NGC 6791

NGC 6791Click image for full size version

December 24, 2020

NGC 6791, in Lyra, is one of the most studied open clusters in the sky. This cluster has a mass of about 5,000 times that of our Sun. It is old, 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.

This image was taken on a night illuminated by a nearly full Moon in July 2015, and shows why I love shooting open clusters: the good ones punch right through even when the skies are bright.

Tekkies:

SBIG STL-11000M camera, Baader R, G and B filters, 10″ f/6.8 ASA astrograph, Paramount MX.  Guided with QHY5 guide camera and 80 mm f/6 Stellar-Vue refractor.  Acquisition, guiding and mount control with TheSkyX. Focusing with FocusMax. Automation with CCDCommander. All preprocessing and post-processing in PixInsight. Shot from my SkyShed in Guelph, Ontario. Acquired July 2-3, 2015. Good to excellent transparency, and good to very good seeing throughout acquisition.

11 x 10m R
11 x 10m G
9 x 10m B
Total: 5hr10m
 
Image scale 1.1 arcsec per pixel

Data Reduction and Initial Processing
Preprocessing: The WeightedBatchPreProcessing script was used to create integrated master files.

Gradient Removal: DBE was applied to each master using Subtraction.

Luminance Creation:  The masters were integrated with ImageIntegration, using Average combination, weighted for noise with no pixel rejection.

Colour
Colour Balancing:  Colour was balanced with ColorCalibration.

Linear Noise Reduction: MultiscaleLinearTransform was used to reduce noise in the background areas, using an internal mask to protect bright structures. Layer settings for threshold and strength: Layer 1: 5.0 0.85, 2 iterations; Layer 2: 3.5, 0.75, 2 iterations.

Stretching:  HistogramTransformation was applied to make a pleasing, bright image, with background set to an intensity of approximately 0.10.

Luminance

Linear Noise Reduction:  MultiscaleLinearTransform was used to reduce noise in the background areas, using an internal mask to protect bright stars. Layer settings for threshold and strength: Layer 1: 3.0 0.85, 1 iterations; Layer 2: 2.0, 0.75, 2 iterations.

Stretching:  HistogramTransformation was applied to make a pleasing, bright image, with background set to an intensity of approximately 0.10.

Combining Lightness and Colour Images

LRGB Combination: The lightness image was applied to the RGB image using LRGBCombination with default settings.

Additional Processing
Nonlinear Noise Reduction: TGVDenoise was used in L*a*b* mode to reduce noise with a mask used to target the background areas and protect the stars (max. 1,000 iterations and convergence selected for both lightness and chrominance). 

Final Steps: Background and star brightness, contrast, and colour saturation were adjusted in several iterations using CurvesTransformation with masks as required. ICCProfileTransformation (sRGB IEC61966-2.1; Relative Colorimetric with black point compensation) was applied prior to saving in jpg format.