The Coathanger

The CoathangerClick image for full size version

September 13, 2021

The Coathanger is an asterism — a pattern of stars — that looks unmistakably like its namesake. It is also catalogued as Brocchi’s Cluster and Collinder 399. I’ve looked at this hundreds of times in binoculars; it is a summer favourite. It can even be seen with the naked eye in a dark sky.  

The Coathanger was long thought to be a star cluster — a group of gravitationally-bound stars. However, it has recently been concluded that it is a chance alignment of stars. The many dark lanes within the image are part of the Milky Way’s dust lane. Several are included in the LDN and LBN catalogues. I prepared an annotated image that identifies some of them. The pretty star cluster at right in the image is NGC 6802.

Acquisition, focusing, and control of Sky-Watcher HEQ6 mount with N.I.N.A., EQMOD and PHD2. Focus with Optec DirectSync motor and controller. Equipment control with PrimaLuce Labs Eagle 4 Pro computer. All pre-processing and processing in PixInsight. Acquired from my SkyShed in Guelph. Average transparency and seeing. Data acquired August 23-25, 2021 in a mostly moonless sky.

Takahashi FSQ-106 ED IV @ f/5 and QHY367C Pro one-shot colour camera with Optolong UV/IR filter

34x5m UV-IR = 2hr50m
Image scale 1.9 arcsec per pixel

Data Reduction and Linear Processing Steps
Preprocessing: The WeightedBatchPreProcessing script was used to create a master frame. The OSC subs were integrated with DrizzleIntegration (CFA Drizzle x1).

Gradient Removal: DBE was applied using Subtraction.

Colour Balancing:  Colour was balanced with PhotometricColorCalibration.

Linear Noise Reduction: MultiscaleLinearTransform was used to reduce noise in the background areas, using an internal mask to protect stars. 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.

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). 

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

Final Steps: Background and star brightness, contrast, and colour saturation were adjusted in several iterations using CurvesTransformation with masks as required. The DarkStructureEnhance script was applied with a strength of 0.2. ICCProfileTransformation (sRGB IEC61966-2.1; Relative Colorimetric with black point compensation) was applied prior to saving in jpg format.