NGC 7023 (Iris Nebula) and Molecular Clouds in Cepheus

Images below show a dusty 6.25° × 4.0° large region in Cepheus containing NGC 7023 (Iris Nebula) and VdB 4141 (Ghost Nebula). It is a result from a test shots with new instruments for the Northern hemisphere survey. A report about the newly used SDSS filters including a comparison with RGB can be found at here.

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NGC 7023 (Iris Nebula) and Molecular Clouds in Cepheus with SDSS I', R' and G' filters
Color composite from SDSS I' (mapped to red), R' (mapped to green) and G' (mapped to blue) filters.

NGC 7023, also known as Iris Nebula, is the bright blue region top left of the center. The bright nebula that lies left to it, is VdB 4141 (Ghost Nebula). The deep red star between these two nebulae is T Cephei, a Mira variable star. These stars are dying a red giants characterized by strong infrared emission (that's why it appears red here). In near infrared T Cephei is about 100 times brighter than in visible light (-0.496 mag in J band vs. 4.644 mag in G band). Another bright red giant is AC Draconis in top right corner, a long periodic variable star.

The red and green artifacts below AC Draconis are probably caused by reflections on a part within the lens (due to the strange shape). The artifacts around the bright stars are residuals from the aggressive star reduction (by up to factor 40) and the fact that the PSF (point spread function) strongly varies across the field of view.

The color of the nebulae is influenced by scattering which makes dense region more opaque for blue light than for (infra)red radiation. For that reason regions known as dark nebulae are reddish in that image.

Image data

FOV: 6.25° × 4.0°
Date: 12/2021 to 01/2022
Location: Pulsnitz, Germany
Instrument: 2-3 × 100mm lens at f=300mm (3× array was not fully operational at beginning)
Camera Sensor: 2-3 × IMX455 (3× array was not fully operational at beginning)
Orientation: North is up (exactly)
Scale: 3 arcsec/pixel (at full resolution)
Total exposure times:
SDSS G': 10.5 h
SDSS R': 9.9 h
SDSS I': 6.1 h

Image processing

All image processing steps are deterministic, i.e. there was no manual retouching or any other kind of non-reproducible adjustment. The software which was used can be downloaded here.

Image processing steps where:

  1. Bias correction, dark current subtraction, flatfield correction, noise estimation
  2. Alignment and brightness calibration using stars from reference image
  3. Stacking with masking unlikely values and background correction
  4. Star subtraction
  5. Denoising and deconvolution
  6. Color composition
  7. Dynamic range compression using non-linear high-pass filter
  8. Tonal curve correction

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