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Caldwell 4

The Iris Nebula

Caldwell 4

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The Iris Nebula (also known as NGC 7023 and Caldwell 4) is a bright reflection nebula in the constellation Cepheus. Wikipedia


Acquisition Details

Dates August 28th, 2022 (Waxing Crescent Moon)
Frames 30x300” (5mins), 2h 30m of total integration time
Focal Length 360mm (f/6)
Camera ZWO ASI533MC-Pro (gain 100, cooled at -5°C)
Telescope TS-Optics Photoline 60mm Doublet Apochromatic Refractor
Accessories 1.0x Flattener
Filters TS-Optics 2” UV & IR Cut Filter
Mount Sky-Watcher EQ6R-Pro
Guide Scope Angeleyes Mini Guidescope 30mm f/4
Guide Camera ZWO ASI 120MM mini
Computer ZWO ASIAIR Pro

Processing Details

I stacked the ligths frames in Siril using the OSC Preprocessing Without DBF (Darks, Biases, Flats) script. I decided not to use any Dark, Bias or Flat frames. I might try to create a dark frame library later, but the ASI533MC-Pro is considered a camera that can do without darks given the absence of amp glow.

I then used the following tools:

  • Photometric Color Calibration
  • Asinh Transformation (stretch 600.0, black point 0.001)
  • Histogram Transformation (mid 0.077, low 0.093, high 1.000)
  • SCNR (type 0, amount 1.00, preserve true)
  • Saturation enhancement (amount 0.35)
  • Background extraction (Correction: Subtraction)