NGC6791, Open Cluster

NGC6791Click image for full size version

July 3, 2015, Astronomy Magazine Online Picture of the Day July 6, 2015

Astronomy Mag POD July 6 2015NGC6791 is one of the most studied open clusters in the sky — but not by me up until now.  In fact, I hadn’t heard of it until I began looking for targets to shoot on a night lit up by a nearly full moon.  This cluster is in the constellation Lyra, and has a mass of about 5,000 times that of our Sun.  It is an old cluster, at about 8 billion years, but contains some unusual younger stars (4 and 6 billion years old).  It is very metal-rich (“metals” in astronomy refers to elements other than hydrogen and helium); old open clusters are usually metal-poor.  It covers an area about half the width of the Moon.  Look around the image and you’ll spot a few galaxies that lie far in the background.

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 and guiding with Maxim-DL.  Focusing with FocusMax.  Automation with CCDCommander.  Calibration, cosmetic correction, registration, integration and all processing in PixInsight.  Shot from my SkyShed in Guelph, Ontario.  Full moon, good to very good transparency and seeing.

11x10m R, 11x10m G and 9x10m B unbinned frames (total=5hr10m).

RGB:
Creation and cleanup: R, G and B masters were cropped and DBE was applied to each before combining to make an RGB image.  BackgroundNeutralization and ColorCalibration were applied.

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

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

Stretching: HistogramTransformation was applied to make a pleasing yet bright image.  TGV Noise was applied and the image was re-stretched to reset the black point.

Combining SynthL with RGB:
LRGBCombination was then used to apply the Luminance to the RGB, making a SynthLRGB image.

Final Processing
Colour saturation of the stars was increased using a star mask.  The mask was inverted and saturation of the background was decreased.  Curves was applied to boost contrast.

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

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