NGC 4565, The Needle Galaxy


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February 17, 2106

NGC4565, the Needle Galaxy, lies about 40 million light years from Earth in the constellation Coma Berenices.  It has a diameter of about 100,000 light years, similar to that of our own Milky Way galaxy.  Its length appears about half the Moon’s width in images, and somewhat less through the eyepiece.  Although it is classified as a barred spiral galaxy, the central bar isn’t visible in this image, because the galaxy is edge-on to our line of sight.  However, its presence is evident from Spitzer Space Telescope data.  Several other galaxies also appear in this field.  The most prominent is NGC4562 at lower left.   Two more are to the lower right, and there’s another above the right-hand tip of the galaxy, level with the core.  Several even fainter galaxies are scattered throughout the image.

A previous version of this image was Astronomy Magazine Picture of the Day on August 12, 2014.

SBIG STL-11000M camera, Baader LRGB filters, 10″ f/6.8 ASA astrograph, Paramount MX.  Guided with STL-11000’s internal guider. Focusing with FocusMax.  Acquistion, guiding and calibration using Maxim-DL.  Registration, integration and all processing in PixInsight. Shot from my SkyShed in Guelph, Ontario. No moon.  Average to very good transparency and good seeing.  Acquired over 7 nights during February and March 2014.

24x10m L, and 12x5m R, G and B unbinned frames (total=7hr).

Creation and cleanup:   R, G and B masters were combined with ChannelCombination, 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. 

Synthetic Luminance
Creation and cleanup: The R,G, B and L masters were combined using the ImageIntegration tool (average, additive with scaling, noise evaluation, iterative K-sigma / biweight midvariance, no pixel rejection) 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.

Final Processing
UnsharpMask was applied to the galaxy using a mask.  Contrast, brightness and colour saturation were adjusted in several iterations with the Curves tool, with separate adjustments were made for galaxy, background and stars using a mask.  

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