Double Cluster

Double ClusterClick image for full size version

I’ve imaged the Double Cluster with just about every different scope and camera that I’ve used. The Sky-Watcher Esprit 150 with the QHY-16200 camera really hits the sweet spot, getting the cluster well resolved in a nice frame.

NGC 869 (left) and NGC 884,each contain hundreds of stars and are about 13 million years old (compared to 75-150 million years for the Pleiades). Lying about 7,500 light years away and a few hundred light years apart, they have a combined mass about 6,500 times our Sun’s. However, their combined halo is more than 20,000 solar masses. The clusters are moving towards us at a speed of about 40 km/s.

These open clusters are bright enough that they were originally given star designations (h- and Chi Perseus). They are visible to the naked eye, appearing as an oval foggy patch in a dark sky. They look like sparkling white diamond dust in binoculars or a small telescope. 


Sky-Watcher Esprit 150 f/7 refractor, QHY 16200-A camera, Optolong L, R, G and B filters, Paramount MX. Acquisition with TheSkyX unguided. Focused with TheSkyX @Focus3. Automation with CCDCommander. All pre-processing and processing in PixInsight. Acquired from my SkyShed in Guelph. No moonlight, average transparency and average seeing. Data acquired October 9, 2018.

10x3m L, R, G and B (Total = 2hr).

Data Reduction and Cleanup
The BatchPreProcessing script was used to perform calibration, cosmetic correction and registration of all frames. ImageIntegration was used to create the L, R, G and B masters. DynamicCrop was used to crop all the masters identically. DynamicBackgroundExtraction was applied to each master.

RGB Creation and Processing
Creation and cleanup: ChannelCombination was used to make a colour image from the R, G and B masters. The RGB image was processed with PhotometricColorCalibration using a small preview of background sky as the background reference.  

Linear Noise Reduction:  MultiscaleLinearTransform was used to reduce noise in the RGB image. An internal mask was used, with layer settings for threshold and strength as follows: Layer 1: 3, 0.65   Layer 2: 2, 0.4  Layer 3: 2, 0.25  Layer 4: 1, 0.15.

Stretching:  MaskedStretch was applied to the RGB image to make a pleasing, bright image. This was followed by brightness and contrast enhancement with HistogramTransformation and CurvesTransformation.

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

Linear Noise Reduction: MultiscaleLinearTransform was used to reduce noise in the SynthL image. An internal mask was used, with layer settings for threshold and strength as follows: Layer 1: 3, 0.5   Layer 2: 2, 0.35  Layer 3: 2, 0.2  Layer 4: 1, 0.1.

Stretching:  HistogramTransformation was applied to the SynthL to make a pleasing, bright image. 

Combining SynthL and RGB
The processed SynthL was applied to the RGB image using LRGBCombine.

Additional Processing

Nonlinear Noise Reduction: TGVDenoise was used in L*a*b* mode to reduce noise in the background areas.

Final Steps: Background and star brightness, contrast and saturation were adjusted in several iterations using CurvesTransformation with masks as required. 

Image scale is about 1.15 arcsec per pixel for this camera/telescope combination.