NGC5466, Globular Cluster

NGC5466Click image for full size version

May 2 2015

This image captures an ancient object:  NGC5466 is 12.5 billion years old, give or take 900 million years.  Unlike some of its very splashy globular cluster cousins, NGC5466 has a relatively unconcentrated core.  It belongs to Class 12 of the Shapley–Sawyer Concentration Class for describing the 150 or so globular clusters that are associated with the Milky Way galaxy.  Class 12 is the most sparse. It is unique in containing a particular type of blue star (“horizontal branch”).  It lies about 52,000 light years from us, and about the same distance from the centre of the Milky Way.  There are many small galaxies scattered throughout the image, including one at about the 5 o’clock position just outside the main core of NGC5466.   They all lie much, much farther away than NGC5466, and probably have their own groups of attendant globular clusters.

Tekkies:
SBIG STL-11000M camera, Baader 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.  Acquisition, 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.  Gibbous moon, fair to excellent transparency and very good seeing throughout acquisition.

18x10m R, 15x10m G and 15x10m B unbinned frames (total=8hr).

RGB:
Creation and cleanup: R, G and B masters were cropped and combined to make an RGB image which was processed with DBE and ColourCalibration.

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

Synthetic Luminance:
Creation and cleanup: The cropped R,G and B 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.

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

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
TGVDenoise was applied to the SynthLRGB image in Lab mode followed by a histogram stretch to increase image contrast.  SCNR was applied through a star mask to reduce green cast in a few stars.  A star core mask was made by applying morphological transformation and convolution to a copy of SynthL. Colour saturation in star cores was increased using this mask.

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

Share on Facebook67Tweet about this on TwitterShare on Google+0Pin on Pinterest0