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The Cygnus Loop: A Cosmic Shock Wave

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Mosaic Image of Cygnus Loop

Six-panel (3 x 2) mosaic of the Cygnus Loop

  • Telescope: Askar FRA 500 (90-mm aperture)
  • Mount: ZWO AM5 (strain-wave gear drive)
  • Autoguiding: Off-axis guider with ZWO ASI120-MM Mini camera
  • Controller: ZWO ASIair
  • Optical Configuration: Flat-field quintuplet Petzval refractor (f/5.6 w/o reducer)
  • Filterwheel: RGB (Astronomik 1.25-in.); H-alpha (Baader 7-nm); O-III (Baader 4-nm)
  • Imaging Camera: ZWO ASI1600-MM Pro
  • Camera Gain: 70 (RGB); 0 (H-alpha & O-III)
  • Sensor Temperature: -10° C
  • Light Frames per Panel: ~120 (H-alpha & O-III); 40 (RGB)
  • Calibration Frames: 60 darks, 60 biases, 30 flats per filter
  • Total Exposure Time per Filter per Panel: 8 h (H-alpha & O-III); 0.66 h (RGB)
  • Pre-Processing & Processing: PixInsight
  • Post-processing: Photoshop CC
  • Imaging Locations: Sierra Nevada Mountains (8,600 ft.); Los Angeles, Calif.

The Cygnus Loop, a favorite deep-sky object (DSO) among professional and amateur astronomers alike, is created from the shock wave and remnants of an ancient supernova explosion hurtling through the interstellar medium (ISM) at hypersonic speeds. The energy released into the ISM causes it to glow at wavelengths across the spectrum from x-rays to radio waves. This image, taken in the visible spectrum, captures the continuum emissions of dust/molecules (white) as well as the line emissions from hydrogen atoms (red) and oxygen atoms (teal).

The most recent and reliable distance estimates using Gaia astrometry data (https://academic.oup.com/mnras/article/481/2/1786/5088377) place the Cygnus Loop at about 2,400 light years (735 parsecs) from Earth, where it takes up 3 degrees of our sky (six Moons wide). This means that its actual diameter is about 120 light years.

To photograph such an extensive DSO with my new rig meant shooting a six-panel mosaic. RGB subframes for each panel of the mosaic were shot first at a dark-sky location far away from the city. Back in the city, I shot 8 hours of narrowband subframes for each panel in H-alpha and O-III. All imaging sessions were fully automated with the ASIair. Shooting all of the ~2,100 subframes for this mosaic took about a month and a half.

To ensure the highest color fidelity of the star field and nebulae, RGB color calibration was achieved during processing using Gaia’s latest photometric data (DR3) through PixInsight’s Spectrophotometric Color Calibration tool. Blending the narrowband data into a final HaOiiiRGB image was accomplished using the PixelMath tool. The latest AI image-processing tools were also employed for noise reduction, star-field optimization, and deconvolution.

The full-resolution mosaic image is 10,800 x 10,800 pixels, covers about four square degrees of sky, and reveals the entire Cygnus Loop and its surrounding star field with superlative color and clarity.