining SynthL with RGB:
The luminance channel of the RGB was extracted, processed and then added back into the RGB image as follows:
1. Extract luminance from the RGB image.
2. Apply LinearFit using the SynthL channel as a reference.
3. Use ChannelCombination in Lab mode to replace the RGB’s luminance with the fitted luminance from step 2.
4. LRGBCombine was then used to make a SynthLRGB image.

Final Processing
Final Steps: Background colour saturation was reduced slightly.   A blurred range mask was made and used to increase colour saturation and contrast in the nebula.  A copy of the image was processed to enhance the nebula only, boosting red, brightness and  contrast.  The duplicate was blended back into the original using a mask to protect the background and stars.

Image scale is about 1.1 arcsec per pixel for this camera / telescope combination.

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