SH2-125 (Cocoon Nebula, IC 5146) and VdB 147

SH2-125 (also known as the Cocoon Nebula and IC 5146) is the brightest part of a larger star-forming region in the constellation Cygnus. The object is a mixed reflection / emission nebula illuminated by the star cluster Collinder 470 and primarily ionized by its brightest member, BD +46 3474. VdB 147 is a smaller reflection nebula in the same star-forming region, mostly illuminated by BD +46 3471. Distances of approximately 2500 light-years (770 pc) for both nebulae are known from parallax measurements by the Gaia spacecraft.

SH2-125 (IC 5146, Cocoon Nebula) and VdB 147 in H-alpha and continuum
Hα: on
Click on the image to load it at full resolution in a JavaScript viewer. Use the button to toggle Hα.

In both images, NIR (near infrared) is mapped to red, red is mapped to green and blue is mapped to blue. In the version with Hα, that emission line is added to the red channel. Stars are partially subtracted to improve the visibility of the nebulae.

SH2-125 is the large blue and red (Hα version only) nebula. Its ionizing source, BD +46 3474, is the bright star near the center. The small blue object to the right of SH2-125 is VdB 147.

Image data

FOV (full view in the JavaScript viewer): 0.62° × 0.47°
Position (J2000): RA: 21h53m05s; DEC: 47°16′
Date: 2018-2022
Location: Pulsnitz, Germany
Instrument: 400mm Newton at f=1520mm
Camera Sensor: Panasonic MN34230 (all but red), Sony IMX455 (red)
Orientation: North is up
Scale: 0.8 arcsec/pixel (at full resolution)
Total exposure times:
Hα (3 nm): 10.3 h
Near infrared: 8.3 h
Red (SDSS R' + 400-650 nm band-pass): 2.1 h
Blue: 6.1 h

Image processing

All image processing steps are deterministic and none of the algorithms use machine learning (often referred to as “AI”), which tends to generate plausible looking fake details. The software used can be downloaded here.

The image processing steps were:

  1. Bias correction, dark current subtraction, flatfield correction, noise estimation
  2. Alignment and brightness calibration using stars from reference image
  3. Stacking with outlier rejection, background estimation and optimal weighting based on noise estimation
  4. Star subtraction where star positions and intensities are extracted from continuum images
  5. Denoising and deconvolution of both components (stars and residual)
  6. Dynamic range compression using non-linear high-pass filter
  7. Color composition and tonal curve correction

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